The Role Of Gpcr Oligomerization In Receptor Trafficking To The Cell Surface

The dominant negative effect of receptor mutants on the ability of the wildtype receptor to traffic to the cell surface when co-expressed indicates that a GPCR oligomer must be in an appropriate conformation within the ER to permit cell-surface expression. For other membrane-spanning proteins such as the potassium ion channels, oligomerization is well-established to be important for proper cell-surface expression (57). In this case, correct oligomeriza-tion of ion channel subunits results in the formation of a fully functional ion channel within the ER that is trafficked to the cell surface. Proper oligomeric formations may facilitate cell-surface expression by masking ER retention signals (such as the RXR motif [58]) and exposing export motifs (such as the DXE motif [59]) found on ion channel subunits. These retention and retrieval motifs may also play a role in the alignment of GPCR oligomers, because many of these motifs are found within the primary sequences of several GPCRs.

3.1. Y-Aminobutyric Acid BR1-y-Aminobutyric Acid BR2 Hetero-Oligomerization

The Y-aminobutyric acid B (GABAb) receptors provide a significant example of the importance of GPCR oligomerization in receptor trafficking, as illustrated by the interaction between GABABR1 and GABABR2, which share a 35% overall amino acid homology, that results in the formation of a fully functional GABAB receptor (60-64). When individually expressed, GABABR1 is mostly intracellularly localized within the ER (65), whereas the GABABR2 is expressed at the cell surface and within the cytoplasm

(62.66). Co-expression of both receptors in heterologous cell lines forms GABABR1-GABABR2 hetero-oligomers, which display functional characteristics similar to endogenous GABAB receptors (60-62,64), and increases GABABR1 cell-surface expression (61). Interactions between the coiled-coil domains within the carboxyl termini of the GABABR1 and GABABR2

(60.67) and their transmembrane domains (68,69) participate in hetero-oli-gomer formation (60-64). The coiled-coil domain also masks the ER retention motif RXR on the carboxyl terminus of GABABR1, allowing the receptor to exit the ER to be further processed within the Golgi into a mature, glycosylated receptor before cell-surface localization (61-63). The resulting glycosylation of the GABABR1 becomes resistant to the activity of the enzyme endoglycosidase H (Endo H), which specifically cleaves glycoproteins that have high-mannose oligosaccharides attached at their N-linked glycosylation sites—a characteristic of glycoproteins that are intracellularly retained in the ER (61-63).

3.2. Hetero-Oligomerization Between <x1-AR Subtypes

A further example demonstrating the importance of GPCR oligomerization in receptor cell-surface trafficking is the interaction of the a1B-AR with the a1A- or a1D-ARs, forming a1A-a1B and a1B-a1D hetero-oligomers, respectively (70). Expressed alone, a1A-AR and a1D-AR (71,72) are poorly expressed at the cell surface, whereas the a1B-AR is predominantly expressed at the cell surface (73). The formation of hetero-oligomers resulted in no change in the pharmacology of either receptor but increased cell-surface expression of the a1A- and a1D-ARs, compared to the a1A- and a1D-ARs expressed alone (70). These observations suggest that the a1B-AR facilitates transport of a1A- and a1D-ARs to the cell surface (70). Physical interactions forming hetero-oligomers are not mediated by interactions between the carboxyl or amino terminus of the receptors and are specific, because co-expression of a1A- and a1D-ARs did not lead to formation of hetero-oligomers or change in cell-surface expression of either receptor.

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