Studies on calcium ion-induced conformation changes in the actin-tropomyosin-troponin system by fluorimetry. IV. Conformational changes in the region containing fluorescence-labeled sulfhydryl group(s) of troponin.
Conformational changes associated with the functional states of the molecule of troponin were studied using SH-direct fluorogenic reagents, N-(p-(2-benzimidazolyl)phenyl) maleimide (BIPM) and N-(1-anilinonaphthyl-4) maleimide (ANM). 1. The fluorescence parameters of ANM-troponin, intensity, and polarization, did not change on combining it with tropomyosin alone, but markedly changed when F-actin was further added to the system. 2. The conformation around the dye-labeled sulfhydryl group(s) was shown to be susceptible to Ca2+ in terms of fluorescence intensity of the label, thermal transition of the conformation, and the microenvironment near the label. 3. On addition of Ca2+, the fluorescence characteristics of the two systems, ANM-troponin . tropomyosin and ANM-troponin . tropomyosin . F-actin complexes, were altered in opposite directions. When BIPM was used in place of ANM, similar changes were observed: a simple decrease in the intensity when pCa was decreased from 7.4 to 5.5 in the system without F-actin and a sigmoidal increase in the range from pCa 7 to 6 in the system with F-actin. Heavy meromyosin, when added to the latter complex (the reconstituted thin filaments), made the profile of its Ca2+ concentration dependence of fluorescence similar to that of the former complex. When tropomyosin was labeled in place of troponin, similar results were obtained. The data obtained imply that the Ca2+-induced conformational changes of troponin are markedly modified when detached from actin, and that heavy meromyosin weakens the interaction of the troponin . tropomyosin complex with F-actin.
D000199:Actins; D000818:Animals; D002118:Calcium; D004533:Egtazic Acid; D007700:Kinetics; D046911:Macromolecular Substances; D009124:Muscle Proteins; D009132:Muscles; D011487:Protein Conformation; D011817:Rabbits; D013050:Spectrometry, Fluorescence; D013438:Sulfhydryl Compounds; D013696:Temperature; D014335:Tropomyosin; D014336:Troponin; D014783:Viscosity
D000199:Actins; D046911:Macromolecular Substances; D009124:Muscle Proteins; D013438:Sulfhydryl Compounds; D014335:Tropomyosin; D014336:Troponin; D004533:Egtazic Acid; D002118:Calcium