Biology of Disease

and microtubules (MT). Although the lengths of the fibers are indeterminate

as they are actively extended and shortened during cellular activities, their

diameters are fairly uniform between cell types. Microfilaments and MTs have

diameters of approximately 7 and 25 nm respectively. As their name implies,

IFs have diameters between these values of 8–11 nm.

Microfilaments are made of the protein actin. They are relatively flexible

filaments but, cross-linked into bundles, they can withstand compression.

Microtubules are composed of tubulin proteins arranged into hollow rods that

are rigid and can resist both compression and tension. Intermediate filaments

are built up from a number of types of proteins that are tissue specific, keratins

in epidermal cells, desmin in muscles, for example. They form flexible cables

whose high tensile strength allows the cell to resist excessive stretching.

Microfilaments and MTs form defined tracks within the cell for the transport

of macromolecules and membranous structures. The two most common

methods for this involves the movements of motor proteins along the

filaments that are driven by the hydrolysis of ATP. The motor proteins of the

MTs are dyneins and kinesins; those of the MFs are the myosins. Actin–myosin

complexes are probably best known as the contractile apparatus of skeletal

muscle tissues. Skeletal muscle tissue shows a multinuclear organization or

syncitium arranged into fibers, which are surrounded by a basal lamina of

extracellular matrix proteins, which forms a supporting sheath. Each fiber

contains sarcoplasm (cytoplasm) that houses the contractile fibers of actin

and myosin and is surrounded by a sarcolemma (plasma membrane). A

network of elongated protein molecules about 150 nm long of the protein

dystrophin is found within the sarcoplasm The dystrophin links actin

filaments to a transmembrane complex of proteins that, in turn, is linked to

components of the basal lamina (Figure 16.16). This complex arrangement

X]VeiZg&+/ MEMBRANE, ORGANELLE AND CYTOSKELETAL DISORDERS

W^dad\nd[Y^hZVhZ

Figure 16.16 Schematic indicating how

dystrophin, a transmembrane protein complex

and the basal lamina may all interact to stabilize

the sarcolemma of muscle fibers.

X]VeiZg&+/ MEMBRANE, ORGANELLE AND CYTOSKELETAL DISORDERS

)++ W^dad\nd[Y^hZVhZ

Actin filament

Distrophin

Sarcolemma

Syntrophin

A2 Laminin

Extracellular matrix

Sarcoglycan

complex

Dystroglycan

complex

Sarcoplasm