The Role of Troponin in the Ca²⁺-Regulation of Skeletal Muscle Contraction

Abstract

Vertebrate skeletal muscle contraction is activated by the binding of Ca²⁺ to the low affinity Ca²⁺-pecific (regulatory) sites of troponin C (TnC), the Ca²⁺-binding subunit of troponin, which together with troponin I (TnI), troponin T (TnT) and tropomyosin (Tm) form the regulatory system of the contractile apparatus (Zot and Potter 1987; Moss 1992; Gergely 1998). TnC is necessary for force development in skinned muscle fibers and its selective extraction from muscle fibers results in a permanent relaxation at all Ca²⁺ concentrations. This process is fully reversible and reconstitution of the TnC-depleted fibers with exogenous TnC restores Ca²⁺-dependent contraction (Zot and Potter 1982; Szczesna et al. 1996). The structure of TnC has been solved to atomic resolution by X-ray crystallography (Herzberg and James 1985, 1988; Houdusse et al. 1997). TnC consists of two globular domains corresponding to the NH₂-and COOH-termini and each domain contains two EF-hand divalent cation-binding sites. The low affinity sites of TnC, called the Ca²⁺-specific sites (I and II) (Potter and Gergely 1975), are located in the NH₂-terminal domain of TnC and are separated from the COOH-terminal domain of TnC by a single nineturn α-helix comprising helices D and E (Herzberg and James 1985, 1988). The COOH-terminal region of TnC contains two high affinity Ca²⁺-binding sites designated as sites III and IV and referred to as the Ca²⁺-Mg²⁺ sites (Potter and Gergely 1975). Sites I and II bind Ca²⁺ specifically with K_Ca2+ ≅ 3 × 10⁵M⁻¹, whereas sites III and IV bind Ca²⁺ with K_Ca2+ ≅ 2 × 10⁷M⁻¹ and Mg²⁺ with K_Mg2+ ≅ 2 × 10³M⁻¹ (Potter and Gergely 1975; Grabarek et al. 1992).