Connectin/titin, giant elastic protein of muscle

The structure and function of the giant elastic protein connectin/titin are described on the basis of recent investigations. The 3000 kDa protein links the Z line to the myosin filament in striated muscle sarcomeres. The NH2-terminal region of connectin filament is involved in the Z line binding, and the COOP-terminal region is bound onto the myosin filament with an overlap between the counter-connectin filaments at the M line. The PEVK region in the I band is shown to be mainly responsible for passive tension generation. The longitudinal continuity of myosin-, actin-free sarcomeres is explained by the linkage of freed connectin filaments extending from both sides of the Z lines in a sarcomere. The role of connectin in myofibrillar differentiation and the biodiversity of connectin-related proteins in the animal kingdom are briefly reviewed.