A Direct Regulatory Role for Troponin T and a Dual Role for Troponin C in the Ca Regulation of Muscle Contraction

Troponin (Tn), containing three subunits: Ca binding (TnC), inhibitory (TnI), and tropomyosin binding (TnT), plays a crucial role in the Ca regulation of vertebrate striated muscle contraction. These three subunits function by interacting with each other and with the other thin filament proteins. Pr...

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Veröffentlicht in:The Journal of biological chemistry 1995-02, Vol.270 (6), p.2557
Hauptverfasser: James D. Potter, Zelin Sheng, Bo-Sheng Pan, Jiaju Zhao
Format: Artikel
Sprache:eng
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Zusammenfassung:Troponin (Tn), containing three subunits: Ca binding (TnC), inhibitory (TnI), and tropomyosin binding (TnT), plays a crucial role in the Ca regulation of vertebrate striated muscle contraction. These three subunits function by interacting with each other and with the other thin filament proteins. Previous studies suggested that the primary role of TnT is to anchor the TnI•TnC complex to the thin filament, primarily through its interactions with TnI and tropomyosin. We propose here a new role for TnT. Our results indicate that, when TnT is combined with the TnI•TnC complex, there is an activation of actomyosin ATPase that is Ca -dependent. To determine whether the latter results from a direct 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 of residues 1-57) of TnI, TnI (Sheng et al. (1992) J. Biol. Chem. 267, 25407-25413), which interacts with TnC but not TnT. Both wild type (TnI•TnC•TnT) and mutant (TnI •TnC•TnT) Tn complexes demonstrated equivalent activity in the Ca regulation of actomyosin-S1 ATPase activity. Similarly, both TnI and TnI could equally reconstitute TnI-depleted skinned muscle fibers. Therefore, since TnI does not interact with TnT, these results suggest that TnT reconstitutes native Ca sensitivity via direct interaction with TnC. Thus Ca binding to TnC would have a dual role: 1) release of the ATPase inhibition by TnI and 2) activation of the ATPase through interaction with TnT.
ISSN:0021-9258
1083-351X
DOI:10.1074/jbc.270.6.2557