Introduction

The myosin heavy chain (MyoHC) plays a central role in cell motility and muscle contraction of all species, and a better understanding of the molecular diversity present between different isoforms of this protein is necessary if we are to understand the fine tuning of locomotion at the molecular level. In skeletal muscle the myosin molecule is located in the thick filament of the myofibril and together with actin takes part in the mechanism by which the chemical energy of ATP is converted into mechanical work. In its native state, striated muscle myosin is a hexameric protein consisting of two ‘heavy’ polypeptide chains of approximately 220 kD each and four ‘light’ chains approximately 17–20 kD each (Weeds & Lowey, 1971). Both the heavy and light chains of myosin exist as multiple isoforms which produce distinct isoforms of myosin all with similar, but not identical, structure and function (Gros & Buckingham, 1987).

The two myosin heavy chains intertwine at their more carboxyl regions forming an α-helical coiled-coil, about 150 nm long, termed the rod region of the molecule. The NH2 region of each individual MyoHC molecule consists of a globular head about 19 nm long and 5 nm wide (at the widest point) such that the myosin molecule as a whole contains two head regions. The head region of the MyoHC forms the ‘crossbridge’ between the thick and thin filaments of the sarcomere. It contains an actin binding domain, the sites where the myosin light chains interact and the region where the ATPase activity of the molecule is localized (Craig, 1986).