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Published online 15 May 2006. doi:10.1083/jcb.200601014
 The Rockefeller University Press, 0021-9525 $8.00
The Journal of Cell Biology
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ARTICLE

 M line–deficient titin causes cardiac lethality through impaired maturation of the sarcomere

Stefanie Weinert1, Nora Bergmann1, Xiuju Luo3, Bettina Erdmann2, and Michael Gotthardt1,3

1 Neuromuscular and Cardiovascular Cell Biology and 2 Electron Microscopy, Max-Delbrück-Center for Molecular Medicine, D-13125 Berlin-Buch, Germany
3 Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Washington State University, Pullman, WA 99164

Correspondence to Michael Gotthardt: gotthardt{at}mdc-berlin.de

Titin, the largest protein known to date, has been linked to sarcomere assembly and function through its elastic adaptor and signaling domains. Titin's M-line region contains a unique kinase domain that has been proposed to regulate sarcomere assembly via its substrate titin cap (T-cap). In this study, we use a titin M line–deficient mouse to show that the initial assembly of the sarcomere does not depend on titin's M-line region or the phosphorylation of T-cap by the titin kinase. Rather, titin's M-line region is required to form a continuous titin filament and to provide mechanical stability of the embryonic sarcomere. Even without titin integrating into the M band, sarcomeres show proper spacing and alignment of Z discs and M bands but fail to grow laterally and ultimately disassemble.

The comparison of disassembly in the developing and mature knockout sarcomere suggests diverse functions for titin's M line in embryonic development and the adult heart that not only involve the differential expression of titin isoforms but also of titin-binding proteins.

Abbreviations used in this paper: FHL2, four and a half LIM-only protein 2; MEx, M-line exon; MuRF, muscle-specific RING finger protein; Nbr1, neighbor of BRCA1 gene 1; Sqstm1, sequestosome 1; T-cap, titin cap.


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