Common Characteristics Found by Fitting Divergent Data from TNC Mutations

Abstract

A single point mutation of the TnC protein was seen to have complex effects on the Ca-tension relationship of single muscle fibers (Lee et al., 2010, Am J Physiol Cell Physiol. doi:10.1152/ajpcell.00491.2009). Compared with wild type, three mutant TnC proteins, T70D, I60Q, and V43Q, all shifted the calcium sensitivity of tension in a qualitatively predictable manner based on the affinities of each mutant for calcium. However, a simple change in affinity can not explain why the slopes of the tension responses differed significantly among the fibers reconstituted with mutants and wild type TnC proteins. Similarly, the ktr data were elevated at low calcium compared with wild type for only two of the three mutants. We describe a refinement of our published model (Zot et al., 2009, PLoS One. 4:e8052) that relates all of the physiological measurements to three fundamental interactions, namely, calcium and TnC (affinity), TnC and TnI (switch), and TnI and actin (coupling). From our model, altering affinity causes a parallel shift in Ca-tension relationship, but the reduction in slope observed for all mutants is explained partly or wholly by an impairment of the switch. The elevated ktr is explained by a reduction in the coupling constant. The exponential shape of the ktr data, which is identical for fibers reconstituted with mutant and wild type TnC, fits the cooperative mechanism of our published model. As this mechanism is troponin-independent, we are intrigued by the response of cooperatvity to the switch. Model specifications and supplementary materials may be found online (www.westga.edu/STEMresearch). This work was supported by NSF grant MCB-0508203 (HGZ).