Association of stomatin with lipid-protein complexes in the plasma membrane and the endocytic compartment

The 31 kDa integral membrane protein stomatin (protein 7.2b) has a monotopi c structure and a cytofacial orientation, We have shown previously that sto matin is located in plasma membrane protruding structures and forms high-or der homooligomers in the human epithelial cell line UAC, suggesting that th is protein has a structural function in the cortical morphogenesis of the c ells, It is also present in a pool of juxtanuclear vesicles. In this study, we show that stomatin colocalizes with the GPI-anchored proteins placental alkaline phosphatase (PLAP) and membrane folate receptor alpha (MFR alpha) endogenously expressed in UAC cells, This observation enabled us to demons trate two different aspects of stomatin, First, using anti-FLAP antibody in ternalization, we show that the peri-centrosomal vesicles containing stomat in correspond to a subset of endosomes, which can also be labeled with the late endosomal/lysosomal marker LAMP-2, Secondly, we found that stomatin is partially present in detergentin-soluble membrane domains and co-patches w ith FLAP on the plasma membrane, after cross-linking of FLAP by antibodies. These data indicate that stomatin and GPI-anchored proteins are linked thr ough lipid rafts and undergo the same sorting events. We propose that stoma tin, through its affinity for lipid rafts, functions in concentrating GPI-a nchored proteins in membrane microvillar structures. Consistent with this h ypothesis, we found that stomatin is expressed exclusively in microvilli of the apical membrane in polarized Madin-Darby canine kidney (MDCK) cells.