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

Abstract

Cysteine string proteins (CSPs) belong to the DnaJ-like 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 β-cells, Cspα is localized on insulin containing large dense core vesicles (LDCVs). An isoform closely related to Cspα, Cspβ, has been obtained from testis cell cDNA libraries. To gain insights on this isoform and more generally on the properties of CSPs, we compared Cspα and Cspβ. In pull-down experiments, Cspβ was able to interact to the same extent with two of the known Cspα chaperone partners, Hsc70 and SGT. Upon transient overexpression in clonal β-cells, Cspβ but not Cspα 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 Cspβ remained tightly bound to membranes, intrinsic properties of CSPs are sufficient for interactions with membranes. Indeed, recombinant Cspα and Cspβ were capable to interact with membranes even in their non-palmitoylated forms. Furthermore, overexpressed Cspβ 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.