Species variations in cDNA sequence and exon splicing patterns in the extensible I-band region of cardiac titin: relation to passive tension

Titin is believed to play a major role in passive tension development in cardiac muscle. The cDNA sequence of cardiac titin in the I-band sarcomeric region was determined for several mammalian species. Contiguous sequences of 3749, 12,230, 6602, and 11,850 base pairs have been obtained for the rat N2B, rat N2BA, dog N2B, and dog N2BA isoforms respectively. The length of the PEVK region of the N2B isoform did not correlate with rest tension properties since the only species showing an altered length was the dog that expressed a shorter form. No differences were found between the N2B PEVK lengths in ventricular and atrial muscle. New N2BA splicing pathways in the first tandem Ig region were found in human and dog cardiac muscle. Most of the rat and dog sequences were 85-95% identical with the reported human sequence. However, the N2B unique amino acid sequences of rat and dog were only 51 and 67% identical to human. The rat N2B unique sequence was 526 amino acids in length compared to 572 in human. The difference in length was due to deletion of amino acid segments from six different regions of the N2B unique domain. Patterns of PEVK exon expression were also much different in the dog, human, and rat. Six separate dog N2BA PEVK clones were sequenced, and all had different exon splice combinations yielding PEVK lengths ranging from 703 to 900 amino acids. In contrast a rat N2BA clone had a PEVK length of 525 amino acids, while a human clone had an 908 amino acid PEVK segment. Thus, in addition to the higher proportion of the shorter N2B isoform found in rat compared with dog cardiac muscle observed previously, shorter N2B unique and N2BA PEVK segments may also contribute to the greater passive tension in cardiac muscle from rats.