Amphiphysin I is associated with coated endocytic intermediates and undergoes stimulation-dependent dephosphorylation in nerve terminals

Amphiphysin I is an abundant presynaptic protein that interacts via its COOH-terminal src homology 3 (SH3) domain with the GTPase dynamin I and the inositol-5-phosphatase synaptojanin. Both dynamin I and synaptojanin I have a putative role in synaptic vesicle recycling and undergo rapid dephosphorylation in rat brain synaptosomes stimulated to secrete by a depolarizing stimulus. We show here that amphiphysin I also undergoes constitutive phosphorylation and stimulation-dependent dephosphorylation. Dephosphorylation of amphiphysin I requires extracellular Ca²⁺ and is unaffected by pretreatment of synaptosomes with tetanus toxin. Thus, Ca²⁺ influx, but not synaptic vesicle exocytosis, is required for dephosphorylation. Dephosphorylation of amphiphysin I, like dephosphorylation of dynamin I and synaptojanin I, is inhibited by cyclosporin A and FK-506 (0.5 μM), two drugs that specifically block the Ca²⁺/calmodulin-dependent phosphatase 2B calcineurin, but not by okadaic acid (1 μM), which blocks protein phosphatases 1 and 2B. We also show by immunogold electron microscopy immunocytochemistry that amphiphysin I is localized in the nerve terminal cytomatrix and is partially associated with endocytic intermediates. These include the clathrin-coated buds and dynamin-coated tubules, which accumulate in nerve terminal membranes incubated in the presence of guanosine 5'-3-O- (thio)triphosphate. These data support the hypothesis that amphiphysin I, dynamin I, and synaptojanin I are physiological partners in some step(s) of synaptic vesicle endocytosis. We hypothesize that the parallel Ca²⁺- dependent calcineurin-dependent dephosphorylation of amphiphysin I and of its two major binding proteins is part of a process that primes the nerve terminal for endocytosis in response to a burst of exocytosis.