THE NH2 TERMINUS OF TITIN SPANS THE Z-DISC - ITS INTERACTION WITH A NOVEL 19-KD LIGAND (T-CAP) IS REQUIRED FOR SARCOMERIC INTEGRITY

Titin is a giant elastic protein in vertebrate striated muscles with a n unprecedented molecular mass of 3-4 megadaltons. Single molecules of titin extend from the Z-line to the M-line. Here, we define the molec ular layout of titin within the Z-line; the most NH2-terminal 30 kD of titin is located at the periphery of the Z-line at the border of the adjacent sarcomere, whereas the subsequent 60 kD of titin spans the en tire width of the Z-line. In vitro binding studies reveal that mammali an titins have at least four potential binding sites for alpha-actinin within their Z-line spanning region. Titin filaments may specify Z-li ne width and internal structure by varying the length of their NH2-ter minal overlap and number of alpha-actinin binding sites that serve to cross-link the titin and thin filaments. Furthermore, we demonstrate t hat the NH2-terminal titin Ig repeats Z1 and Z2 in the periphery of th e Z-line bind to a novel 19-kD protein, referred to as titin-cap. Usin g dominant-negative approaches in cardiac myocytes, both the titin Z1- Z2 domains and titin-cap are shown to be required for the structural i ntegrity of sarcomeres, suggesting that their interaction is critical in titin filament-regulated sarcomeric assembly.