The effect of regulatory Ca 2+ on the in situ structures of troponin C and troponin I: a neutron scattering study 1 1Edited by M. F. Moody

Invited review: Probing the structures of muscle regulatory proteins using small‐angle solution scattering

“…Experimentally, the researchers collected SANS data from sTn complexes, in which all possible combinations of the subunits were deuterated, in both the Ca 21 -free and Ca 21 -bound states. No significant Ca 21 -dependent changes in the R g of sTnC and sTnT2 were detected in the complex, while a large increase in the R g of sTnI was observed upon removal of Ca 21 , consistent with the earlier findings of Stone et al 49 The R g value of sTnI alone is larger than R g of the ternary complex, indicating that sTnI spans the entire length of the complex. The neutron-derived models of the Ca 21 -free and Ca 21 -bound sTn complexes indicate that the change between the more compact Ca 21 bound state to the extended Ca 21 free state is a direct consequence of the extension of the sTnI at its C-terminus.…”

“…Although the specific structural features of this extension cannot be deduced from the low-resolution neutron models, the novel finding of this structural change in sTnI has laid a foundation for further high-resolution studies. 15,[63][64][65] All optimized SANS-derived models point to a similar shape for sTnC, that of a fully extended dumbbell shape in both Ca 21 -free and Ca 21 -bound states, consistent with previous interpretations 48,49 as opposed to the somewhat more compact conformation observed in the cTn crystal structure 60 and SANS models for cTn. 50,51 In the SANS-derived model for sTn, the N-terminal domain of Ca 21 -bound sTnC rotates more than 1008 away from the position seen in the crystal structure.…”

“…Upon Ca 21 binding to the regulatory sites of TnC, TnI was observed to undergo a large-scale structural change resulting in a more compact structure with a $ 10% decrease in the radius of gyration R g and a $ 9% increase in the apparent cross-sectional radius of gyration (R c ). By modelling TnI as a two-domain molecule represented by oblate and prolate ellipsoids containing 65% and 35% of the mass, respectively, these researchers concluded there was a decrease in the center-of-mass separation of these domains of $ 15 Å upon Ca 21 addition, a strong indication that TnI acts as a molecular switch consistent with the mechanism proposed by Olah et al 46 Of note, sTnI in the ternary complex 49 was found to be more compact than was observed in for the binary complex, 46,48 suggesting interactions with sTnT were important in determining the structure of sTnI in the complex. Further development of ideas about the proposed Ca 21 -dependent molecular switch in Tn would have to await crystal structure data on Tn (see below).…”

“…The loss of Ca 21 from the two low-affinity Ca 21 binding sites in the N-terminal domain of TnC was postulated to lead to a closing of that domain and the release of the TnI inhibitory sequence so that it could shift to its binding site on actin and the subsequent movement of tropomyosin would inhibit the acto-myosin interaction. 48 Stone et al completed a neutron solvent matching series on ternary sTn 49 that confirmed the extended configuration of sTnC in the Ca 21 -saturated complex. In their study, complexes were prepared with deuterated TnI or TnC, and the unlabeled subunits were then solvent matched (at 41.6% 2 H 2 O) to enable structural evaluation of the deuterated components.…”

The Role of Troponins in Muscle Contraction

“…Therefore, the Ca 2+ -bound TnC neutralizes inhibition of the actin-activated myosin Mg 2+ -ATPase activity and initiates other interactions of TnI with TnT and Tm · actin. The C-terminal domain of TnI containing these two actin-binding sites has been proposed to act as a molecular Ca 2+ -sensitive switch during muscle contraction since it moves between Tm · actin and TnC in response to Ca 2+ binding to TnC (34,35). It has been well accepted that the metaldependent interactions of TnI and TnC play an important role (structural and functional) in the regulation of contraction.…”