Bovine cardiac troponin T: amino acid sequences of the two isoforms

Troponin T (TnT) is the tropomyosin-binding subunit of troponin, the thin filament regulatory complex that confers calcium sensitivity to striated muscle contraction and actomyosin ATPase activity. Bovine cardiac muscle contains two isoforms (TnT-1 and TnT-2) of TnT that differ in sequence near their amino termini. Thin filaments containing TnT-2 require less calcium to activate the MgATPase rate of myosin than do thin filaments containing TnT-1. Using whole troponin T purified from adult bovine cardiac muscle, we have determined the complete amino acid sequence of the larger, more abundant isoform TnT-1. We confirmed that sequence differences between TnT-1 and TnT-2 are confined to the amino-terminal regions and found that TnT-1 makes up approximately 75% of the total troponin T isolated. Partial sequencing of the separated isoforms showed that the difference between them is due solely to residues 15-19 (Glu-Ala-Ala-Glu-Glu) of TnT-1 being absent from TnT-2. The deleted segment may correspond to the product of exon 4 of the chicken cardiac TnT gene [Cooper, T.A., & Ordahl, C.P. (1985) J. Biol. Chem. 260, 11140-11148]. Exon 5, which is developmentally regulated in the chicken, is not expressed in either TnT-1 or TnT-2. TnT-1 contains 284 amino acid residues and has a Mr of 33,808, while TnT-2 contains 279 amino acid residues and has a Mr of 33,279. Bovine cardiac TnT contains the only known thiol group in any isolated TnT (Cys-39 of TnT-1, Cys-34 of TnT-2). Comparison of bovine, rabbit, and chicken cardiac TnT sequences shows near identity of the amino-terminal 13 amino acid residues (exons 2 and 3 of the chicken cardiac gene), many differences in the following 60 residues (exons 4-8), and great similarity in the C-terminal 230 residues (exons 9-18).