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J. Cell Biol.,
Volume 143, Number 4, November 16, 1998 1013-1027
* Departments of Cell Biology and Anatomy, and Molecular and Cellular Biology, University of Arizona, Tucson, Arizona 85724; 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 European Molecular Biology Laboratory, 69012 Heidelberg, Germany; § Institute of Molecular and Cellular Biosciences, University
of Tokyo, Tokyo 113-8657, Japan; Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Washington
State University, Pullman, Washington 99164; and ¶ Max-Planck Institute for Immunobiology, D-79011 Freiburg, Germany
-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 -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.
-actinin;
Z-disc;
titin-cap
(T-cap);
sarcomere
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