The regulation of endocytic traffic by the exocyst complex in polarized epithelial cells

GALLO LI; APODACA G

Pittsburgh

Congreso; Local Traffic Symposium; 2010

Carnegie Mellon University

The recycling of proteins back to the plasma membrane is essential for maintaining the proper composition of the apical and basolateral domains in the cell surface. Defects in the polarized trafficking of proteins, such as the epithelial sodium channel (ENaC) or aquaporin-2 (AQ2), can cause various kidney-associated diseases, such as hypertension, and polycystic kidney disease. The understanding of the protein machinery involved in regulating endocytic and recycling pathways may help explain these defects. The exocyst is an evolutionarily conserved tethering complex, comprised of eight subunits: Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70 and Exo84. This complex is known to regulate apical and basal recycling pathways, as well as basolateral-to-apical transcytosis. Whether there is one octomeric holocomplex or different subcomplexes is not completely understood. I hypothesize that the two isoforms, Sec15A and Sec15B, form distinct exocyst complexes that regulate different endocytic trafficking pathways by interaction with different Rab11 isoforms in polarized epithelial cells. My studies in filter-grown polarized Madin-Darby canine kidney (MDCK) cells show that Sec15A or Sec15B colocalizes with Sec8 but they partially overlap with each other, as assessed by confocal laser microscopy. These results are consistent with the presence of different Sec15A and Sec15B complexes. Moreover, Sec15B colocalizes with Rab11b, a member of Rab11 subfamily of GTPases, which regulates recycling pathways. Further, in polarized mouse cortical collecting duct (mCCD) cells which express ENaC endogenously, Sec15B colocalizes with ENaC. These results indicate that Sec15B is likely to be involved in the apical recycling of ENaC, a Rab11b- dependent pathway.