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

The alpha(2A)-adrenergic receptor (alpha(2A)AR) is polarized to the basolateral membrane of Madin-Darby canine kidney cells via direct targeting. Examination of mutant <alpha >(2A)AR reveals that direct delivery is independent of NH,-terminal glycosylation, COOH-terminal acylation, or protein sequences within the large third cytoplasmic loop or COOH-terminal tail. Combined mutation of these structural features also does not perturb alpha(2A)AR delivery, suggesting that a three-dimensional structure imparted by non-contiguous endofacial sequences does not confer alpha(2A)AR targeting and that motifs in or near the bilayer must be involved in targeting of the alpha(2A)AR. Mutation of a conserved Asp residue in transmembrane two that alters receptor-G-protein interactions also does not impair alpha(2A)AR targeting. Finally, modification of sequences in transmembrane seven that resemble tyrosine-containing 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 altering the ability of the alpha(2A)AR to couple to G-proteins. These data suggest that although targeting of the alpha(2A)AR likely involves bilayer sequences, the third cytoplasmic loop may contain structural features that promote stabilization of the alpha(2A)AR on the basolateral surface of Madin-Darby canine kidney cells.