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