A new troponin T and cDNA clones for 13 different muscle proteins, found by shotgun sequencing

Complete amino acid sequences have been established for 19 muscle-related proteins and these proteins are each sufficiently abundant to suggest that their mRNA levels are about 0.4% or higher. Based on these considerations, a simple theoretical analysis shows that clones for most of these proteins can be identified within a complementary DNA library by sequencing cDNA inserts from 150-200 randomly selected clones. This procedure should not only rigorously identify specific clones, but it could also uncover amino acid sequence variants of major muscle proteins such as the troponins. We have determined sequences for about 20,000 nucleotides within 178 randomly selected clones of a rabbit muscle cDNA library, and report here that in addition to finding sequences encoding the two known skeletal muscle isotypes of troponin C, we have discovered sequences encoding two forms of troponin T. Over the region of nucleotide sequence overlap in the troponin T clones, the new isotype diverges significantly from its counterpart. Altogether, clones for 13 of the 19 known muscle-specific proteins were identified, in addition to the clone for the new troponin T isotype.