Regulatory properties of the NH2- and COOH-terminal domains of troponin T. ATPase activation and binding to troponin I and troponin C.
The contraction of skeletal muscle is regulated by Ca2+ binding to troponin C, which results in an internal reorganization of the interactions within the troponin-tropomyosin complex. Troponin T is necessary for Ca2+-dependent inhibition and activation of actomyosin. Troponin T consists of an extended NH2-terminal domain that interacts with tropomyosin and a globular COOH-terminal domain that interacts with tropomyosin, troponin I, and troponin C. In this study we used recombinant troponin T and troponin I fragments to delimit further the structural and regulatory interactions with the thin filament. Our results show the following: (i) the NH2-terminal region of troponin T activates the actomyosin ATPase in the presence of tropomyosin; (ii) the interaction of the globular domain of troponin T with the thin filament blocks ATPase activation in the absence of Ca2+; and (iii) the COOH-terminal region of the globular domain anchors the troponin C-troponin I binary complex to troponin T through a direct Ca2+-independent interaction with the NH2-terminal region of troponin I. This interaction is required for Ca2+-dependent activation of the actomyosin ATPase activity. Based on these results we propose a refined model for the troponin complex and its interaction with the thin filament.
Malnic
B
B
Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, CP 26.077, 05599-970 São Paulo SP, Brazil.
Farah
C S
CS
Reinach
F C
FC
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002645:Chickens; D004789:Enzyme Activation; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology; D016297:Mutagenesis, Site-Directed; D009218:Myosins / Q000378:metabolism*; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014336:Troponin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019209:Troponin C / Q000378:metabolism*; D019210:Troponin I / Q000378:metabolism*; D020107:Troponin T
D011994:Recombinant Proteins; D014336:Troponin; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T; D009218:Myosins; D002118:Calcium