Functional Significance of C-Terminal Mobile Domain of Cardiac Troponin I

Abstract

C-terminal mobile domain of cTnI (cTnI-MD, residues 168-210) is a highly conserved region which plays important roles in regulating myofilament function. Due to the unresolved structure of the region, it is still unclear how cTnI-MD plays a role in modulating the sarcomere length (SL)-dependent and Ca2+-induced effects on cardiac contractility. In this study, the functional significance of the MD in contractile behavior of skinned rat papillary muscle fibers and SL dependent activation was studied by truncation of the entire MD (cTnI(1-167)) and the C-terminus of MD (cTnI(1-193)). We hypothesized that different truncations may have different impacts on SL-dependent Ca2+-regulation of tension development and cross-bridge cycling. To test this hypothesis, the tension-pCa relationships and myosin cross-bridge kinetics were measured as cTnI(1-167) and cTnI(1-193) were exchanged into skinned rat papillary muscle fibers at short (SL=1.9µm) and long SLs (SL=2.2µm). Our results indicate that the SL induced enhancement of Ca2+-sensitivity and cooperativity observed in wild-type fibers is preserved in cTnI(1-193) and is diminished in cTnI(1-167) fibers. cTnI(1-167) truncation resulted in enhanced maximum tension and cooperativity of Ca2+-activation compared to cTnI(WT) and cTnI(1-193) and enhanced Ca2+-sensitivity compared to cTnI(WT) at both short and long SLs. cTnI(1-193) truncation resulted in enhanced myofilament Ca2+-sensitivity at both SLs and a depressed maximum tension just at long SL. Both cTnI-MD truncations showed a faster cross-bridge attachment rate at both SLs. Compared to cTnI(WT), cTnI(1-167) showed a slower cross-bridge detachment rate while cTnI(1-193) showed a faster cross-bridge detachment rate at long SL. The results of our study indicate that the truncation of cTnI-MD at different regions causes destabilization of the closed-state position of tropomyosin suggested by the observed increase in the cross-bridge attachment rate to thin filament as well as Ca2+-sensitivity of tension development at both sarcomere lengths.