Cloned rat cardiac titin class I and class II motifs. Expression, purification, characterization, and interaction with F-actin.

Titin (connectin) is a giant protein that forms a single-molecule elastic filament extending from the M-line to the Z-line in the striated muscle sarcomere. The sequence of titin consists mainly of repeats of two types of approximately 100-amino acid motifs (class I and class II that show homology to the fibronectin type III and immunoglobulin-C2 domains, respectively). To investigate the functions of the two classes of titin motifs as the basic units of this large sarcomere organizer molecule, titin cDNA segments encoding single class I or class II or linked class I-II motifs were cloned by polymerase chain reaction from a rat cardiac cDNA library into the T7 RNA polymerase-based pAED4 vector to express non-fusion titin fragments in Escherichia coli. High level expression of the three titin fragments was achieved, and effective rapid purification procedures were developed. The purified titin fragments were verified by their amino acid composition, apparent molecular mass, and charge. Antibodies raised against the genetically expressed titin motifs specifically recognized intact rat cardiac and skeletal muscle titins in Western blotting and immunofluorescence microscopy, confirming the authenticity of the cloned fragments. High beta-sheet contents of these titin motifs indicate a folding state very similar to that of intact native titin. Solid-phase protein-binding assays demonstrated that a single class I motif was able to bind both myosin and F-actin. In comparison, a single class II motif had weaker binding to only F-actin but the fragment containing linked class I and class II motifs showed significantly stronger interactions with both myosin and F-actin. The binding of titin motifs to myosin supports the proposed association of A-band titin with the thick filament, and the novel titin-F-actin interaction was confirmed by F-actin cosedimentation assays, suggesting that titin may also be involved in the structure and/or function of the thin filament.