A calcyclin-associated protein is a newly identified member of the Ca²⁺/phospholipid-binding proteins, annexin family

A calcyclin-associated protein with an apparent molecular weight of 50,000 (CAP-50) was purified from rabbit lung. The procedure included ammonium sulfate precipitation, anion and cation ion-exchange, and calcyclin affinity chromatographies. Interestingly, partial amino acid sequences of lysyl- endpeptidase-digested fragments indicated that CAP-50 was a member of the Ca²⁺/phospholipid-binding proteins, the annexin family. The sequence of a proteolytic peptide with Staphylococcus aureus V8 protease on NH₂-terminal region is not homologous with any other annexin family proteins. Phospholipid binding studies showed that CAP-50 bound to phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidic acid- containing vesicles, in a Ca²⁺-dependent manner. In the presence of Ca²⁺/calcyclin, CAP-50 formed a complex with calcyclin and bound to the PS- containing vesicles. The apparent K(d) value of calcyclin for CAP-50 was calculated to be 1.61 x 10^-6 M. Zero-length cross-linking studies indicated that 1 mol of CAP-50 bound to an equimolar unit of calcyclin. CAP-50 inhibited the phospholipase A₂ activity, dose-dependently (IC₅₀ = 0.2 μM), however, calcyclin did not alter the inhibitory effect. With the ¹²⁵I-calcyclin gel overlay method, calcyclin bound tightly to CAP-50 in a Ca²⁺-dependent manner after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These results suggest that rabbit lung CAP-50 is a newly identified member of the annexin family. Ca²⁺/calcyclin apparently regulates the function of CAP-50 on cytosolic face of the plasma membrane.