TRANSPORT OF VESICULAR STOMATITIS-VIRUS G-PROTEIN TO THE CELL-SURFACEIS SIGNAL MEDIATED IN POLARIZED AND NONPOLARIZED CELLS

Current models propose that in nonpolarized cells, transport of plasma membrane proteins to the surface occurs by default. In contrast, comp elling evidence indicates that in polarized epithelial cells, plasma m embrane proteins are sorted in the TGN into at least two vectorial rou tes to apical and basolateral surface domains. Since both apical and b asolateral proteins are also normally expressed by both polarized and nonpolarized cells, we explored here whether recently described basola teral sorting signals in the cytoplasmic domain of basolateral protein s are recognized and used for post-TGN transport by nonpolarized cells . To this end, we compared the inhibitory effect of basolateral signal peptides on the cytosol-stimulated release of two basolateral and one apical marker in semi-intact fibroblasts (3T3), pituitary (GH3), and epithelial (MDCK) cells. A basolateral signal peptide (VSVGp) correspo nding to the 29-amino acid cytoplasmic tail of vesicular stomatitis vi rus G protein (VSVG) inhibited with identical potency the vesicular re lease of VSVG from the TGN of all three cell lines. On the other hand, the VSVG peptide did not inhibit the vesicular release of HA in MDCK cells nor of two polypeptide hormones (growth hormone and prolactin) i n GH3 cells, whereas in 3T3 cells (influenza) hemagglutinin was inhibi ted, albeit with a 3x lower potency than VSVG. The results support the existence of a basolateral-like, signal-mediated constitutive pathway from TGN to plasma membrane in all three cell types, and suggest that an apical-like pathway may be present in fibroblasts. The data suppor t cargo protein involvement, not bulk flow, in the formation of post-T GN vesicles and predict the involvement of distinct cytosolic factors in the assembly of apical and basolateral transport vesicles.