INTERACTION BETWEEN CALTROPIN AND THE C-TERMINAL REGION OF SMOOTH-MUSCLE CALDESMON

Caltropin (CaT) binds caldesmon (CaD) in a Ca2+-dependent manner with an affinity higher than that of calmodulin (CaM). Photo-crosslinking b etween CaT and a benzophenone-labeled C-terminal CaD fragment (27K) re sults in a 35-kDa protein that corresponds to the 1:1 adduct between C aT and 27K. In the absence of Ca2+, no crosslinking is obtained. This result is similar to that obtained with CaM and 27K. The apparent affi nity of CaM for GS17C, a CaM-binding peptide of CaD, is weakened by Ca T, suggesting CaT competes with CaM for the peptide. In contrast to Ca M, CaT does not induce changes in the tryptophan fluorescence of GS17C . Thus although the two Ca2+-binding proteins behave similarly, there are differences in with CaD. (C) 1995 Academic Press, Inc.