Ca2+-induced conformational changes in the troponin complex detected by crosslinking.

In an attempt to detect Ca2+-induced conformational changes by crosslinking, rabbit muscle troponin complex was reacted with the bifunctional reagents 1,3-difluoro-4,6-dinitrobenzene, 4,4'-difluoro-3,3'-dinitrodiphenylsulfone and dimethyl suberimidate under various conditions and the products were analyzed by dodecyl sulfate gel electrophoresis. In the absence of divalent cations, with the two aromatic reagents at a low reagent/protein ratio, the main cross-link products were troponin T-I and I-C. With dimethyl suberimidate the only major crosslink product was conjugate T-I. Ca2+, alone as well as in the presence of Mg2+, prevented the formation of I-C crosslinks with both aromatic reagents, but it did not affect crosslinking with dimethyl suberimidate. Ca2+ also decreased the number of NH2 groups of troponin that are highly reactive towards 2,4,6-trinitrobenzene sulfonate. Both effects of Ca2+ can be interpreted in terms of a conformational change in the troponin complex elicited by the binding of the specific divalent cation.