Synaptic vesicles are the secretory organelles responsible for the regulated secretion of neurotransmission. Proteins associated with or integral components of the lipid bilayer likely represent important components in regulated secretion. CSP (cysteine string protein) is a 34 kDa protein which copurifies with cholinergic synaptic vesicles from the marine ray Torpedo californica. In Drosophila melanogaster deletion of the CSP gene causes impaired presynaptic neuromuscular transmission. A rat brain complementary DNA (cDNA) encoding CSP was isolated and sequence analysis predicts a protein with 86% identity to Torpedo CSP. Rat CSP contains a "J domain" as well as a cysteine rich "string" region. The J domain "fingerprints" the CSP family as members of the universally conserved DnaJ/hsp40 (heat shock protein) chaperone family. Polyclonal antisera raised against a seventeen amino acid peptide representing the carboxy terminus of rat CSP detected a 35 kDa immunoreactive protein in a rat brain synaptic vesicle enriched preparation. A 35 kDa immunoreactive protein that comigrates electrophoretically with rat brain CSP was also detected in zymogen granule membranes. These results establish the presence of a CSP in rat brain and in the zymogen granule of the rat pancreas and suggest that CSPs have a role in exocrine and neural secretion.