CAP-50, a calcyclin-associated protein with an apparent molecular mass or 50 kDa, was purified and proved to be a novel annexin [Tokumitsu, H. et al. (1992) J. Biol. Chem. 267, 8919–8924]. We examined the binding of CAP-50 to other Ca2+-binding proteins which have two or four EF-hand structures, by a co-precipitation assay with phospholipid (phosphatidylserine). Among nine Ca2+-binding proteins (calcyclin, S-100 proteins, p11, calgizzarin, calvasculin, calmodulin and troponin C) examined, only calcyclin interacted with CAP-50. These results clearly show that the interaction of CAP-50 to calcyclin is specific, i.e. other Ca2+-binding proteins with the EF-hand structure could not substitute for calcyclin, thereby suggesting the possible role in specific regulation of the function of CAP-50 by Ca2+/calcyclin.
