Smooth muscle myosin heavy chain isoforms and their role in muscle physiology

Unlike vertebrate skeletal muscle, smooth muscle myosin heavy chain isoforms are encoded by a single gene. Alternative splicing of the primary transcript from a single gene generates four smooth muscle myosin heavy chain isoforms. These isoforms differ both at the carboxyl terminus (SM1 and SM2 isoforms) and at the amino terminus (SM-A and SM-B isoforms). The smooth muscle myosin heavy chain isoforms are differentially expressed during smooth muscle development and in different smooth muscle cell types. The mechanical properties of smooth muscle may be correlated with the myosin heavy chain content/isoform expression. However, the precise function of each smooth muscle myosin heavy chain isoform to muscle contraction remains to be determined. This review mainly focuses on the molecular basis of smooth muscle myosin heavy chain isoform diversity, its expression during development and disease, and its role in muscle physiology.