Skeletal muscle Myosin or myosin II is the motor protein that generates force to drive muscle contraction. It is a 520 kDa hexamer comprised of two heavy chains and four light chains. Myosin heavy chain is 220 kDa in size and consists of a long coiled-coil domain tail that mediates dimerization of the two heavy chains and a globular head region that mediates ATP-dependent sliding of actin filaments. Myosin heavy chain can be proteolytically cleaved to produce heavy meromyosin, which includes the S1 motor domain (head region) and first third of the coiled coil domain, and light meromyosin, which includes the C-terminal two thirds of the coiled coil domain.