Contents of this page:
Classes of myosin & basic structure
Motor domain function & structure
Bipolar assemblies of myosin II
Roles & mechanisms of myosins I, V, & VI
Note: In these notes references are given to page numbers in the Molecular Biology of the Cell textbook by Alberts et al. (A).
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 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.
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.
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:
belts of actin filaments that encircle epithelial cells, associated with adhering junctions (A p. 1071-1072).
the contractile ring of cytokinesis, located just inside the plasma membrane at the division furrow (A p. 1054).
the cortical web of actin filaments, located just inside the plasma membrane in many cells.
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:
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
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.
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.
Profilin promotes ADP/ATP exchange by G-actin, to yield the ATP-bound form competent to polymerize, at the leading edge of an advancing cell.
Arp2/3, a complex that includes actin related proteins 2 & 3, binds to the sides of actin filaments and nucleates growth of new filaments within lamellipodia.
Capping protein adds to the plus ends of actin filaments shortly after they are nucleated by Arp2/3, keeping actin filaments at the leading edge short and highly branched.
Myosin I binds to the plasma membrane, and may pull the membrane forward as it walks actin filaments toward the plus end (diagram above).
and gelsolin may sever actin filaments,
providing new plus ends for nucleation of actin filament growth and
helping to keep actin filament branches short within the lamellipodium.
Cofilin also promotes depolymerization of actin filaments further back from the leading edge within a lamellipodium.
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:
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.