Accumulation of Hsp70/Hsc70 molecular chaperone regulator BAG-1 on COPI-coated structures in gastric epithelial cells

The multifunctional protein BAG-1 binds to molecular chaperones Hsc70/Hsp70 and regulates their activity. While a role for BAG-1 in regulating Hsc70 chaperone activity is firmly established in vitro, physiological roles of this regulation remain obscure. Here we show that BAG-1 is highly expressed in gastrointestinal epithelial cells and accumulates at the Golgi apparatus, probably through interaction with COPI coated structure. Subcellular fractionation of gastric mucosa revealed that BAG-1 was mainly recovered in the Golgi-enriched fractions. Confocal immunofluorescence studies revealed that BAG-1 was overlapped with a cis-Golgi matrix protein GM130 and a COPI component beta-COP in the vicinity of the Golgi. In response to brefeldin A, which blocks recruitment of COPI coats to the Golgi membrane, BAG-1 was dispersed throughout the cytoplasm as beta-COP was. Extensive overlap of BAG-1 immunofluorescence with beta-COP was still observed when cells were treated with nocodazole, which depolymerizes microtubules and induces fragmentation of the Golgi stacks. Immunoelectron microscopy revealed that BAG-1 signals were predominantly found on vesicular membrane structures adjacent to Golgi stacks, but not on mitochondrial membrane. Taken together, we suggest that BAG-1 is targeted to the COPI coated structures, implying its contribution toward the COPI vesicular transport in gastrointestinal epithelial cells.