v-SNARE cellubrevin is required for basolateral sorting of AP-1 B-dependent cargo in polarized epithelial cells

The epithelial cell-specific adaptor complex AP-1B is crucial for correct delivery of many transmembrane proteins from recycling endosomes to the baso-lateral plasma membrane. Subsequently, membrane fusion is dependent on the formation of complexes between SNARE proteins located at the target membrane and on transport vesicles. Although the t-SNARE syntaxin A has been localized to the basolateral membrane, the v-SNARE operative in the AP-1B pathway remained unknown. We show that the ubiquitously expressed v-SNARE cellubrevin localizes to the basolateral membrane and to recycling endosomes, where it colocalizes with AP-1B. Furthermore, we demonstrate that cellubrevin coimmunoprecipitates preferentially with syntaxin 4, implicating this v-SNARE in basolateral fusion events. Cleavage of cellubrevin with tetanus neurotoxin (TeNT) results in scattering of AP-1B localization and missorting of AP-1B-dependent cargos, such as transferrin receptor and a truncated low-density lipoprotein receptor, LDLR-CT27. These data suggest that cellubrevin and AP-1B cooperate in basolateral membrane trafficking.