Characterization of a 5.4 kb cDNA fragment from the Z-line region of rabbit cardiac titin reveals phosphorylation sites for proline-directed kinases

Titin is an approximately 3 MDa protein that spans from the M- to the Z-line in the sarcomeres of vertebrate striated muscle. The protein is presumably encoded by unusually large mRNAs of 70–80 kb. Although titin has been studied by several laboratories, barely more than half of the cDNA sequence (approximately 45 kb) has been published, most of it obtained from the A-band and M-line region (corresponding to the C-terminal half of the molecule). A special cDNA library was constructed using size selected total RNA from adult rabbit cardiac muscle in order to obtain sequence data from titin's unknown N-terminal region. A monoclonal antibody (T12), which binds to an epitope close to the Z-line, was used to identify initial cDNA clones. Additional overlapping clones were isolated and sequenced yielding a 5.4 kb contig. The encoded polypeptide contains 16 Type-II domains and four unique intervening segments. Polyclonal sera, raised against an expressed protein fragment encoded by the 5′ end of the contig, strongly stained the Z-line of myofibrils of different species. However, the sequence of this fragment is 83% identical at the amino acid level with the previously reported C-terminal (i.e. M-line) end of chicken embryonic skeletal muscle titin. The expressed protein fragment could be phosphorylated in vitro by embryonic skeletal muscle extract and by the purified proline-directed kinase ERK1, presumably at the xSPxR recognition sites located in the first interdomain segment.