CALBINDIN D-28K FORMS A CA2-DISSOCIABLE COMPLEX WITH MELLITIN IN-VITRO()

Calbindin D-28K (CB), a cytosolic calcium binding protein (CBP), forms a macromolecular complex with the polypeptide mellitin (ME) the absen ce of calcium, which can be reversibly dissociated by the addition of Ca2+. The molar ratio of CB:ME constituted in this complex is 1:4, sug gesting that CB interacts with the tetrameric form of ME. Like free te trameric ME, the CB:ME complex does not migrate into 15% non-denaturin g polyacrylamide electrophoretic gels, although both constituents migr ate normally after irreversible complex denaturation by heating in sod ium dodecyl sulphate (SDS). The interaction of these two proteins can be distinguished from the association of calmodulin (CM) with ME, whic h forms a reversibly dissociable, equimolar complex in the presence of Ca2+ and a stable non-migrating complex (molar ratio = 1:12) in its a bsence. Thus, CB and CM appear to bind ME under different Ca2+ regulat ory control, suggesting possible roles for CB as a Ca2+-dependent regu latory binding protein.