RAB3A AND RAB3B CARBOXY-TERMINAL PEPTIDES ARE BOTH POTENT AND SPECIFIC INHIBITORS OF PROLACTIN-RELEASE BY RAT CULTURED ANTERIOR-PITUITARY-CELLS

Chimeric polypeptides composed of the homeodomain of Antennapedia and of the C-terminus of several low molecular weight GTP-binding proteins of the rab family have been found to translocate through the membrane of cells in culture and to accumulate in the cytoplasm and nucleus. W e have used these chimeric peptides to study, in intact endocrine cell s, a putative role for the C-terminal domain of rab proteins in the ex ocytotic process. We show that the internalization of 33- and 32-amino acid polypeptides corresponding to the C-terminal domains of rab3A an d rab3B blocks calcium-triggered PRL release from adult rat anterior p ituitary cells maintained in primary culture. This effect is specific to rab3 since it is not observed after internalization of either rab1 or rab2 C-terminal peptides. In addition, we demonstrate that this inh ibition does not require the geranylgeranylation of the internalized C -termini. As rab3B normally shows a permissive effect on exocytosis in PRL-secreting cells, we suggest that the C-terminal domains of rab3A and rab3B contain structural elements that compete with endogeneous ra b3 necessary for calcium-induced exocytosis.