The complete gene sequence of titin, expression of an unusual approximate to 700-kDa titin isoform, and its interaction with obscurin identify a novel Z-line to I-band linking system

Titin is a giant vertebrate striated muscle protein with critical importanc e for myofibril elasticity and structural integrity. We show here that the complete sequence of the human titin gene contains 363 exons, which togethe r code for 38 138 residues (4200 kDa). In its central I-band region, 47 nov el PEVK exons were found, which contribute to titin's extensible spring pro perties. Additionally, 3 unique I-band titin exons were identified (named n ovex-1 to -3). Novex-3 functions as an alternative titin C-terminus. The no vex-3 titin isoform is approximate to 700 kDa in size and spans from Z1-Z2 (titin's N-terminus) to novex-3 (C-terminal exon). Novex-3 titin specifical ly interacts with obscurin, a 721-kDa myofibrillar protein composed of 57 I g/FN3 domains. followed by one IQ, SH3, DH, and a PH domain at its C-termin us. The obscurin domains Ig48/Ig49 bind to novex-3 titin and target to the Z-line region when expressed as a GFP fusion protein in live cardiac myocyt es. Immunoelectron microscopy detected the C-terminal Ig48/Ig49 obscurin ep itope near the Z-line edge. The distance from the Z-line varied with sarcom ere length, suggesting that the novex-3 titin/obscurin complex forms an ela stic Z-disc to I-band linking system. This system could link together calci um-dependent, SH3-, and GTPase-regulated signaling pathways in close proxim ity to the Z-disc, a structure increasingly implicated in the restructuring of sarcomeres during cardiomyopathies.