Sec12p is a type II membrane glycoprotein in the endoplasmic reticulum (ER) of Saccharomyces cerevisiae which is essential for transport vesicle budding. It is the guanine nucleotide exchange factor for the small GTP-binding protein Sar1p which is a constituent of COP II ER to Golgi vesicles. We report the sequence and localization of the human homologue to yeast Rer1p, which has recently been identified genetically as an essential component for retention of Sec12p in the ER. Reverse polymerase chain reaction was used to obtain cDNAs from HeLa cells. They code for a protein of 196 amino acids, corresponding to a molecular mass of 23 kDa. The translated sequence is 44% identical and 65% similar to yeast Rer1 protein. The four putative transmembrane domains are predicted to form a W-topology with both N- and C-terminus facing the cytosol. The functional activity of myc-tagged human Rer1 was demonstrated by the complementation of the RER1 deletion in S. cerevisiae. Mislocalization of the Sec12-reporter protein was reduced similar to the results obtained with yeast Rer1p. Human Rer1 protein was expressed in HeLa cells and the subcellular distribution analyzed by double immunofluorescence and immunoelectron microscopy of thawed cryosections. The tagged protein was localized to the Golgi apparatus and peripheral elements of the ER-Golgi interface. High overexpression leads to relocation of human Rer1 to ER-like structures together with KDEL-receptor and affects the structural organization of the Golgi apparatus. Under conditions of brefeldin A treatment, human Rer1 distributes together with recycling Golgi proteins.