Cooperative Changes in Troponin Structure Explained without Cooperative Calcium Binding

Abstract

To explain paradoxical changes in the intensity of the meridional 1/38.5 nm−1 reflection corresponding to the structure of troponin during single and paired twitches, we employ a model that assumes muscle regulation by simple non-interacting Ca2+ binding sites of troponin (Tn) in the thin filament (Zot, H.G. and Hasbun, J.E. (2016) Frontiers Physiol. 7:406). In muscle stretched beyond overlap and stimulated once, the intensity of the meridional 1/38.5 nm−1 reflection peaks slower and decays faster than expected for the Ca2+ association properties of isolated Tn, which may be explained by a model in which Ca2+-bound Tn regulates the position of tropomyosin (Tm) cooperatively (Matsuo, T., Iwamoto, H., and Yagi, N. (2010). Biophys. J. 99, 193-200). However, Tn in regulated actin binds Ca2+ non-cooperatively. In muscle receiving paired stimuli, crossbridges elicit a much larger peak tension than the change in 1/38.5 nm−1 reflection intensity, suggesting cooperativity owing to cycling crossbridges (Matsuo, T., Iwamoto, H., and Yagi, N. (2008). J Mol. Biol. 383, 1019-1036). Given standard pulses of free Ca2+, we fit the tension data of single and paired pulses with a model that permits cooperativity owing to cycling crossbridges but not Ca2+-bound Tn. The model produces time courses of Ca2+-bound Tn and the position of the Tm-Tn complex that follow single and paired transients of the meridional 1/38.5 nm−1 reflection intensity changes for both overlap and nonoverlap preparations. The model predicts that a fraction of Ca2+-bound Tn correlates in time and amplitude with the tension transient. This fraction represents Tn positioned away from possible interaction with actin by cycling crossbridges acting cooperatively on the position of Tm in the thin filament. Thus, this most parsimonious model of cooperativity is also the one most capable of explaining experimental observations.