Conformational modulation of slow skeletal muscle troponin T by an NH₂-terminal metal-binding extension

Troponin T (TnT) is an essential element in the thin filament Ca²⁺-regulatory system controlling striated muscle contraction. Alternative RNA splicing generates developmental and muscle type-specific TnT isoforms differing in the hypervariable NH₂-terminal region. Using avian fast skeletal muscle TnT containing a metal-binding segment, we have demonstrated a role of the NH₂-terminal domain in modulating the conformation of TnT (Wang J and Jin JP. Biochemistry 37: 14519-14528, 1998). To further investigate the structure-function relationship of TnT, the present study constructed and characterized a recombinant protein in which the metal-binding peptide present in avian fast skeletal muscle TnT was fused to the NH₂ terminus of mouse slow skeletal muscle TnT. Metal ion or monoclonal antibody binding to the NH₂-terminal extension induced conformational changes in other domains of the model TnT molecule. This was shown by the altered affinity to a monoclonal antibody against the COOH-terminal region and a polyclonal antiserum recognizing multiple epitopes. Protein binding assays showed that metal binding to the NH₂-terminal extension had effects on the interaction of TnT with troponin I, troponin C, and most significantly, tropomyosin. The data indicate that the NH₂-terminal Tx [4-7 repeats of a sequence motif His-(Glu/Ala)-Glu-Ala-His] extension confers a specific conformational modulation in the slow skeletal muscle TnT.