ANNEXIN VI BINDS TO A SYNAPTIC VESICLE PROTEIN, SYNAPSIN-I

Annexin VI bound to > 14 species of proteins in the whole homogenate o f rat forebrain in a Ca2+/phosphatidylserine- or phosphatidic acid-dep endent manner. When the subcellular fractions of rat forebrain were ex amined with a blot from a sodium dodecyl sulfate-polyacrylamide gel, e ach annexin VI-binding protein showed a different distribution, sugges ting that annexin VI is a multifunctional protein. Of these proteins, the doublers of M(r) 80,000 were enriched in the purified synaptic ves icles and were identified as synapsin I. Annexin VI bound to the head domain of synapsin I. When the binding of annexin VI to synapsin I was characterized in the native state, the affinity of the binding for Ca 2+ (K-Ca) was 12.6 mu M and the affinity for annexin VI (K-D) was simi lar to 270 nM. Phosphorylation of synapsin I by cyclic AMP-dependent p rotein kinase and by Ca2+/calmodulin-dependent protein kinase II inhib ited the annexin VI binding. The mode of the inhibition was different between the two kinases. These results indicate that annexin VI may mo dulate the function of synapsin I in a Ca2+- and phospholipid-dependen t manner.