Prenylated Rab acceptor (PRA1) is a protein that binds Rab GTPases and the v-SNARE VAMP2. The protein is localized to the Golgi complex and post-Golgi vesicles. To determine its functional role, we generated a number of point mutations and divided them into three classes based on cellular localization. Class A mutants were retained in the endoplasmic reticulum (ER) and exerted an inhibitory effect on transport of vesicular stomatitis virus envelope glycoprotein (VSVG) from the ER to Golgi as well as to the plasma membrane. Class B mutants exhibited a highly condensed Golgi complex and inhibited exit of anterograde cargo from this organelle. Class C mutants exhibited an intermediate phenotype with Golgi and ER localization along with extensive tubular structures emanating from the Golgi complex. There was a direct correlation between the cellular phenotype and binding to Rab and VAMP2. Class A and C mutants showed a significant decrease in Rab and VAMP2 binding, whereas an increase in binding was observed in the class B mutants. Thus, PRA1 is required for vesicle formation from the Golgi complex and might be involved in recruitment of Rab effectors and SNARE proteins during cargo sequestration.Rab GTPases constitute the largest group within the Ras superfamily. They regulate vesicle trafficking by cycling through active membrane-bound GTP-and inactive cytosolic GDP-bound states. Membrane localization requires modification of the cysteine-containing motif at the carboxyl terminus by prenyl residues. Cycling between the membrane and cytosol is mediated by GDP dissociation inhibitor (GDI), 1 which extracts GDP-bound Rab from the membrane. Activation through guanine nucleotide exchange at the membrane is catalyzed by a guanine nucleotide exchange factor, of which a number have been identified in mammals (1, 2).Vesicular transport through the secretory pathway undergoes a number of discrete steps each involving budding, membrane remodeling, targeting, docking, and fusion. In ER to Golgi transport, anterograde cargo proteins such as VSVG are selectively transported to the Golgi along with resident ER proteins with the latter retrieved by a salvage process that recognizes distinct motifs within the protein (3). These transport vesicles contain an electron-dense coat assembled under the control of the small GTPase ARF (4). The fungal metabolite brefeldin A (BFA) inhibits ARF activation by stabilizing the inactive ARF-guanine nucleotide exchange factor (GEF) complex (5) resulting in retrograde transport of Golgi content to the ER. At the Golgi complex, cargo proteins destined for postGolgi locations are sorted into distinct carriers upon exit from the trans face, whereas Golgi resident proteins such as mannosidase II (Man II) are selectively retrieved in COPI-coated vesicles and returned to the cis face (6).Vesicle fusion is mediated by the core SNARE complex consisting of the vesicle protein VAMP, or synaptobrevin, and two other membrane proteins, syntaxin and SNAP-25 (7). Rab effectors play a regulatory role in this pro...