A human cDNA encoding an 841-aa guanine nucleotide-exchange protein (GEP) for ADP-ribosylation factors (ARFs), named ARF-GEP(100), which contains a Sec7 domain, a pleckstrin homology (PH)-like domain, and an incomplete IQ-motif, was identified. On Northern blot analysis of human tissues, a approximately 8-kb mRNA that hybridized with an ARF-GEP(100) cDNA was abundant in peripheral blood leukocytes, brain, and spleen. ARF-GEP(100) accelerated [(35)S]GTPgammaS binding to ARF1 (class I) and ARF5 (class II) 2- to 3-fold, and to ARF6 (class III) ca. 12-fold. The ARF-GEP(100) Sec7 domain contains Asp(543) and Met(555), corresponding to residues associated with sensitivity to the inhibitory effect of the fungal metabolite brefeldin A (BFA) in yeast Sec7, but also Phe(535) and Ala(536), associated with BFA-insensitivity. The PH-like domain differs greatly from those of other ARF GEPs in regions involved in phospholipid binding. Consistent with its structure, ARF-GEP(100) activity was not affected by BFA or phospholipids. After subcellular fractionation of cultured T98G human glioblastoma cells, ARF6 was almost entirely in the crude membrane fraction, whereas ARF-GEP(100), a 100-kDa protein detected with antipeptide antibodies, was cytosolic. On immunofluorescence microscopy, both proteins had a punctate pattern of distribution throughout the cells, with apparent colocalization only in peripheral areas. The coarse punctate distribution of EEA-1 in regions nearer the nucleus appeared to coincide with that of ARF-GEP(100) in those areas. No similar coincidence of ARF-GEP(100) with AP-1, AP-2, catenin, LAMP-1, or 58K was observed. The new human BFA-insensitive GEP may function with ARF6 in specific endocytic processes.