SORTING AND INTRACELLULAR TRAFFICKING OF A GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEIN AND 2 HYBRID TRANSMEMBRANE PROTEINS WITH THE SAME ECTODOMAIN IN MADIN-DARBY CANINE KIDNEY EPITHELIAL-CELLS

We compared the trafficking of the glycosylphosphatidylinositol (GPI)- anchored placental alkaline phosphatase (FLAP) and two chimeric transm embrane proteins containing the FLAP ectodomain in stably transfected Madin-Darby canine kidney epithelial cells to determine whether differ ent mechanisms might be used in apical sorting of GPI-anchored and tra nsmembrane proteins. PLAP-G, which contained the transmembrane and cyt oplasmic domains of the vesicular stomatitis virus glycoprotein, was d elivered directly to the basolateral surface. FLAP-HA contained the tr ansmembrane and cytoplasmic domains of influenza hemagglutinin. Both F LAP and FLAP-HA were delivered directly to the apical membrane. FLAP b ecomes insoluble in Triton X-100 during biosynthetic transport, as it associates with detergent-resistant membranes. Neither hybrid protein was detergent insoluble, though the small amount of FLAP that was miss orted to the basolateral surface was insoluble. We examined the effect s of three drugs known to interfere with membrane trafficking on sorti ng and delivery of FLAP and the hybrid proteins. Monensin had no effec t on sorting or surface expression of any of the proteins. Nocodazole affected the sorting of both FLAP and FLAP-HA but not of PLAP-G. Brefe ldin A appeared to disrupt the sorting of FLAP and FLAP-HA but not of PLAP-G. This conclusion was tempered by the observation that this drug affected the distribution of proteins at the cell surface. Thus, sort ing and transport of GPI-anchored and apical transmembrane proteins ar e similar in a number of respects.