Variable surface loops and myosin activity: Accessories to a motor

The catalytic head of myosin is a globular structure that has historically been divided into three segments of 25, 50, and 20 kDa. The solvent-exposed , proteolytically-sensitive surface loops of myosin that join these three s egments are highly variable in their sequences. While surface loops have no t traditionally been thought to affect enzymatic activities, these loops li e near the ATP and actin-binding sites and have been implicated in the modu lation of myosin's kinetic activities. In this work we review the wealth of data regarding the loops that has accumulated over the years and discuss t he roles of the loops in contributing to the different activities displayed by different myosin isoforms.