A DIRECT REGULATORY ROLE FOR TROPONIN-T AND A DUAL ROLE FOR TROPONIN-C IN THE CA2-CONTRACTION( REGULATION OF MUSCLE)

Troponin (Tn), containing three subunits: Ca2+ binding (TnC), inhibito ry (TnI), and tropomyosin binding (TnT), plays a crucial role in the C a2+ regulation of vertebrate striated muscle contraction. These three subunits function by interacting with each other and with the other th in filament proteins. Previous studies suggested that the primary role of TnT is to anchor the TnI.TnC complex to the thin filament, primari ly through its interactions with TnI and tropomyosin. We propose here a new role for TnT. Our results indicate that, when TnT is combined wi th the TnI.TnC complex, there is an activation of actomyosin ATPase th at is Ca2+-dependent. To determine whether the latter results from a d irect effect of TnC on TnT or indirectly from an effect of TnC on TnI which is transmitted to TnT, we prepared a deletion mutant (deletion o f residues 1-57) of TnI, TnI(d57) (Sheng et al. (1992) J. Biol. Chem. 267, 25407-25413), which interacts with TnC but not TnT. Both wild typ e (TnI.TnC.TnT) and mutant (TnI(d57).Tnc.TnT) Tn complexes demonstrate d equivalent activity in the Ca2+ regulation of actomyosin-S1 ATPase a ctivity. Similarly, both TnT and TnI(d57) could equally reconstitute T nI-depleted skinned muscle fibers. Therefore, since TnI(d57) does not interact with TnT, these results suggest that TnT reconstitutes native Ca2+ sensitivity via direct interaction with TnC. Thus Ca2+ binding t o TnC would have a dual role: 1) release of the ATPase inhibition by T nI and 2) activation of the ATPase through interaction with TnT.