GBF1, a Guanine Nucleotide Exchange Factor for ADP‐Ribosylation Factors, is Localized to the cis‐Golgi and Involved in Membrane Association of the COPI Coat

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Abstract
Formation of coated carrier vesicles, such as COPI‐coated vesicles from the cis‐Golgi, is triggered by membrane binding of the GTP‐bound form of ADP‐ribosylation factors. This process is blocked by brefeldin A, which is an inhibitor of guanine nucleotide exchange factors for ADP‐ribosylation factor. GBF1 is one of the guanine nucleotide‐exchange factors for ADP‐ribosylation factor and is localized in the Golgi region. In the present study, we have determined the detailed subcellular localization of GBF1. Immunofluorescence microscopy of cells treated with nocodazole or incubated at 15 °C has suggested that GBF1 behaves similarly to proteins recycling between the cis‐Golgi and the endoplasmic reticulum. Immunoelectron microscopy has revealed that GBF1 localizes primarily to vesicular and tubular structures apposed to the cis‐face of Golgi stacks and minor fractions to the Golgi stacks. GBF1 overexpressed in cells causes recruitment of class I and class II ADP‐ribosylation factors onto Golgi membranes. Furthermore, overexpressed GBF1 antagonizes various effects of brefeldin A, such as inhibition of membrane recruitment of ADP‐ribosylation factors and the COPI coat, and redistribution of Golgi‐resident and itinerant proteins. These observations indicate that GBF1 is involved in the formation of COPI‐coated vesicles from the cis‐Golgi or the pre‐Golgi intermediate compartment through activating ADP‐ribosylation factors.