Probing the functional roles of titin ligands in cardiac myofibril assembly and maintenance.

Sarcomeres of cardiac muscle are comprised of numerous proteins organized in an elegantly precise order. The exact mechanism of how these proteins are assembled into myofibrils during heart development is not yet understood, although existing in vitro and in vivo model systems have provided great insight into this complex process. It has been proposed by several groups that the giant elastic protein titin acts as a "molecular template" to orchestrate sarcomeric organization during myofibrillogenesis. Titin's highly modular structure, composed of both repeating and unique domains that interact with a wide spectrum of contractile and regulatory ligands, supports this hypothesis. Recent functional studies have provided clues to the physiological significance of the interaction of titin with several titin-binding proteins in the context of live cardiac cells. Improved models of cardiac myofibril assembly, along with the application of powerful functional studies in live cells, as well as the characterization of additional titin ligands, is likely to reveal surprising new functions for the titin third filament system.