PHOSPHORYLATION-DEPENDENT EFFECTS OF SYNAPSIN IIA ON ACTIN POLYMERIZATION AND NETWORK FORMATION

The synapsins are a family of synaptic vesicle phosphoproteins which p lay a key role in the regulation of neurotransmitter release and synap se formation, In the case of synapsin I, these biological properties h ave been attributed to its ability to interact with both synaptic vesi cles and the actin-based cytoskeleton. Although synapsin II shares som e of the biological properties of synapsin I, much less is known of it s molecular properties. We have investigated the interactions of recom binant rat synapsin IIa with monomeric and filamenious actin and the s ensitivity of those interactions to phosphorylation, and found that: i ) dephosphorylated synapsin II stimulates actin polymerization by bind ing to actin monomers and forming actively elongating nuclei and by fa cilitating the spontaneous nucleation/elongation processes; ii) dephos phorylated synapsin II induces the formation of thick and ordered bund les of actin filaments with greater potency than synapsin I; iii) phos phorylation by protein kinase A markedly inhibits the ability of synap sin II to interact with both actin monomers and filaments. The results indicate that the interactions of synapsin II with actin are similar but not identical to those of synapsin I and suggest that synapsin II may play a major structural role in mature and developing nerve termin als, which is only partially overlapping with the role played by synap sin I.