Biochemistry. Long-stretched protein molecule, that makes up the major part of the protein of muscle cells. Myosin has a central function in muscle contraction and is able to bind actin molecules. In the cell, it is bundled in functional units, called filaments. Myosins consist of heavy chains providing energy for locomotion, and light chains regulating muscle activity. The heavy chain is composed of the head, neck and tail regions. The head region contains the actin and ATP binding domains. The neck region is involved in binding the light-chains. The tail region provides the anchoring point that maintains the position of the heavy chain.