MYOSIN POWERED CELL MOVEMENTS

TOPICS TO COVER

Why Myosin Types of Myosin Structure of Myosin Activity of Myosin Actin – Myosin Motion

WHY MYOSIN?• Interactions between myosin II and actin filaments

are responsible for muscle contraction.• As biologists investigated various types of cell

movements, it became clear that myosin II mediates only a few types, such as cytokinesis and muscle contraction.

• Other types of cell movements, including vesicle transport, membrane extension, and the movement of chromosomes, require either other myosin isoforms, other motor proteins such as kinesin or dynein, or actin polymerization.

CONTRACTION OF MUSCLES

• Contraction, the special form of movement resulting from the interaction of actin and myosin II, is most highly evolved in skeletal muscle cells. However, somewhat similar contractile events entailing less organized systems are found in nonmuscle cells.

Myosins Are a Large Superfamilyof Mechanochemical Motor Proteins

• Eight members of the myosin gene family have been identified by genomic analysis. Three family members—myosin I, myosin II, and myosin V—are present in nearly all eukaryotic cells and are the best understood.

• Although the specific activities of these myosins differ, they all function as motor proteins.– myosin II powers muscle contraction, as well as cytokinesis. – Myosins I and V take part in cytoskeleton–membrane interactions, such

as the transport of membrane vesicles.– myosins VI, VII, and XV have functions associated with hearing and hair

cell stereocilia structure.

• Plants do not have the same myosins as animal cells. o Three myosins (VII, XI, and XIII) are exclusively expressed in plants. o Myosin XI, which may be the fastest myosin of all, is implicated in the

cytoplasmic streaming seen in green algae and higher plants

STRUCTURE OF MYOSIN

• Myosin II is assembled into so-called bipolar thick filaments, containing hundreds of individual myosin II proteins.

• They interdigitate with actin filaments to bring about muscle contraction.

• Dissolving myosin in a solution of ATP and high salt yields a protein of 6 polypeptides– 2 heavy chains – 4 regulatory light chains• The soluble myosin has ATPase activity.

But which domain of Myosin is responsible for this ATPase activity?

Figure 34.3. Myosin Dissection. Treatment of muscle myosin with proteases forms stable fragments, including subfragments S1 and S2 and light meromyosin. Each S1 fragment includes the head (shown in yellow and pink) from the heavy chain and one copy of each light chain (shown in blue and orange).

Figure 34.19. Thick Filament. (A) An electron micrograph of a reconstituted thick filament reveals the presence of myosin head domains at each end and a relatively narrow central region. (B) A schematic view shows how myosin molecules come together to form the thick filament. [Part A courtesy of Dr. Hugh Huxley

Myosin Heads Walk Along ActinFilamentsin Discrete Steps

• The mechanism of myosin-powered movement was greatly aided by development of in vitro motility assays.

• In one such assay, the sliding-filament assay, the movement of fluorescence-labeled actin filaments along a bed of myosin molecules is observed in a fluorescence microscope. Because the myosin molecules are tethered to a coverslip, they cannot move; thus any force generated by interaction of myosin heads with actin filaments forces the filaments to move relative to the myosin.

• If ATP is present, added actin filaments can be seen to glide along the surface of the coverslip; if ATP is absent, no filament movement is observed.

• This movement is caused by a myosin head (bound to the coverslip) “walking” toward the (+) end of a filament; thus filaments move with the (-) end in the lead. [The one exception is myosin VI, which moves in the opposite direction, toward the (-) end; so the (+) end of a moving filament is in the lead.]

• The rate at which myosin moves an actin filament can be determined from video camera recordings of sliding-filament assays.

• The velocity of filament movement can vary widely, depending on the myosin tested and the assay conditions (e.g., ionic strength, ATP and Ca2+ concentrations, temperature).

• The most critical feature of myosin is its ability to generate a force that powers movements. Researchers have used a device called an optical trap to measure the forces generated by single myosin molecules.

• The results of optical-trap studies show that myosin II moves in discrete steps, approximately 5–10 nm long, and generates 3–5 piconewtons (pN) of force, approximately the same force as that exerted by gravity on a single bacterium.

Actin – Myosin Motion

Figure 34.18. Myosin Motion Along Actin. A myosin head (yellow) in the ADP form is bound to an actin filament (blue). The exchange of ADP for ATP results in (1) the release of myosin from actin and (2) substantial reorientation of the lever arm of myosin. Hydrolysis of ATP (3) allows the myosin head to rebind at a site displaced along the actin filament (4). The release of Pi (5) accompanying this binding increases the strength of interaction between myosin andactin and resets the orientation of the lever arm.

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