Ca(2+)-dependence of structural changes in troponin-C in demembranated fibers of rabbit psoas muscle.
The Ca(2+)-dependence of structural changes in troponin-C (TnC) has been detected by monitoring the fluorescence from TnC labeled at Methionine-25, in the NH2-terminal domain, with danzylaziridine (TnC-DANZ) and then exchanged for endogenous TnC in glycerinated single fibers. The fluorescence-pCa relation obtained from fibers stretched to a sarcomere length greater than 4.0 microns evidenced two transitions: a small one, attributable to the binding of Ca2+ to the high affinity, Ca(2+)-Mg(2+)-binding sites of TnC; and a large one, attributable to the binding of Ca2+ to the low affinity, Ca(2+)-specific binding sites of TnC. In the fluorescence-pCa relation determined with fibers set to a sarcomere length of 2.4 microns, hence obtained in the presence of cycling cross-bridges, the large transition had the same Ca(2+)-dependence as did the development of tension. These results indicate that the NH2-terminal globular domain of TnC is modified by the binding of Ca2+ to sites located in both globular domains and that the structural changes in TnC resulting from the binding of Ca2+ to the low-affinity sites, but not to the high-affinity sites, are directly associated with the triggering of contraction.
Allen
T S
TS
Department of Physiology and Biophysics, University of Washington, Seattle 98195.
Yates
L D
LD
Gordon
A M
AM
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D066298:In Vitro Techniques; D015394:Molecular Structure; D009119:Muscle Contraction / Q000502:physiology; D009132:Muscles / Q000378:metabolism*; D011487:Protein Conformation; D011817:Rabbits; D013050:Spectrometry, Fluorescence; D014336:Troponin / Q000737:chemistry / Q000378:metabolism*; D019209:Troponin C
D014336:Troponin; D019209:Troponin C; D000255:Adenosine Triphosphate; D002118:Calcium