Prenylated Rab acceptor domain family member 1 is involved in stimulated ACTH secretion and inhibition

Dexamethasone (Dex) inhibits stimulated adrenocorticotrophic hormone (ACTH) secretion in AtT-20 cells, a mouse corticotroph tumor cell line. Dexras1 protein expression is induced in corticotrophs by Dex. The function of Dexras1 is unknown; however, it may be involved in corticotrophic negative feedback. Here we report the identification of a Dexras1 interactor, prenylated Rab acceptor domain family member 1 (PRAF1), a protein that localizes to the Golgi complex, post-Golgi vesicles, and endosomes. We determined that amino acids 54-175 of PRAF1 are essential for interaction with Dexras1 and that specific point mutations located within this region enhance PRAF1-Dexras1 interactions. AtT-20 cells stably transfected with truncated or mutated PRAF1 constructs had altered responses to corticotrophin-releasing hormone and Dex, upregulated expression of the ACTH prohormone pro-opiomelanocortin (POMC), altered POMC processing, and altered Golgi complex morphology with decreased intra-Golgi and intracellular co-localization of PRAF1 and ACTH proteins. Our findings indicate that PRAF1 plays a novel role in ACTH stimulated secretion. We propose a model whereby Dexras1 interaction with PRAF1 may lock the sites necessary for PRAF1-Rab3A-VAMP2 interaction resulting in Dex-mediated inhibition of ACTH secretion.