Roles for the troponin tail domain in thin filament assembly and regulation - A deletional study of cardiac troponin T

Striated muscle contraction is regulated by Ca2+ binding to troponin, which has a globular domain and an elongated tail attributable to the NH2-termin al portion of the bovine cardiac troponin T (TnT) subunit. Truncation of th e bovine cardiac troponin tail was investigated using recombinant TnT fragm ents and subunits TnI and TnC. Progressive truncation of the troponin tail caused progressively weaker binding of troponin-tropomyosin to actin and of troponin to actin-tropomyosin. A sharp drop-off in affinity occurred with NH2-terminal deletion of 119 rather than 94 residues. Deletion of 94 residu es had no effect on Ca2+-activation of the myosin subfragment 1-thin filame nt MgATPase rate and did not eliminate cooperative effects of Ca2+ binding. Troponin tail peptide TnT1-153 strongly promoted tropomyosin binding to ac tin in the absence of TnI or TnC. The results show that the anchoring funct ion of the troponin tail involves interactions with actin as well as with t ropomyosin and has comparable importance in the presence or absence of Ca2. Residues 95-153 are particularly important for anchoring, and residues 95 -119 are crucial for function or local folding. Because striated muscle reg ulation involves switching among the conformational states of the thin fila ment, regulatory significance for the troponin tail may arise from its prom inent contribution to the protein-protein interactions within these conform ations.