Myosin head, motor domain <p>Muscle contraction is caused by sliding between the thick and thin filaments of the myofibril. Myosin is a major component of thick filaments and exists as a hexamer of 2 heavy chains [<cite idref="PUB00002350"/>], 2 alkali light chains, and 2 regulatory light chains. The heavy chain can be subdivided into the N-terminal globular head and the C-terminal coiled-coil rod-like tail, although some forms have a globular region in their C-terminal. There are many cell-specific isoforms of myosin heavy chains, coded for by a multi-gene family [<cite idref="PUB00001579"/>]. Myosin interacts with actin to convert chemical energy, in the form of ATP, to mechanical energy [<cite idref="PUB00004629"/>]. The 3-D structure of the head portion of myosin has been determined [<cite idref="PUB00005168"/>] and a model for actin-myosin complex has been constructed [<cite idref="PUB00005169"/>].</p><p>The globular head is well conserved, some highly-conserved regions possibly relating to functional and structural domains [<cite idref="PUB00004601"/>]. The rod-like tail starts with an invariant proline residue, and contains many repeats of a 28 residue region, interrupted at 4 regularly-spaced points known as skip residues. Although the sequence of the tail is not well conserved, the chemical character is, hydrophobic, charged and skip residues occuring in a highly ordered and repeated fashion [<cite idref="PUB00004601"/>].</p>