A 56-kDa selenium-binding protein participates in intra-Golgi protein transport

Transport of proteins between intracellular membrane compartments is a high ly regulated process that depends on several cytosolic factors. By using th e well characterized intra-Golgi cell-free transport assay, we purified fro m bovine brain cytosol a 56-kDa protein that shows a significant transport activity. Partial sequencing of four tryptic peptides obtained from the 56- kDa protein revealed its identity to a cytosolic protein previously charact erized as a selenium-binding protein, SBP56. Recombinant SBP56 expressed in Escherichia coli exhibited transport activity when added to the cell-free intra-Golgi transport. Affinity purified anti-SBP56 polyclonal antibodies s pecifically inhibited intra-Golgi transport in vitro. Although SBP56 is pre dominantly localized in the cytosol, a significant amount is associated wit h membranes. Subcellular fractionation showed that this protein is peripher ally associated with the Golgi membrane. The experiments presented in this study indicate that SBP56 participates in late stages of intra-Golgi protei n transport.