IN-VITRO PHOSPHORYLATION OF ANNEXIN-2 HETEROTETRAMER BY PROTEIN-KINASE-C - COMPARATIVE PROPERTIES OF THE UNPHOSPHORYLATED AND PHOSPHORYLATED ANNEXIN-2 ON THE AGGREGATION AND FUSION OF CHROMAFFIN GRANULE MEMBRANES

Heterotetrameric annexin 2 phosphorylated ''in vitro'' by rat brain protein kinase C is purified and obtained devoid of unphosphorylated protein; it contains 2 mol of phosphate/mol of heterotetramer. The aggregative and binding properties of the phosphorylated annexin 2 toward purified chromaffin granules are compared with those of the unphosphorylated annexin 2, Annexin 2 binds to chromaffin granules with high affinity, Phosphorylation of annexin 2 decreases the affinity of this binding without affecting the maximum binding capacity, The binding curves are strongly cooperative, It is suggested that a surface oligomerization of the proteins may take place upon binding. Besides, phosphorylation of annexin 2 is followed by a dissociation of the light chains from the heavy chains in the heterotetramer. Whereas annexin 2 induces the aggregation of chromaffin granules at mu M calcium concentration, the phosphorylated annexin 2 does not induce aggregation at any concentration of calcium either at pH 6 or 7, The phosphorylation of annexin 2 by protein kinase C, MgATP, and 12-O-tetradecanoylphorbol-13-acetate on chromaffin fin granules induces a fusion of chromaffin granules membranes observed in electron microscopy. The fusion requires the activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate. Given these results and since annexin 2 is phosphorylated by protein kinase C under stimulation of chromaffin cells, it is suggested that phosphorylated annexin 2 may be implicated in the fusion step during exocytosis of chromaffin granules.