Dynamin I is a GTPase enzyme required for the retrieval of synaptic vesicles after exocytosis and functions in endocytosis by stimulating assembly of invaginating synaptic vesicles (1). Dynamin I is phosphorylated in nerve terminals exclusively in the cytosolic compartment and in vitro by protein kinase C (PKC) (2–5). The phosphorylation site in PKC-phosphorylated Dynamin I is a single site at Serine 795, which is located near a binding site for the SH3 domain of p85, the regulatory subunit of phosphatidylinositol 3-kinase (2–5). Dephosphorylation is required for synaptic vesicle retrieval, suggesting that phosphorylation affects the subcellular localization of Dynamin I (5). Mouse, rat and human Dynamin I are phosphorylated on serine residues, including Ser 778, by Cdk5, regulating PACSIN1 recruitment and enabling synaptic vesicle endocytosis.