EXPORT OF CELLUBREVIN FROM THE ENDOPLASMIC-RETICULUM IS CONTROLLED BYBAP31

Cellubrevin is a ubiquitously expressed membrane protein that is local ized to endosomes throughout the endocytotic pathway and functions in constitutive exocytosis, We report that cellubrevin binds with high sp ecificity to BAP31, a representative of a highly conserved family of i ntegral membrane proteins that has recently been discovered to be bind ing proteins of membrane immunoglobulins. The interaction between BAP3 1 and cellubrevin is sensitive to high ionic strength and appears to r equire the transmembrane regions of both proteins, No other proteins o f liver membrane extracts copurified with BAP31 on immobilized recombi nant cellubrevin, demonstrating that the interaction is specific, Syna ptobrevin I bound to BAP31 with comparable affinity, whereas only weak binding was detectable with synaptobrevin II, Furthermore, a fraction of BAP31 and cellubrevin was complexed when each of them was quantita tively immunoprecipitated from detergent extracts of fibroblasts (BHK 21 cells), During purification of clathrin-coated vesicles or early en dosomes, BAP31 did not cofractionate with cellubrevin. Rather, the pro tein was enriched in ER-containing fractions., When BHK cells were ana lyzed by immunocytochemistry, BAP31 did not overlap with cellubrevin, but rather colocalized with resident proteins of the ER, In addition, immunoreactive vesicles were clustered in a paranuclear region close t o the microtubule organizing center, but different from the Golgi appa ratus, When microtubules were depolymerized with nocodazole, this accu mulation disappeared and BAP31 was confined to the ER. Truncation of t he cytoplasmic tail of BAP31 prevented export of cellubrevin, but not of the transferrin receptor from the ER, We conclude that BAP31 repres ents a novel class of sorting proteins that controls anterograde trans port of certain membrane proteins from the ER to the Golgi complex.