Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Inhibition of striated muscle contraction at resting Ca2+ depends on the C-terminal half of troponin I (Tnl) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin's core-and thus tropomyosin-is presumably needed. We generated recombinant, human cardiac troponins containing one of two Tnl constructs: either an 8 amino acid linker between Md and the rest of troponin (cTn(Link8)), or an Md deletion (cTnI(1-163)). Motility assays revealed that Ca2+-sensitivity of reconstituted thin filament sliding was markedly increased with cTnI(Link8) (similar to 0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (similar to 1.4 pCa unit shift). Cardiac Tn's ability to turn off filament sliding at diastolic Ca2+ was mostly (61%), but not completely eliminated with cTnI(1-163). TnI's Md is required for full inhibition of unloaded filament sliding, although other portions of troponin-presumably including Ip-are also necessary. We also confirm that TnI's Md is not responsible for superactivation of actomyosin cycling by troponin. (C) 2016 Elsevier Inc. All rights reserved.