Synapsin I, a major neuron-specific substrate for cAMP-dependent and Ca2+/calmodulin-dependent protein kinases, associates in in vitro assays with brain integral membrane protein site(s) distinct from secretory vesicles and with the neurofilament Mr = 68,000 subunit. The membrane sites for synapsin involve protein(s) and are likely to have physiological relevance since the binding of 125I-labeled synapsin is abolished by digestion with chymotrypsin, is displaced by unlabeled synapsin, is of high affinity (KD = 10 nM), and has a capacity (42 pmol/mg membrane protein) that is comparable to the amount of synapsin in brain, optimal binding occurs at physiological pH (6.8-7.2) and salt concentrations (50 mM), and synapsin binding to membranes is inhibited by phosphorylation with Ca2+/calmodulin-dependent protein kinase. The brain membrane protein sites for synapsin are not due to synaptic vesicles, since synaptic vesicles do not sediment under the conditions of the binding assay. Association between synapsin and the Mr = 68,000 neurofilament subunit has also been demonstrated. The binding of synapsin with the neurofilament subunit is specific since this binding interaction is saturable, with a 1:1 stoichiometry, the binding involves only certain proteolytically derived domains of synapsin, and is therefore not a simple electrostatic interaction between the basic domains of synapsin and the acidic regions in the neurofilament subunit, and Ca2+/calmodulin-dependent phosphorylation of synapsin inhibits this interaction. Synapsin promotes cross-linking of synaptic vesicles to brain membranes, and these complexes are reduced by phosphorylation of synapsin. This interconnecting function of synapsin may be a general characteristic of synapsin binding, with a membrane (synaptic vesicle or nonsecretory vesicle)-bound synapsin associating with microtubules, neurofilaments, or spectrin.