Guanine-nucleotide exchange factor (GEF) for members of the Arf family of small GTPases involved in trafficking in the early secretory pathway; its GEF activity initiates the coating of nascent vesicles via the localized generation of activated ARFs through replacement of GDP with GTP. Recruitment to cis-Golgi membranes requires membrane association of Arf-GDP and can be regulated by ARF1, ARF3, ARF4 and ARF5. Involved in the recruitment of the COPI coat complex to cellular membranes such as the endoplasmic reticulum exit sites (ERES), and the endoplasmic reticulum-Golgi intermediate (ERGIC) and cis-Golgi compartments implicating ARF1 activation. Involved in COPI vesicle-dependent retrograde transport from the ERGIC and cis-Golgi compartments to the endoplasmic reticulum (ER). Involved in the trans-Golgi network recruitment of GGA1, GGA2, GGA3, BIG1, BIG2, and the AP-1 adaptor protein complex related to chlathrin-dependent transport; the function requires its GEF activity (probably at least in part on ARF4 and ARF5). Has GEF activity towards ARF1 (By similarity). Has in vitro GEF activity towards ARF5 (PubMed:10402461). Involved in the processing of PSAP. Required for the assembly of the Golgi apparatus (PubMed:19182783). The AMPK-phosphorylated form is involved in Golgi disassembly during mitotis and under stress conditions. May be involved in the COPI vesicle-dependent recruitment of PNPLA2 to lipid droplets. In neutrophils, involved in G protein-coupled receptor (GPCR)-mediated chemotaxis und superoxide production. Proposed to be recruited by phosphatidylinositol-phosphates generated upon GPCR stimulation to the leading edge where it recruits and activates ARF1, and is involved in recruitment of GIT2 and the NADPH oxidase complex (By similarity). Plays a role in maintaining mitochondrial morphology (By similarity). Inhibited by brefeldin A (BFA) (By similarity). Inhibited by golgicide A (GCA) (PubMed:19182783). Can form homodimers and probably homotetramers (PubMed:17640864). Interacts with COPG1; the interaction is independent on ARF1 activation. Interacts with ARF1, ARF3, ARF4 and ARF5. Interacts with RAB1B (GTP-bound form); required for GBF1 membrane association. Interacts with GGA1, GGA2 and GGA3. Interacts with USO1. Interacts (via SEC7 domain) with PNPLA2 (via C-terminus); the interaction is direct. Can form homodimers and probably homotetramers. Interacts with COPG1; the interaction is independent on ARF1 activation. Interacts with ARF1, ARF3, ARF4 and ARF5. Interacts with RAB1B (GTP-bound form); required for GBF1 membrane association. Interacts with GGA1, GGA2 and GGA3. Interacts with USO1. Interacts (via SEC7 domain) with PNPLA2 (via C-terminus); the interaction is direct. Interacts with ARMH3 (By similarity). Cycles rapidly on and off early Golgi membranes. Stabilized on membranes when complexed with ARF1-GDP and is released from both ARF1 and membranes after it catalyzes GDP displacement and ARF1 binds GTP. Continuous cycles of recruitment and dissociation of GBF1 to membranes are required for sustained ARF activation and COP I recruitment (By similarity). The DCB (dimerization and cyclophiln-binding) and HUS (homology upstream of Sec7) domains are necessary for dimerization. The DCB domain is proposed to support constitutive homodimerization; the HUS domain interacts with the DCB domain which may occur intramolecular or intermolecular.