Protein trafficking to the membrane requires formation of coated carrier vesicles, such as COPI-coated vesicles from the cis-golgi, a process triggered by membrane binding of the GTP-bound form of ADP-ribosylation factors. Normally, brefeldin A (BFA) blocks this action by inhibiting guanine nucleotide exchange factors (GEFs) for ADP-ribosylation factor. However, GBF1, a member of the sec7-domain family of GEFs, allows cells to maintain normal golgi morphology and grow in the presence of BFA. The gene encoding the human GBF1 protein maps to chromosome 10q24, with the sec7-domain centrally positioned, and a proline-rich C-terminus. Based on mutatagenesis analysis, this proline rich region appears to interact with p115 in a functionally significant manner.