Myosin

Introduction

     Myosin is a motor protein that has many functional roles but plays an integral part in muscle functioning and cytokinesis. Myosin works closely with actin, using ATP to move along actin filaments in muscle fibers. There are approximately 20 classes of myosin which are apparent in its rather dendritic phylogenetic tree. These different types of myosin are coded for by at least 39 genes currently identified in the human genome. Myosin is categorized by differences in its structure.

Video on General Muscle Contraction

Structure

     The basic structure of myosin includes head, neck, and tail domains. The globular head, also known as the motor domain, consists of Actin and ATP-binding sites and is found in every heavy chain. The heavy chain can also include a neck. The neck is the joint in which the head is bound to the tail and can serve as a binding site for a light chain. The tail interacts with other molecules and can help mediate myosin activity. The structure and function of these domains can be very diverse. Although they have many commonalities, some differences include direction of movement, velocity, regulation techniques, and variations in its primary movement called the powerstroke.

Shown below are the structures of several of the most common forms of myosin:

Myosin I                             Myosin II

      Myosin III                     Myosin V                      Myosin VI

    Myosin VII                     Myosin IX

Mechanism

     Although different forms of myosin have subtle differences in the execution of its movement or powerstroke, a more general description can encompass the approach used by most myosins.

The Power Stroke:

     -As the head attaches to an actin filament by ATP hydrolysis, ADP is released causing a conformational change in they myosin to a cocked position mounted on the actin.

      -When a new ATP binds the head of the myosin it is released from the actin filament

     -The actin filament does not return to its original position at this point due to the attachment of many other myosin proteins

Video on Myosin Mechanism

Current/Future Research

    There is extensive current research on myosin such as exploring the role of myosin in linking actin cytoskeleton with cell membranes and studying the mechanism of a reverse recovery-step in myosin II mechanism. Research on the myosin protein has revealed links between myosin deficiencies and diseases such as hypertrophy of the heart, sarcomeric defects such as heart enlargement, and problems with blood platelets. Usher 1B syndrome is caused by a defect in myosin VIIA and results in deafness and blindness.  With future research it may be possible to cure the diseases and understand more about muscle function and causes of muscle dystrophy.