Muscle contraction. Muscle myosin is a hexameric protein that consists of 2 heavy chain subunits (MHC), 2 alkali light chain subunits (MLC) and 2 regulatory light chain subunits (MLC-2). Thick filaments of the myofibrils. Both isoform 1 and isoform 2 are detected in small and large intestine, uterus, bladder, aorta, stomach, and at much lower levels in lung. All three isoforms are coexpressed in smooth muscle. Isoform 2 predominates in most of the smooth muscles tested, with the exception of intestine and urinary bladder, which show greater expression of isoform 1. The rodlike tail sequence is highly repetitive, showing cycles of a 28-residue repeat pattern composed of 4 heptapeptides, characteristic for alpha-helical coiled coils. Limited proteolysis of myosin heavy chain produces 1 light meromyosin (LMM) and 1 heavy meromyosin (HMM). HMM can be further cleaved into 2 globular subfragments (S1) and 1 rod-shaped subfragment (S2). Belongs to the TRAFAC class myosin-kinesin ATPase superfamily. Myosin family.