Biochemistry of Metabolism: Cell Biology

Myosin

Contents of this page:
Classes of myosin & basic structure

Motor domain function & structure
Bipolar assemblies of myosin II
Regulation
Roles & mechanisms of myosins I, V, & VI
Ameboid movement

Note: In these notes references are given to page numbers in the Molecular Biology of the Cell textbook by Alberts et al. (A).

Myosins  are a large superfamily of motor proteins that move along actin filaments, while hydrolyzing ATP. About 20 classes of myosin have been distinguished on the basis of the sequence of amino acids in their ATP-hydrolyzing motor domains. The different classes of myosin also differ in structure of their tail domains. Tail domains have various functions in different myosin classes, including dimerization and other protein-protein interactions. Only a few of the known classes of myosin will be discussed here.
See diagrams in A. p. 950, 951, a diagram accessible from the Myosin Home Page at Cambridge University, and diagrams depicting the motor domain and neck region below.

Myosin II was first studied for its role in muscle contraction, but it functions also in non-muscle cells.

  • Myosin II includes two heavy chains.

    • The globular motor domain of each heavy chain catalyzes ATP hydrolysis, and interacts with actin.

    • Each heavy chain continues into a tail domain in which heptad repeat sequences promote dimerization by interacting to form a rod-like a-helical coiled coil.

  • Two light chains, designated essential and regulatory, wrap around the neck region of each myosin II heavy chain. In addition to regulatory roles, light chains may help to stiffen the neck regions.
    • The binding site for each light chain is an IQ (isoleucine, glutamine) sequence motif: IQxxxRGxxxR.
    • Myosin II light chains are similar in structure to calmodulin, but in many organisms have lost the ability to bind Ca++. However, the calmodulin-like light chains of some myosins do bind Ca++.

Myosin I has only one heavy chain with a single globular motor domain. Its relatively short tail lacks the heptad repeats that would be involved in dimerization via formation of a coiled coil.

The Myosin VI tail domain includes a short segment of heptad repeats. Myosin VI is found to be either monomeric or dimeric under different conditions.

Myosin V has two heavy chains like myosin II. But myosin V has a longer neck region that has 6 binding sites for calmodulin light chains. Its shorter coiled coil region is followed by a globular domain at the end of each heavy chain tail.

Motor domains of most myosins move along actin filaments toward the plus ends of the filaments. This movement is ATP-dependent and is accompanied by ATP hydrolysis

An exception is myosin VI, which moves toward the minus ends of actin filaments.

Proof that the head domain with attached neck is sufficient to drive movement has been obtained in studies of isolated myosin heads, using fluorescence microscopy. Myosin heads, detached from myosin tails by protease treatment and fixed to a glass surface, promote gliding of actin filaments labeled with fluorescent rhodamine-phalloidin. This movement is ATP-dependent. See Alberts et al. p. 951.

See also a movie and animation of actin filament movement driven by immobilized myosin, in University of Vermont website.

Myosin II heads interact with actin filaments in a reaction cycle that may be summarized as follows (diagram  in A p. 955):

An animation may be viewed at a website of the Vale Lab at University of California, San Francisco.
 
ATP binds to the myosin head adjacent to a 7-stranded b-sheet. Loops extending from b-strands interact with the adenine nucleotide. 

The nucleotide-binding pocket of myosin is opposite a deep cleft that bisects the actin-binding domain (diagram in A p. 953). Opening and closing of the cleft is proposed to cause the head to pivot about the neck region, as occupancy of the nucleotide-binding site changes and as myosin interacts with and dissociates from actin.

Consistent with the predicted conformational cycle, different conformations of the myosin head & neck have been found in crystal structures. Two examples are shown. The b-sheet adjacent to the nucleotide-binding site is colored magenta; light chains are displayed as backbone, in green & red.

Explore at right an example of a crystal structure of the myosin head with associated light chains. 


Myosin S1-ADP

Similarities in structure of the ADP/ATP-binding site in myosin and the nucleotide binding site in the family of small GTP-binding proteins such as Ras, have led to the suggestion that myosin may be distantly related to the GTP-binding proteins. There is little sequence homology, but the structural similarity suggests a common ancestor.

Explore at right the structure of the nucleotide-binding domain of the proto-oncogene product Ras with bound GDP. Compare to the structure of the myosin head above.


Ras-GDP

Bipolar complexes of myosin II form by interaction of antiparallel coiled coil tail domains. These complexes may contain many myosin molecules, as in the thick filaments of skeletal muscle (diagram in A p. 950).

Antiparallel actin filaments may be caused to move relative to one another, as motor domains at the opposite ends of bipolar myosin II complexes walk toward the plus ends of adjacent actin filaments.

Muscle sarcomere structure and the role of myosin II in muscle contraction will not be discussed in detail here, since it is covered in other courses at Rensselaer. (If you are not familiar with the role of myosin II in muscle see A p. 961-964.) 

In non-muscle cells, myosin II (the type in muscle sarcomeres) is often found to be associated with actin filament bundles. Existence of bipolar myosin assemblies has been postulated. Contraction of actin filament bundles is postulated to involve myosin-mediated sliding of antiparallel actin filaments, e.g., in each of the following:

Regulation by phosphorylation:

Regulation by Ca++ varies, depending on the type of myosin, the tissue and the organism. For example:

Myosins I, V, & VI bind to membranes or to macromolecular complexes via globular tail domains. They have roles, e.g., in movements of organelles or plasma membranes relative to actin filaments:

Movement of myosin V along actin is processive, meaning that myosin V remains attached to an actin filament as it walks along that filament. In contrast, myosin II is a non-processive motor that detaches from actin at a stage of each reaction cycle (see above). The processive movement of myosin V is appropriate for its role in transporting organelles along actin filaments.

In the hand over hand stepping mechanism of myosin V, one head domain dissociates from an actin filament only when the other head domain binds to the next subunit with the correct orientation along the helical actin filament. Since there are 13 actin subunits per helical turn, myosin V has a relatively long step length of 74 nm. By stepping the length of the actin helical repeat, myosin V maintains a straight path along an actin filament, rather than spiraling around it.

Myosin V step length has been measured by monitoring movement of individual fluorescent labeled calmodulin light chains associated with the myosin V neck domain.  For diagrams, see article by Yildiz et al. and a University of Illinois website on research of P. Selvin.

High resolution electron microscopy has detected conformations consistent with the hand-over-hand stepping mechanism.

Animation: This animation of myosin V walking along an actin filament is based on electron microscopic images of myosin V fragments, consisting of part of the tail domain with two attached heads, attached to actin filaments in what is interpreted as different stages of the reaction cycle.  (By M. L. Walker, S. A. Burgess, J. R. Sellers, F. Wang, J. A. Hammer, J. Trinick & P. J. Knight.)

Ameboid movement: At the leading edge of a moving cell is the lamellipodium. Forward extension of a lamellipodium is driven by actin polymerization. Lamellipodia contain an extensively branched network of actin filaments, with their plus ends oriented toward the plasma membrane.

Localization of proteins that participate in generating forward movement, at the leading edge or other regions of an advancing cell, has been demonstrated, e.g., by fluorescent labeling. See A p. 974-977.
Some examples discussed above and in the notes on actin:

  • Various cross-linking proteins stabilize the actin network in lamellipodia. Pulse labeling has shown that the newly formed actin filaments are stable, as an advancing lamellipodium moves past them, until they disassemble further back from the edge. 

  • Myosin II is located predominantly at the rear end of a moving cell, or in regions being retracted.
    Contraction in these regions probably involves sliding of antiparallel actin filaments driven by bipolar myosin assemblies.
    When a focal adhesion fails to detach, a fragment of cytoplasm is sometimes left behind.

  • Calpains (intracellular Ca++-activated proteases) may degrade constituents of focal adhesions at the rear of a moving cell as it is pulled forward.

See FishScope website with movies.
See a website of the Institute of Molecular Biology at Salzburg with movies, an animation & a diagram.

Signaling in ameboid movement is complex and only a few aspects of this regulation will be summarized here. For example:

Regulatory roles of members of the Rho family of GTP-binding proteins include:

PIP2 (phosphatidylinositol-4,5-bisphosphate) hydrolysis by signal-activated Phospholipase C may result in localized increases in profilin, cofilin, gelsolin, and Ca++ (due to IP3 release).

Ca++ indicator dyes have been used to show that cytosolic [Ca++] is highest at the rear of an advancing cell, where it may activate Myosin Light Chain Kinase and calpains. Cytosolic [Ca++] is relatively low at the leading edge of an advancing cell, where movement is driven more by actin filament assembly.

A summary of roles of some cell constituents in ameboid movement is presented at right.

See also diagrams by Vicente-Manzanares et al. in J. Cell Science.

For more details, see the Myosin Home Page, which provides links to additional sites with information relating to myosin.

Copyright 1998-2007 by Joyce J. Diwan. All rights reserved.

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