Minireview: Differential Targeting and Retention of G Protein-Coupled Receptors in Polarized Epithelial Cells

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Abstract
Localization of receptors in discrete cellular microdomains undoubtedly contributes to their interaction with particular effectors and receptor targets. For G protein-coupled receptors, virtually nothing is known about the mechanisms and structural features responsible for their targeting to and retention in varying surface domains. We have shown that the Gi/ Go-coupled alpha 2A-adrenergic receptor (alpha 2AAR) is directly targeted to the lateral subdomain of MDCK II cells. Mutational analysis has revealed that regions in or near the bilayer are likely critical for alpha 2AAR targeting, whereas endofacial domains contribute to alpha 2AAR retention on the lateral surface. Although the alpha 2BAR also is enriched on the lateral subdomain at steady-state, its polarization occurs after initial random delivery to both apical and basolateral surfaces followed by a selective accumulation on the lateral subdomain. The alpha 2CAR also is expressed on the lateral subdomain and achieves its localization via direct delivery to the basolateral surface; however, the alpha 2CAR also exists in an as yet not fully characterized intracellular compartment. Interestingly, another Gi/Go-coupled receptor, the A1 adenosine receptor, is enriched on the apical surface of MDCK II calls and achieves this localization by direct apical delivery. These findings indicate that receptor delivery to polarized surfaces is not determined by receptor coupling to a specific subpopulation of G proteins.