Myosin

Betsy Garratt '03 and Kate Holland '03

Biomolecules Index

I. Introduction

Myosin is a mechanochemical enzyme, also known as a motor protein, that converts chemical energy in the form of ATP, into mechanical energy in order to move along actin filaments. Many cellular movements, such as organelle/vesicle transport and muscle contraction, depend on the interactions between actin filaments and myosin. Sixteen members of the myosin gene family have been identified by genomic analysis. Of these 16, myosin I and myosin II are the most abundant seen in the cell. The functions for most of the myosin gene family are still being determined. Myosin II powers muscle contraction and cytokinesis while Myosin I and V are involved in membrane interactions such as the transport of membrane vesicles. All of the proteins in the myosin gene family are composed of one or two heavy chains and several light chains. The most conseved region in various myosins proteins is the globular head domain which is responsible for generating force. All of the myosin proteins have an effector tail domain which is unique to each type of myosin and determines it location and function in the cell (Lodish, 2000).

Myosin is one of three motor proteins in the cell, the other two being kinesin and dynein both of which move along on microtubules. Even though myosin and kinesin have little sequence homology, the core of the myosin motor domain is structurally similar to kinesin and both have the Ras-fold, in which a nucleotide molecule is bound to loops at one end of a beta-sheet domain (Lodish, 2000) .

One of the best studied functions of myosin is its action in powering muscle contraction. Muscle contraction occurs when 2 sets of interdigitating filaments, thin actin and thick myosin, slide past each other. One theory for how this occurs is that as ATP is hydrolyzed myosin tightly binds to actin. Globular regions of the myosin molecule form crossbridges. These croosbridges extend from the myosin filaments and interact cyclically in a rowing motion with action filaments resulting in muscle contraction. (Rayment et al., 1993)

II. General Structure

Shown here is the Scallop (stage III) conformation of the S1 subunit of myosin, also known as the globular head. The head consists of three sections: a heavy chain fragment, a regulatory light chain (RLC), and the essential light chain (ELC) <>. This specific conformation is formed when ADP is bound to the molecule <>.

A. Heavy Chain <>

The heavy chain makes up the bulk of the myosin head. This domain contains the nucleotide binding site <> and the actin binding region <>. Though 48% of the heavy chain is alpha helices, one major motif in this domain is a large 7-stranded ß-sheet <>. Also protruding from the bottom of the globular structure is a small six-stranded anti-parallel ß-sheet motif <>.

There are four specific subdomains that make up the heavy chain molecule: 25 kDa NH2 terminus, the upper and lower 50 kDa subunits, and the converter. While the first three subdomains are very rigid, the converter <> functions in increasing the conformational changes of the motor domain. Its mobility comes from the fact that it only interacts with the rest of the structure through hydrogen bonds at three residues: Arg-719, Tyr-720, and Glu-771 <>.

Connecting these subdomains are three joints: Switch II, the relay, and the SH1 helix <>. Note that the SH1 helix is disordered and thus does not fully appear in the structure (Houdusse et al., 1999).

Switch II connects the upper and lower 50 kDa subdomains and is found at the bottom of the cleft that is seen in the head. Three glutamate residues of the relay interact with residues of the converter <>. Note the Tryptophan-507 sidechain orientation, which changes in different conformational states <>.

The lever arm <> is a part of the long alpha helix (residues 775-835). This structure is stablized by the binding of the two light chains. The first few turns interact specifically with the ELC (Houdusse et al., 1999).

Reset Molecule.

B. Light chains

The two light chains, the regulatory and essential light chain <>, that comprise the rest of the myosin head are identical throughout their 142 COOH-terminus residues. The regulatory light chain has an additional 41 amino acids at the NH2-terminus end. The two molecules arise from the same gene, but are alternatively spliced to yield two structures. These are wrapped around a single alpha-helix of the heavy chain (the lever arm) <>, but do not overlap.

The essential light chain interacts with the long alpha-helix of the heavy chain at Leu-783 to Met-806 <>. The NH2-terminus domain of the regulatory light chain wraps around the COOH-terminus of the heavy chain between Asn-825 and Leu-842 <>, while the COOH-terminus interacts with residues Glu-808 to Val-826 <>.

Reset Molecule

III. Structural Differences without ADP

When ADP is released from the nucleotide recognition domain, a conformational change occurs, yielding the striated conformation (stage I). Note that both the converter and the lever arm attain a new orientation with respect to the head of the subunit <>. Also, without the presence of ADP, the cleft that separates the upper and lower subdomains is affected. It is open at the bottom, but partially closed at the top where actin binds. Compare striated and scallop confomations.

The SH1 helix is more tightly wound in this striated structure <>(26), which affects the position of the converter <>. In the relay, the side chain of Tryptophan-507is flipped out 90 degrees from the scallop structure, resulting in fewer interactions with the converter <>. In switch II, there is an alternate bending within the joint <>.

In this state, one can see a bend of ~30 degrees in the lever arm - the helix appears to be untwisted at this point <> (Rayment et al., 1993). In the scallop conformation seen earlier, recall that the lever arm at this location is straight.

Both stage I and III are weak actin-binding states.

Reset Molecule

IV. References

Berman, H.M. J. Westbrook, Z. Feng, G. Gilliland, T.N. Bhat, H. Weissig, I.N. Shindyalov, P.E. Bourne: The Protein Data Bank. Nucleic Acids Research, 28 pp. 235-242 (2000)
< http://www.rcsb.org/pdb/> PDB files 1B7T and 2MYS.

Houdusse, Anne, Vassilios N. Kalabokis, Daniel Himmel, Andrew G. Szent-Gyorgyi, and Carolyn Cohen. 1999. Atomic structure of scallop myosin subfragment S1 complexed with MgADP: a novel conformation of the myosin head. Cell 97:459-470.

Lodish, Harvey, Arnold Berk, S. Lawerence Zipursky, Paul Matsudaira, David Baltimore, and James Darnell. 2000. Molecular Cell Biology. W. H. Freeman and Company. 1084 pp.

Rayment, Ivan, Hazel M. Holden, Michael Whittaker, Christopher B. Yohn, Michael Lorenz, Kenneth C. Holmes, and Ronald A. Milligan. 1993. Structure of the Actin-Myosin Comples and Its Implications for Muscle Contraction. Science 261:58-64.