A PROTEIN TARGETING SIGNAL THAT FUNCTIONS IN POLARIZED EPITHELIAL-CELLS IN-VIVO

Eukaryotic membrane-associated polypeptides often contain a glycosylph osphatidylinositol (GPI) anchor that signals the attachment of GPI lip ids to these proteins. The GPI anchor can function as a basolateral or apical targeting signal in mammalian cells cultured in vitro, althoug h the function of the GPI anchor in vivo remains to be elucidated. In this study we have evaluated the effect of fusing a GPI anchor sequenc e to a prokaryotic reporter protein on the cellular location of the po lypeptide in polarized epithelial cells of transgenic mice. The bacter ial enzyme, when fused to a eukaryotic signal peptide, was secreted th rough the basolateral membrane of small-intestinal enterocytes; howeve r, when the enzyme was linked to the GPI anchor sequence the polypepti de was redirected to the apical surface of the epithelial cells. These data provide the first direct evidence that the GPI anchor functions as an apical membrane protein sorting signal in polarized epithelial c ells in vivo.