アイテム詳細

要旨

Titin is a giant elastic protein in vertebrate striated muscles with an unprecedented molecular mass of 3−4 megadaltons. Single molecules of titin extend from the Z−line to the M−line. Here, we define the molecular 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 entire width of the Z−line. In vitro binding studies reveal that mammalian titins have at least four potential binding sites for alpha−actinin within their Z−line spanning region. Titin filaments may specify Z−line width and internal structure by varying the length of their NH2−terminal overlap and number of alpha−actinin binding sites that serve to cross−link the titin and thin filaments. Furthermore, we demonstrate that the NH2−terminal titin Ig repeats Z1 and Z2 in the periphery of the Z−line bind to a novel 19−kD protein, referred to as titin−cap. Using dominant−negative approaches in cardiac myocytes, both the titin Z1−Z2 domains and titin−cap are shown to be required for the structural integrity of sarcomeres, suggesting that their interaction is critical in titin filament−regulated sarcomeric assembly