The role of the NH(2)- and COOH-terminal domains of the inhibitory region of troponin I in the regulation of skeletal muscle contraction.
The role of the inhibitory region of troponin (Tn) I in the regulation of skeletal muscle contraction was studied with three deletion mutants of its inhibitory region: 1) complete (TnI-(Delta96-116)), 2) the COOH-terminal domain (TnI-(Delta105-115)), and 3) the NH(2)-terminal domain (TnI-(Delta95-106)). Measurements of Ca(2+)-regulated force and relaxation were performed in skinned skeletal muscle fibers whose endogenous TnI (along with TnT and TnC) was displaced with high concentrations of added troponin T. Reconstitution of the Tn-displaced fibers with a TnI.TnC complex restored the Ca(2+) sensitivity of force; however, the levels of relaxation and force development varied. Relaxation of the fibers (pCa 8) was drastically impaired with two of the inhibitory region deletion mutants, TnI-(Delta96-116).TnC and TnI-(Delta105-115).TnC. The TnI-(Delta95-106).TnC mutant retained approximately 55% relaxation when reconstituted in the Tn-displaced fibers. Activation in skinned skeletal muscle fibers was enhanced with all TnI mutants compared with wild-type TnI. Interestingly, all three mutants of TnI increased the Ca(2+) sensitivity of contraction. None of the TnI deletion mutants, when reconstituted into Tn, could inhibit actin-tropomyosin-activated myosin ATPase in the absence of Ca(2+), and two of them (TnI-(Delta96-116) and TnI-(Delta105-115)) gave significant activation in the absence of Ca(2+). These results suggest that the COOH terminus of the inhibitory region of TnI (residues 105-115) is much more critical for the biological activity of TnI than the NH(2)-terminal region, consisting of residues 95-106. Presumably, the COOH-terminal domain of the inhibitory region of TnI is a part of the Ca(2+)-sensitive molecular switch during muscle contraction.
Szczesna
D
D
Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, Miami, Florida 33101, USA.
Zhang
R
R
Zhao
J
J
Jones
M
M
Potter
J D
JD
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D004789:Enzyme Activation; D008969:Molecular Sequence Data; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism; D011485:Protein Binding / Q000235:genetics; D011817:Rabbits; D017384:Sequence Deletion; D014335:Tropomyosin / Q000378:metabolism; D019209:Troponin C / Q000378:metabolism; D019210:Troponin I / Q000235:genetics / Q000378:metabolism*; D020107:Troponin T / Q000378:metabolism
D014335:Tropomyosin; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T; D009218:Myosins; D002118:Calcium