Palmitoylation and testis-enriched expression of the cysteine-string protein beta isoform

Cysteine-string protein alpha (CSPalpha) is a DnaJ chaperone that is associated with secretory vesicles in diverse cell types. The cysteine-string region is the signature domain of CSPalpha and contains 14 closely spaced cysteines, the majority of which are palmitoylated; this post-translational modification mediates stable membrane attachment. CSPalpha has been proposed to function in regulated exocytosis pathways throughout the body and has an additional neuroprotective function. Two novel CSP isoforms, beta and gamma, were identified recently, although the expression profile, properties, and functions of these proteins are not clear and in some cases are subject to debate. Here, we report that CSPbeta is enriched in rat testis and was not detected in any other tissue that was examined, including brain. Although the cysteine-string domain of CSPbeta is distinct from that found in CSPalpha, the endogenous beta isoform expressed in testis is membrane-associated and palmitoylated. However, in agreement with earlier work, we find that the palmitoylation efficiency of CSPbeta is reduced compared with that of CSPalpha. Subsequent analysis of chimeric proteins reveals that regions upstream of the cysteine-string domains of CSPalpha and CSPbeta underlie this difference in palmitoylation efficiency between the two isoforms.