Calcium Structural Transition of Troponin in the Complexes, on the Thin Filament, and in Muscle Fibres, as Studied By Site-Directed Spin-Labelling EPR

Abstract

We have measured the intersite distance, side-chain mobility and orientation of specific site(s) of troponin (Tn) complex on the thin filaments or in muscle fibres as well as in solution by means of site-directed spin labeling electron paramagnetic resonance (SDSL-EPR). We have examined the Ca(2+)-induced movement of the B and C helices relative to the D helix in a human cardiac (hc)TnC monomer state and hcTnC-hcTnI binary complex. An interspin distance between G42C (B helix) and C84 (D helix) was 18.4 angstroms in the absence of Ca2+. The distance between Q58C (C helix) and C84 (D helix) was 18.3 angstroms. Distance changes were observed by the addition of Ca2+ and by the formation of a complex with TnI. Both Ca2+ and TnI are essential for the full opening -3 angstroms of the N-domain in cardiac TnC. We have determined the in situ distances between C35 and C84 by measuring pulsed electron-electron double resonance (PELDOR) spectroscopy. The distances were 26.0 and 27.2 A in the monomer state and in reconstituted fibres, respectively. The addition of Ca2+ decreased the distance to 23.2 angstroms in fibres but only slightly in the monomer state, indicating that Ca2+ binding to the N-lobe of hcTnC induced a larger structural change in muscle fibres than in the monomer state. We also succeeded in synthesizing a new bifunctional spin labels that is firmly fixed on a central E-helix (94C-101C) of skeletal(sk)TnC to examine its orientation in reconstituted muscle fibres. EPR spectrum showed that this helix is disordered with respect to the filament axis. We have studied the calcium structural transition in skTnI and tropomyosin on the filament by SDSL-EPR. The spin label at a TnI switch segment (C133) showed three motional states depending on Ca2+ and actin. The data suggested that the TnI switch segment binds to TnC N-lobe in +Ca2+ state, and that in -Ca2+ state it is free in TnC-I-T complex alone while fixed to actin in the reconstituted thin filaments. In contrast, the side chain spin labels along the entire tropomyosin molecule showed no Ca(2+)-induced mobility changes.