Cysteine-string protein isoform beta (Cspβ) is targeted to the trans-Golgi network as a non-palmitoylated CSP in clonal β-cells

Cysteine string proteins (CSPs) belong to the DnaJ-Iike chaperone family and play an important role in regulated exocytosis in neurons and endocrine cells. The palmitoylation of several residues in a cysteine string domain may anchor CSPs to the exocytotic vesicle surface and in pancreatic beta-cells, Csp alpha is localized on insulin containing large dense core vesicles (LDCVs). An isoform closely related to Csp alpha, Csp beta, has been obtained from testis cell cDNA libraries. To gain insights on this isoform and more generally on the properties of CSPs, we compared Cspa and Csp beta. In pull-down experiments, Csp beta was able to interact to the same extent with two of the known Csp alpha chaperone partners, Hsc70 and SGT. Upon transient overexpression in clonal beta-cells, Csp beta but not Csp alpha was mainly produced as a non-palmitoylated protein and mutational analysis indicated that domains distinct from the cysteine string are responsible for this difference. As Csp beta remained tightly bound to membranes, intrinsic properties of CSPs are sufficient for interactions with membranes. Indeed, recombinant Csp alpha and Csp beta were capable to interact with membranes even in their non-palmitoylated forms. Furthermore, overexpressed Csp beta was not associated with LDCVs, but was localized at the trans-Golgi network. Our results suggest a possible correlation between the specific membrane targeting and the palmitoylation level of CSPs. (c) 2006 Flsevier B.V. All rights reserved.