Cysteine string protein beta is prominently associated with nerve terminals and secretory organelles in mouse brain

Cysteine string proteins (CSPs) are associated with regulated secretory organelles in organisms ranging from fruit flies to man. Mammals have three csp genes (alpha, beta and gamma), and previous work indicated that expression of the csp-beta and -gamma genes was restricted to the testes. For the current investigation, antibodies specific for CSP-beta were developed. Unexpectedly, immunoblot analysis indicated that CSP-beta was prominently expressed throughout mouse brain. Upon sub-cellular fractionation, CSP-beta was enriched in synaptosomes and synaptic vesicle fractions. Interestingly, CSP-beta existed almost exclusively as part of a high mass complex both in testis and brain. This complex required aggressive denaturation to release monomeric CSP-beta. By Northern analysis CSP-beta mRNA was present at very low abundance as a approximately 1.0kb species in mouse brain. Collectively, the enrichment of CSP-beta in synaptosomes and the association of CSP-beta with synaptic vesicles suggest that CSP-beta, like CSP-alpha, may be an important component of the regulated secretory machinery in mouse brain.