Synapsin I is a neuron-specific phosphoprotein that is concentrated in the presynaptic nerve terminal in association with the cytoplasmic surface of synaptic vesicles. It has been demonstrated to bundle F- actin in a phosphorylation-dependent manner in vitro, a property consistent with its proposed role in linking synaptic vesicles to the cytoskeleton and its involvement in the regulation of neurotransmitter release. Synapsin I is composed of two distinct domains, a COOH terminal, collagenase-sensitive, hydrophilic, and strongly basic tail region, and an NH2 terminal, collagenase-resistant head region relatively rich in hydrophobic amino acids. To elucidate the structural basis for the interactions between synapsin I and F-actin and how it relates to other characteristics of synapsin I, we have performed a structure-function analysis of fragments of synapsin I produced by cysteine-specific cleavage with 2-nitro-5-thiocyanobenzoic acid. The fragments were identified and aligned with the parent molecule using the deduced primary structure of synapsin I and the known phosphorylation sites as markers. We have purified these fragments and examined their interactions with F-actin. Two distinct fragments, a 29- kD NH2-terminal fragment and a 15-kD middle fragment, were shown to contain F-actin binding sites. A 51/54-kD middle/tail fragment retained the F-actin binding and bundling activity of synapsin I, but the isolated tail fragment did not retain either activity. In contrast to phosphorylation of sites two and three in intact synapsin I, which abolishes F-actin bundling activity, phosphorylation of these sites in the middle/tail fragment failed to abolish this activity. In conclusion, three domains of synapsin I appear to be involved in F- actin binding and bundling.