UNIQUE STRUCTURAL FEATURES IMPORTANT FOR STABILIZATION VERSUS POLARIZATION OF THE ALPHA(2A)-ADRENERGIC RECEPTOR ON THE BASOLATERAL MEMBRANEOF MADIN-DARBY CANINE KIDNEY-CELLS

The alpha(2A)-adrenergic receptor (alpha(2A)AR) is polarized to the ba solateral membrane of Madin-Darby canine kidney cells via direct targe ting. Examination of mutant <alpha >(2A)AR reveals that direct deliver y is independent of NH,-terminal glycosylation, COOH-terminal acylatio n, or protein sequences within the large third cytoplasmic loop or COO H-terminal tail. Combined mutation of these structural features also d oes not perturb alpha(2A)AR delivery, suggesting that a three-dimensio nal structure imparted by non-contiguous endofacial sequences does not confer alpha(2A)AR targeting and that motifs in or near the bilayer m ust be involved in targeting of the alpha(2A)AR. Mutation of a conserv ed Asp residue in transmembrane two that alters receptor-G-protein int eractions also does not impair alpha(2A)AR targeting. Finally, modific ation of sequences in transmembrane seven that resemble tyrosine-conta ining endocytosis motifs utilized for targeting by some proteins does not perturb alpha(2A)AR sorting. Interestingly, deletion of the large third cytoplasmic loop of the alpha(2A)AR decreases receptor half-life on the basolateral surface from approximately 11 to 4.5 h without alt ering the ability of the alpha(2A)AR to couple to G-proteins. These da ta suggest that although targeting of the alpha(2A)AR likely involves bilayer sequences, the third cytoplasmic loop may contain structural f eatures that promote stabilization of the alpha(2A)AR on the basolater al surface of Madin-Darby canine kidney cells.