The calcium-sensing protein synaptotagmin 7 is expressed on different endosomal compartments in endocrine, neuroendocrine cells or neurons but not on large dense core vesicles
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 beta-cells, Cspalpha is localized on insulin containing large dense core vesicles (LDCVs). An isoform closely related to Cspalpha, Cspbeta, has been obtained from testis cell cDNA libraries. To gain insights on this isoform and more generally on the properties of CSPs, we compared Cspalpha and Cspbeta. In pull-down experiments, Cspbeta was able to interact to the same extent with two of the known Cspalpha chaperone partners, Hsc70 and SGT. Upon transient overexpression in clonal beta-cells, Cspbeta but not Cspalpha 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 Cspbeta remained tightly bound to membranes, intrinsic properties of CSPs are sufficient for interactions with membranes. Indeed, recombinant Cspalpha and Cspbeta were capable to interact with membranes even in their non-palmitoylated forms. Furthermore, overexpressed Cspbeta 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.