Phosphorylation of annexin II tetramer by protein kinase C inhibits aggregation of lipid vesicles by the protein

Annexin II tetramer (A-IIt) is a member of the annexin family of Ca²⁺ and phospholipid-binding proteins. The ability of this protein to aggregate both phospholipid vesicles and chromaffin granules has suggested a role for the protein in membrane trafficking events such as exocytosis. A-IIt is also a major intracellular substrate of both pp60(src) and protein kinase C; however, the effect of phosphorylation on the activity of this protein is unknown. In the current report we have examined the effect of phosphorylation on the lipid vesicle aggregation activity of the protein. Protein kinase C catalyzed the incorporation of 2.1 ± 0.8 mol of phosphate/mol of A-IIt. Phosphorylation of A-IIt caused a dramatic decrease in the rate and extent of lipid vesicle aggregation without significantly effecting Ca²⁺-dependent lipid binding by the phosphorylated protein. Phosphorylation of A-IIt increased the A(50%) (Ca²⁺) of lipid vesicle aggregation from 0.18 μM to 0.65 mM. Activation of A-IIt phosphorylation, concomitant with activation of lipid vesicle aggregation, inhibited both the rate and extent of lipid vesicle aggregation but did not cause disassembly of the aggregated lipid vesicles. These results suggest that protein kinase C-dependent phosphorylation of A-IIt blocks the ability of the protein to aggregate phospholipid vesicles without affecting the lipid vesicle binding properties of the protein.