Medical Physiology, 3rd Edition

Cellular Organelles and the Cytoskeleton

The cell is composed of discrete organelles that subserve distinct functions

When a eukaryotic cell is viewed through a light microscope, a handful of recognizable intracellular structures can be discerned. The intracellular matrix, or cytoplasm, appears grainy, suggesting the presence of components that are too small to be discriminated by this technique. With the much higher magnifications available with an electron microscope, the graininess gives way to clarity that reveals the cell interior to be remarkably complex. Even the simplest nucleated animal cell possesses a wide variety of intricate structures with specific shapes and sizes. These structures are the membrane-enclosed organelles, the functional building blocks of cells.

Figure 2-10 illustrates the interior of a typical cell. The largest organelle in this picture is the nucleus, which houses the cell's complement of genetic information. This structure, which is visible in the light microscope, is usually round or oblong, although in some cells it displays a complex, lobulated shape. Depending on the cell type, the nucleus can range in diameter from 2 to 20 µm. With some exceptions, including skeletal muscle and certain specialized cells of the immune system, each animal cell has a single nucleus.

image

FIGURE 2-10 Ultrastructure of a typical animal cell.

Surrounding the nucleus is a web of tubules or saccules known as the endoplasmic reticulum (ER). This organelle can exist in either of two forms, rough or smooth. The surfaces of the rough ER tubules are studded with ribosomes, the major sites of protein synthesis. Ribosomes can also exist free in the cytosol. The surfaces of the smooth ER, which participates in lipid synthesis, are not similarly endowed. The ER also serves as a major reservoir for calcium ions. The ER membrane is equipped with a Ca pump that uses the energy released through ATP hydrolysis to drive the transport of Ca2+ from the cytoplasm into the ER lumen (see p. 118). This Ca2+ can be rapidly released in response to messenger molecules and plays a major role in intracellular signaling (see p. 60).

The Golgi complex resembles a stack of pancakes. Each pancake in the stack represents a discrete, flat saccule. The number and size of the saccules in the Golgi stack vary among cell types. The Golgi complex is a processing station that participates in protein maturation and targets newly synthesized proteins to their appropriate subcellular destinations.

Perhaps the most intriguing morphological appearance belongs to the mitochondrion, which is essentially a balloon within a balloon. The outer membrane and inner membrane define two distinct internal compartments: the intermembrane space and the matrix space. The surface of the inner membrane is thrown into dramatic folds called cristae. This organelle is ~0.2 µm in diameter, which is at the limit of resolution of the light microscope. The mitochondrion is the power plant of the cell, a critical manufacturer of ATP. Many cellular reactions are also catalyzed within the mitochondrion.

The cell's digestive organelle is the lysosome. This large structure frequently contains several smaller round vesicles called exosomes within its internal space.

The cytoplasm contains numerous other organelles whose shapes are not quite as distinguishing, including endosomes, peroxisomes, and transport vesicles.

Despite their diversity, all cellular organelles are constructed from the same building blocks. Each is composed of a membrane that forms the entire extent of its surface. The membranes of the subcellular organelles are what can be visualized in electron micrographs. The biochemical and physical properties of an organelle's limiting membrane dictate many of its functional properties.

The nucleus stores, replicates, and reads the cell's genetic information

The nucleus serves as a cell's repository for its complement of chromosomal DNA. To conceive of the nucleus as simply a hermetically sealed vault for genetic information, however, is a gross oversimplification. All of the machinery necessary to maintain, to copy, and to transcribe DNA is in the nucleus, which is the focus of all of the cellular pathways that regulate gene expression and cell division. Transcriptional control is discussed in Chapter 4. The focus of this section is nuclear structure.

The nucleus is surrounded by a double membrane (see Fig. 2-10). The outer membrane is studded with ribosomes and is continuous with the membranes of the rough ER. The inner membrane is smooth and faces the intranuclear space, or nucleoplasm. The space between these concentric membranes is continuous with the lumen of the rough ER. The inner and outer nuclear membranes meet at specialized structures known as nuclear pores, which penetrate the nuclear envelope and provide a transport pathway between the cytoplasm and the nuclear interior (see p. 110). All RNA transcripts that are produced in the nucleus must pass through nuclear pores to be translated in the cytoplasm. Similarly, all the signaling molecules that influence nuclear function as well as all proteins of the nuclear interior (which are synthesized in the cytoplasm) enter the nucleus through nuclear pores.

Nuclear pores are selective in choosing the molecules that they allow to pass. Cytoplasmic proteins destined for the nuclear interior must be endowed with a nuclear-localization sequence to gain entry. Several nuclear localization sequences have been characterized, and all seem to share common structural elements. For example, they all have short stretches of four to eight basic amino acids that can be located anywhere in the protein's sequence. Evidence implies that the ability of these signals to mediate nuclear localization can be modulated by phosphorylation, which suggests that the entry of proteins into the nucleus may be under the control of the cell's second-messenger systems.

The selectivity of the nuclear pore is surprising, considering its size. The outer diameter of the entire nuclear pore is ~100 nm, considerably larger than the proteins whose passage it controls. The nuclear pore's specificity is provided by the nuclear pore complex (NPC), an intricate matrix of protein that is distributed in a highly organized octagonal array. The NPC is composed of several hundred proteins, of at least 30 different types, that form a pore of variable internal diameter. In its resting state, the NPC forms an aqueous channel that is ~9 nm in diameter, restricting the movement of any protein larger than 60 kDa. However, when it is confronted with a protein bearing a nuclear-localization signal or a messenger RNA (mRNA) transcript, the pore complex can dilate to many times this size. The structures of the proteins lining the pore appear to be uniquely flexible, perhaps permitting the cell to regulate the pore's permeability. The NPC acts as a barrier that prevents the random diffusion of integral membrane proteins between the outer and inner membranes of the nuclear envelope. However, newly synthesized membrane proteins destined for residence in the inner nuclear membrane appear to carry signals that allow them to diffuse from their sites of synthesis in the outer membrane to the inner membrane via the NPC. Thus, although the inner and outer nuclear membranes are continuous with one another at nuclear pores, their protein contents remain tightly controlled and distinct.

Between mitoses, the chromosomal DNA is present in the nucleus as densely packed heterochromatin and more loosely arrayed euchromatin. Chromatin is a complex between DNA and numerous DNA-binding proteins, which organize the chromosome into a chain of tightly folded DNA-protein assemblies called nucleosomes (see pp. 75–76). Interspersed within the nucleoplasm are round, dense nucleoli, where the transcription of ribosomal RNA and the assembly of ribosomal subunits appear to occur.

The interior surface of the inner nuclear membrane is apposed to a fibrillar protein skeleton referred to as the nuclear lamina. This meshwork, composed of proteins known as lamins, is presumably involved in providing structural support to the nuclear envelope. Mutations in the gene encoding lamin A are responsible for progeria, a genetic disease associated with rapid, premature aging. The nuclear lamina may also play a role in orchestrating nuclear reassembly. During mitosis, the nuclear envelope breaks down into small vesicles, and the contents of the nucleoplasm mix with the cytoplasm. After mitosis, these vesicles fuse with one another to regenerate the double-walled nuclear membrane. The means by which these vesicles find one another and assemble correctly is the subject of intense study. Similarly, the mechanisms involved in maintaining the compositional discreteness of the inner and outer membranes during vesiculation and reassembly have yet to be determined. After reconstitution of the nuclear envelope, the proteins of the nucleoplasm are reimported from the cytoplasm through the nuclear pores by virtue of their nuclear-localization sequences.

Lysosomes digest material derived from the interior and exterior of the cell

In the course of normal daily living, cells accumulate waste. Organelles become damaged and dysfunctional. Proteins denature and aggregate. New materials are constantly being brought into the cells from the extracellular environment through the process of endocytosis (discussed below). In specialized cells of the immune system, such as macrophages, the collection of foreign materials (in the form of pathogens) from the extracellular milieu is the cellular raison d'être. If this material were allowed to accumulate indefinitely, it would ultimately fill the cell and essentially choke it to death. Clearly, cells must have mechanisms for disposing of this waste material.

The lysosome is the cell's trash incinerator. It is filled with a broad assortment of degradative enzymes that can break down most forms of cellular debris. Proton pumps embedded within the lysosome's limiting membrane ensure that this space is an extremely acidic environment, which aids in protein hydrolysis. A rare group of inherited disorders, called lysosomal storage diseases (see Box 2-2), result from the deficiency of lysosomal enzymes that are involved in the degradation of a variety of substances.

The lysosomal membrane is specially adapted to resist digestion by the enzymes and the acid that it encapsulates, which ensures that the harsh conditions necessary for efficient degradation are effectively contained. Loss of lysosomal membrane integrity may underlie some clinically important inflammatory conditions, such as gout.

Material that has been internalized from the cell exterior by endocytosis is surrounded by the membrane of an endocytic vesicle. To deliver this material to the lysosome, the membranes of the endocytic vesicles fuse with the lysosomal membrane and discharge their cargo into the lysosomal milieu.

Intracellular structures that are destined for degradation, such as fragments of organelles, are engulfed by the lysosome in a process called autophagy. Autophagy results in the formation of membrane-enclosed structures within the lysosomal lumen; hence, the lysosome is often referred to as a multivesicular body. Autophagy allows the cell to degrade old or damaged components. Increased metabolic needs also can stimulate autophagy, allowing the cell to recycle and “burn” its own structural components to generate energy.

The mitochondrion is the site of oxidative energy production

Oxygen-dependent ATP production—or oxidative phosphorylation—occurs in the mitochondrion. Like the nucleus, the mitochondrion (see Fig. 2-10) is a double-membrane structure. The inner mitochondrial membrane contains the proteins that constitute the electron transport chain, which generates pH and voltage gradients across this membrane. According to the “chemiosmotic” model (see p. 118), the inner membrane uses the energy in these gradients to generate ATP from ADP and inorganic phosphate.

The mitochondrion maintains and replicates its own genome. This circular DNA strand encodes mitochondrial transfer RNAs and (in humans) 13 mitochondrial proteins. Several copies of the mitochondrial genome are located in the inner mitochondrial matrix, which also has all of the machinery necessary to transcribe and to translate this DNA, including ribosomes. Whereas the proteins encoded in mitochondrial DNA contribute to the structure and function of the mitochondrion, they account for a relatively small fraction of total mitochondrial protein. Most mitochondrial proteins are specified by nuclear DNA and are synthesized on cytoplasmic ribosomes.

The two mitochondrial membranes enclose two distinct compartments: the intermembrane space and the inner mitochondrial matrix space. The intermembrane space lies between the two membranes; the inner mitochondrial matrix space is completely enclosed by the inner mitochondrial membrane. These compartments have completely different complements of soluble proteins, and the two membranes themselves have extremely different proteins.

In addition to playing a role in energy metabolism, the mitochondrion also serves as a reservoir for intracellular Ca2+. The mitochondrial Ca2+ uniporter MCU, together with the regulatory protein MICU, mediates the uptake of Ca2+ across the mitochondrial inner membrane and into the matrix via facilitated diffusion. MCU exploits the mitochondrial membrane potential to drive the accumulation of very high Ca2+ levels. Elevated cytosolic [Ca2+] can lead to the release of Ca2+ from this reservoir and thereby amplify signaling processes. The mitochondrial Ca2+ stores are also released as a consequence of energy starvation, which leads to cell injury and death. Finally, the mitochondrion plays a central role in the process called apoptosis, or programmed cell death (see p. 1241). Certain external or internal signals can induce the cell to initiate a signaling cascade that leads ultimately to the activation of enzymes that bring about the cell's demise. One of the pathways that initiates this highly ordered form of cellular suicide depends on the participation of the mitochondrion. Apoptosis plays an extremely important role during tissue development and is also involved in the body's mechanisms for identifying and destroying cancer cells.

Mitochondriopathies—disorders of mitochondrial function—appear to play a role in several degenerative diseases (e.g., Huntington disease, dysautonomic mitochondrial myopathy).

The cytoplasm is not amorphous but is organized by the cytoskeleton

Our discussion thus far has focused almost exclusively on the cell's membranous elements. We have treated the cytoplasm as if it were a homogeneous solution in which the organelles and vesicles carry out their functions while floating about unimpeded and at random. Rather, the cytoplasm is enormously complex with an intricate local structure and the capacity for locomotion.

The cytoplasmic cytoskeleton is composed of protein filaments that radiate throughout the cell, serving as the beams, struts, and stays that determine cell shape and resilience. On the basis of their appearance in the electron microscope, these filaments were initially divided into several classes (Table 2-2): thick, thin, and intermediate filaments as well as microtubules. Subsequent biochemical analysis has revealed that each of these varieties is composed of distinct polypeptides and differs with respect to its formation, stability, and biological function.

TABLE 2-2

Components of the Cytoskeleton

 

SUBUNITS

DIAMETER (nm)

Intermediate filaments

Tetramer of two coiled dimers

8–10

Microtubules

Heterodimers of α and β tubulin form long protofilaments, 5 nm in diameter

25

Thin filaments

Globular or G-actin, 5 nm in diameter, arranged in a double helix to form fibrous or F-actin

5–8

Thick filaments

Assembly of myosin molecules

10

Intermediate filaments provide cells with structural support

Intermediate filaments are so named because their 8- to 10-nm diameters, as measured in the electron microscope, are intermediate between those of the actin thin filaments and the myosin thick filaments. As with all of the cytoskeletal filaments that we will discuss, intermediate filaments are polymers that are assembled from individual protein subunits. There is a very large variety of biochemically distinct subunit proteins that are all structurally related to one another and that derive from a single gene family. The expression of these subunit polypeptides can be cell-type specific or restricted to specific regions within a cell. Thus, vimentin is found in cells that are derived from mesenchyme, and the closely related glial fibrillary acidic protein (GFAP) is expressed exclusively in glial cells (see pp. 287–288). Neurofilament proteins are present in neuronal processes. The keratins are present in epithelial cells as well as in certain epithelially derived structures. The nuclear lamins that form the structural scaffolding of the nuclear envelope are also members of the intermediate-filament family.

Intermediate-filament monomers are themselves fibrillar in structure. They assemble to form long, intercoiled dimers that in turn assemble side to side to form the tetrameric subunits. Finally, these tetrameric subunits pack together, end to end and side to side, to form intermediate filaments. Filament assembly can be regulated by the cell and in some cases appears to be governed by phosphorylation of the subunit polypeptides. Intermediate filaments appear to radiate from and to reinforce areas of a cell subject to tensile stress. They emanate from the adhesion plaques that attach cells to their substrata. In epithelial cells, they insert at the desmosomal junctions that attach neighboring cells to one another. The toughness and resilience of the meshworks formed by these filaments is perhaps best illustrated by the keratins, the primary constituents of nails, hair, and the outer layers of skin.

Microtubules provide structural support and provide the basis for several types of subcellular motility

Microtubules are polymers formed from heterodimers of the proteins α and β tubulin (Fig. 2-11A). These heterodimers assemble head to tail, creating the circumferential wall of a microtubule, which surrounds an empty lumen. Because the tubulin heterodimers assemble with a specific orientation, microtubules are polar structures, and their ends manifest distinct biochemical properties. At one tip of the tubule, designated the plus end, tubulin heterodimers can be added to the growing polymer at three times the rate that this process occurs at the opposite minus end. The relative rates of microtubule growth and depolymerization are controlled in part by an enzymatic activity that is inherent in the tubulin dimer. Tubulin dimers bind to GTP, and in this GTP-bound state they associate more tightly with the growing ends of microtubules. Once a tubulin dimer becomes part of the microtubule, it hydrolyzes the GTP to GDP, which lowers the binding affinity of the dimer for the tubule and helps hasten disassembly. Consequently, the microtubules can undergo rapid rounds of growth and shrinkage, a behavior termed dynamic instability. Various cytosolic proteins can bind to the ends of microtubules and serve as caps that prevent assembly and disassembly, and thus stabilize the structures of the microtubules. A large and diverse family of microtubule-associated proteins appears to modulate not only the stability of the tubules but also their capacity to interact with other intracellular components.

image

FIGURE 2-11 Microtubules. A, Heterodimers of α and β tubulin form long protofilaments, 13 of which surround the hollow core of a microtubule. The microtubule grows more rapidly at its plus end. The molecular motor dynein moves along the microtubule in the plus-to-minus direction, whereas the molecular motor kinesin moves in the opposite direction. ATP is the fuel for each of these motors. B, The microtubules originate from a microtubule-organizing center or centrosome, which generally consists of two centrioles (green cylinders). C, A motile cilium can actively bend as its microtubules slide past each other. The molecular motor dynein produces this motion, fueled by ATP.

In most cells, all of the microtubules originate from the microtubule-organizing center or centrosome. This structure generally consists of two centrioles, each of which is a small (~0.5 µm long × 0.3 µm in diameter) assembly of nine triplet microtubules that are arranged obliquely along the wall of a cylinder (upper portion of Fig. 2-11B). The two centrioles in a centrosome are oriented at right angles to one another. The minus ends of all of a cell's microtubules are associated with proteins that surround the centrosome, whereas the rapidly growing plus ends radiate throughout the cytoplasm in a star-like arrangement (“astral” microtubules).

Microtubules participate in a multitude of cellular functions and structures. For example, microtubules project down the axon of neurons. Microtubules also provide the framework for the lacy membranes of the ER and Golgi complex. Disruption of microtubules causes these organelles to undergo dramatic morphological rearrangements and vesicularization. Microtubules also play a central role in cell division. Early in mitosis, the centrioles that make up the centrosomes replicate, forming two centrosomes at opposite poles of the dividing nucleus. Emanating from these centrosomes are the microtubules that form the spindle fibers, which in turn align the chromosomes (lower portion of Fig. 2-11B). Their coordinated growth and dissolution at either side of the chromosomes may provide the force for separating the genetic material during the anaphase of mitosis. A pair of centrioles remains with each daughter cell.

The architectural and mechanical capacities of microtubules are perhaps best illustrated by their role in motility. An electron microscopic cross section of a cilium demonstrates the elegance, symmetry, and intricacy of this structure (see Fig. 2-11C). Every cilium arises out of its own basal body, which is essentially a centriole that is situated at the ciliary root. Cilia come in two varieties—motile and nonmotile. Whereas motile cilia move and develop force, generating directional fluid flow in a number of organs, nonmotile cilia do not move on their own and instead serve sensory functions. We discuss motile cilia here and nonmotile cilia on page 43.

Cilia are present on the surfaces of many types of epithelial cells, including those that line the larger pulmonary airways (see p. 597). In the airway epithelial cells, their oar-like beating motions help propel foreign bodies and pathogens toward their ultimate expulsion at the pharynx. At the center of a cilium is a structure called the axoneme, which is composed of a precisely defined “9 + 2” array of microtubules. Each of the 9 (which are also called outer tubules) consists of a complete microtubule with 13 tubulin monomers in cross section (the A tubule) to which is fused an incomplete microtubule with 11 tubulin monomers in cross section (the B tubule). Each of the 2, which lie at the core of the cilium, is a complete microtubule. This entire 9 + 2 structure runs the entire length of the cilium. The same array forms the core of a flagellum, the serpentine motions of which propel sperm cells (see Fig. 56-1).

Radial spokes connect the outer tubules to the central pair, and outer tubules attach to their neighbors by two types of linkages. One is composed of the protein dynein, which acts as a molecular motor to power ciliary and flagellar motions. image N2-4Dynein is an ATPase that converts the energy released through ATP hydrolysis into a conformational change that produces a bending motion. Because dynein attached to one outer tubule interacts with a neighboring outer tubule, this bending of the dynein molecule causes the adjacent outer tubules to slide past one another. It is this sliding-filament motion that gives rise to the coordinated movements of the entire structure. To some extent, this coordination is accomplished through the action of the second linkage protein, called nexin. The nexin arms restrict the extent to which neighboring outer tubules can move with respect to each other and thus prevent the dynein motor from driving the dissolution of the entire complex.

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Molecular Motors

Contributed by Alisha Bouzaher

Molecular motors, a class of molecular machinery, convert chemical energy into mechanical force and motion necessary to carry out important mechanisms of translocation and organization within the cell. By harnessing chemical free energy released from the hydrolysis of ATP, molecular motors bind and translocate a substrate in a unidirectional motion along a polarized track; an example is myosin “walking” along actin filaments. The directionality of molecular motor movement along these tracks depends on track polarity and the interaction of the track with the head domain on the motor protein, the site of ATP hydrolysis. This movement is typically linear or rotational.

The families of molecular motors exhibit diverse amino-acid sequence, structure, and motile properties.

Family

Motor

Tract

Motion

Physiological Function

Cytoskeletal motors

Myosin image N2-14 (see pp. 233–234)

Actin filaments image N2-6

Linear

Muscle contraction, cell locomotion, ciliary function

 

Dynein (see p. 25)

Microtubules (see p. 23–25)

Retrograde transport

Cilia, flagella movement

 

Kinesin (see p. 25)

Microtubules

Anterograde transport

Intracellular transport of membrane-bound organelles, mitotic/meiotic spindle formation

Nucleic acid motors

DNA polymerase

DNA

Linear

DNA replication

RNA polymerase (see p. 85)

DNA

Linear

DNA transcription

 

Helicase

DNA

Linear

DNA transcription

 

Topoisomerase

DNA

Linear

DNA transcription

   

   

References

Kolomeisky AB, Fisher ME. Molecular motors: A theorist's perspective. Annu Rev Phys Chem. 2007;58:675–695.

Schliwa M, Woehlke G. Molecular motors. Nature. 2003;422:759–765.

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Muscle Myosin

Contributed by Ed Moczydlowski

For historical reasons, the parts of the myosin II molecule in muscle often have more than one name.

• Myosin heavy chains (MHCs) consist of the following:

• The N-terminal head

• A neck or lever or linker or hinge

• The C-terminal rod or tail

• Essential myosin light chains (ELCs or MLC-1) are also called alkali chains.

• Regulatory myosin light chains (RLCs or MLC-2).

The utility of the dynein motor protein is not restricted to its function in cilia and flagella. Cytoplasmic dynein, which is a close relative of the motor molecule found in cilia, and a second family of motor proteins called kinesins provide the force necessary to move membrane-bound organelles through the cytoplasm along microtubular tracks (see Fig. 2-11A). The ability of vesicular organelles to move rapidly along microtubules was first noted in neurons, in which vesicles carrying newly synthesized proteins must be transported over extremely long distances from the cell body to the axon tip. Rather than trust this critical process to the vagaries of slow, nondirected diffusion, the neuron makes use of a kinesin motor, which links a vesicle to a microtubule. Kinesins hydrolyze ATP and, like dynein, convert this energy into mechanical transitions that cause kinesins to “walk” along the microtubule. Kinesins will move only along microtubules and thereby transport their vesicle cargoes in the minus-to-plus direction (orthograde). Thus, in neurons, kinesin-bound vesicles move from the microtubular minus ends, originating at the centrosome in the cell body, toward the plus ends in the axons. This direction of motion is referred to as anterograde fast axonal transport. Cytoplasmic dynein moves in the plus-to-minus direction (retrograde).

The motor-driven movement of cellular organelles along microtubular tracks is not unique to neurons. This process, involving both kinesins and cytoplasmic dynein, appears to occur in almost every cell and may control the majority of subcellular vesicular traffic.

Thin filaments (actin) and thick filaments (myosin) are present in almost every cell type

Thin filaments, also called microfilaments, are 5 to 8 nm in diameter. They are helical polymers composed of a single polypeptide called globular actin or G-actin. Thin filaments are functionally similar to microtubules in two respects: (1) the actin polymers are polar and grow at different rates at their two ends, and (2) actin binds and then hydrolyzes a nucleotide. However, whereas tubulin binds GTP and then hydrolyzes it to GDP, actin binds ATP and then hydrolyzes it to ADP. After G-actin binds ATP, it may interact with another ATP-bound monomer to form an unstable dimer (Fig. 2-12A). Adding a third ATP-bound monomer, however, yields a stable trimer that serves as a nucleus for assembly of the polymer of fibrous actin or F-actin. Once it is part of F-actin, the actin monomer hydrolyzes its bound ATP, retaining the ADP and releasing the inorganic phosphate. The ADP-bound actin monomer is more likely to disengage itself from its neighbors, just as GDP-bound tubulin dimers are more likely to disassemble from tubulin (see p. 23). Even though the length of the F-actin filament may remain more or less constant, the polymer may continually grow at its plus end but disassemble at its minus end (see Fig. 2-12B). This “treadmilling” requires the continuous input of energy (i.e., hydrolysis of ATP) and illustrates the unique dynamic properties of actin filament elongation and disassembly.

image

FIGURE 2-12 Thin filaments. A, Single molecules of G-actin form F-actin filaments. B, F-actin can grow at the plus end while shrinking at the minus end, with no change in length. Pi, inorganic phosphate.

Thick filaments are composed of dimers of a remarkable force-generating protein called myosin. All myosin molecules have helical tails and globular head groups that hydrolyze ATP and act as motors to move along an actin filament. The energy liberated by ATP hydrolysis is invested in bending the myosin molecule around a pivot point called the hinge region, which marks the junction between the globular and tail regions. By means of this bending, myosin, like the dynein and kinesin that interact with microtubules, acts as a molecular motor that converts chemical into mechanical energy. image N2-4

In muscle, the myosin molecules are in the myosin II subfamily and exist as dimers with their long tails intertwined (Fig. 2-13A). In muscle, each of the two myosin II heads binds two additional protein subunits referred to as myosin light chains. Nonmuscle cells, in addition to myosin II, may have a variety of other, smaller myosin molecules. These other myosins, the most widely studied of which is myosin I, have shorter tails and, at least in some cases, act as molecular motors that move vesicles along actin filaments.

image

FIGURE 2-13 Thick filaments. A, Myosin I is one of a large number of widely distributed myosins that have short tails. Myosin II is the myosin that participates in muscle contraction. B, The pivoting action of the myosin head, fueled by ATP, moves the thick filament past the thin filament. C, In skeletal and cardiac muscle, the sarcomere is the fundamental contractile unit. S1 and S2 are subfragments of heavy meromyosin.

In muscle, the myosin II dimers stack as antiparallel arrays to form a bipolar structure with a central region that contains only tails (see Fig. 2-13A). The ends of the thick filament contain the heads that bend toward the filament's central region. The pivoting action of the myosin head groups drags the neighboring thin filament (see Fig. 2-13B), which includes other molecules besides actin. This sliding-filament phenomenon underlies muscle contraction and force generation (see Fig. 2-13C).

Actin as well as an ever-growing list of myosin isoforms is present in essentially every cell type. The functions of these proteins are easy to imagine in some cases and are less obvious in many others. Many cells, including all of the fibroblast-like cells, possess actin filaments that are arranged in stress fibers. These linear arrays of fibers interconnect adhesion plaques to one another and to interior structures in the cell. They orient themselves along lines of tension and can, in turn, exert contractile force on the substratum that underlies the cell. Stress-fiber contractions may be involved in the macroscopic contractions that are associated with wound healing. Frequently, actin filaments in nonmuscle cells are held together in bundles by cross-linking proteins. Numerous classes of cross-linking proteins have been identified, several of which can respond to physiological changes by either stabilizing or severing filaments and filament bundles.

In motile cells, such as fibroblasts and macrophages, arrays of actin-myosin filaments are responsible for cell locomotion. Assembly of actin filaments can drive the directional extension of the cell membrane at the cell's leading edge, creating structures known as lamellipodia. A Ca2+-stimulated myosin light chain kinase regulates the assembly of myosin and actin filaments, which produce contractile force and retraction at the cell's trailing edge. To generate directional motion, these cytoskeletal elements must be able to form transient traction-generating connections to the cell's substratum. image N2-5 These connections are established through integrins (see p. 17).

N2-5

Cell Locomotion

Contributed by Michael Caplan

It should be noted that at least in some cases, the interactions that contribute to cellular motility are geometrically and biochemically quite distinct from those which underlie muscle contraction. For example, the amoebic cells of the bread mold Dictyostelium discoideum are able to continue crawling despite the elimination—by genetic deletion—of all of their myosin. Although motility in these altered cells is not normal, that it persists at all provides ample proof that the paradigms that apply to smooth and striated muscle are by no means the only schemes that nature has devised to generate actin filament–based motion.

In contrast to fibroblasts and circulating cells of the immune system, cells such as neurons and epithelial cells generally do not move much after their differentiation is complete. Despite this lack of movement, however, these cells are equipped with remarkably intricate networks of actin and myosin filaments. In some cases, these cytoskeletal elements permit the cell to extend processes to distant locations. This is the case in neurons, in which the growth and migration of axons during development or regeneration of the nervous system bear a striking morphological resemblance to the crawling of free-living amoebae. The tip of a growing axon, known as a growth cone, is richly endowed with contractile fibers and is capable of the same types of crawling motions that characterize motile cells.

In epithelial cells, the role of the actin-myosin cytoskeleton is somewhat less obvious but still important to normal physiological function. The microvilli at the apical surfaces of many epithelial-cell types (e.g., those that line the renal proximal tubule and the small intestine) are supported by an intricate scaffolding of actin filaments that form their cores (Fig. 2-14). This bundle of actin fibers is held together and anchored to the overlying plasma membrane by a variety of cross-linking proteins, including various myosin isoforms. The roots of the microvillar actin filament bundles emerge from the bases of the microvilli into a dense meshwork of cytoskeletal filaments known as the terminal web. Included among the components of the terminal web are fodrin (the nonerythroid homolog of spectrin) and myosin. The myosin in the terminal web interconnects the actin filaments of neighboring microvilli. In addition, this actin-myosin complex is capable of generating contractile movements that can alter the spatial relationships among microvilli.

image

FIGURE 2-14 Actin filaments at the brush border of an epithelial cell.

Actin and myosin filaments also form an adhesion belt that encircles the cytoplasmic surface of the epithelial plasma membrane at the level of the tight junctions that interconnect neighboring cells. These adhesion belts are apparently capable of contraction and thus cause epithelial cells that normally form a continuous sheet to pull away from one another, temporarily loosening tight junctions and creating direct passages that connect the luminal space to the ECF compartment.

Actin and myosin also participate in processes common to most if not all cell types. The process of cytokinesis, in which the cytoplasm of a dividing cell physically separates into two daughter cells, is driven by actin and myosin filaments. Beneath the cleavage furrow that forms in the membrane of the dividing cell is a contractile ring of actin and myosin filaments. Contraction of this ring deepens the cleavage furrow; this invagination ultimately severs the cell and produces the two progeny. image N2-6

N2-6

Other Roles of Actin and Myosin

Contributed by Michael Caplan

In addition to having the functions discussed in the text, actin and myosin may play important roles in other processes. Actin filaments and various newly discovered isoforms of myosin may be involved in the shuttling of intracellular cargoes in much the same way that microtubules and their associated motor proteins participate in this function. Certain types of myosin appear to serve as motors that drive the movements of vesicles and other organelles along tracks composed of actin filaments. The precise role and relative importance of these movements in the physiology of the cell has yet to be fully elucidated. Despite this uncertainty, it is clear that the actin and myosin cytoskeleton subserves a multitude of functions, ranging from its classical role in the macroscopic contractions of skeletal muscles to its contributions to motility at subcellular scales.