Synaptic vesicle proteins share a specialized and common fate throughout the life cycle of the vesicle, and thus may need to respond to some common signals. It is therefore expected that these proteins will share some common motifs. However, sequence comparison among many of these proteins has not revealed any obvious motifs. Such a motif may be formed by the relative abundance of proline residues, which are not randomly distributed along the sequence but rather are clustered at the cytoplasmatic face of vesicular proteins. We propose that proline clusters serve as structural spacers between functional domains as well as potential target sites for protein-protein interactions. In view of the proline-rich nature of SH3 binding proteins, some of the proline-rich synaptic vesicle proteins may also participate in SH3 binding. Such binding may modulate certain signalling pathways in nerve terminals. Surprisingly, the consensus sequence between the proline clusters of synaptic vesicle proteins is found in a large family of abundant proline-rich proteins of the secretory organelles of the parotid exocrine gland.