Structure of the Actin-containing Filaments in Vertebrate Skeletal Muscle

Abstract

In the skeletal muscles of vertebrates the chain of processes coupling excitation with contraction terminates in the release of the actin subunits in the thin filaments from an inhibited state. While the muscle is relaxed, the actin subunits are prevented from interacting with myosin: Cross-bridges cannot be formed, and tension cannot be generated. The state of the actin subunits is regulated by a multi-component protein system which is believed to be situated in the thin filaments and which is itself controlled by the level of free Ca++ ions in the myofibrillar compartment of the muscle cell. On excitation Ca++ ions enter the myofibril and are captured by one component of the regulatory protein system, with the result that the actin subunits are released from inhibition. The regulatory system consists of tropomyosin (TM) and at least two other components, collectively known as troponin (TP), one of which binds the triggering Ca++