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Specific Interactions Between gC1qR and α1‐Adrenoceptor Subtypes

Abstract The multi‐functional protein gC1qR has been reported to interact with an arginine‐rich motif in the C‐tail of hamster α1B‐adrenoceptors (ARs), controlling their expression and subcellular localization. Since a similar motif is present in α1D‐, but not α1A‐ARs, we studied the specificity of this interaction. Human α1‐ARs, tagged at their amino termini with Flag epitopes, were coexpressed in HEK293 cells with gC1qR containing a hemaglutinin (HA) tag at its carboxy terminus. Immunoprecipitation studies showed that Flag‐α1B‐ or α1D‐, but not α1A‐ARs, caused coimmunoprecipitation of HA‐gC1qR, while immunoprecipitation of HA‐gC1qR caused coimmunoprecipitation of Flag‐α1B‐ or α1D‐, but not α1A‐ARs, supporting specific interactions between subtypes. C‐terminal truncation of Flag‐α1‐ARs prevented interaction with HA‐gC1qR, supporting previous conclusions about the role of the C‐terminal arginine‐rich motif. These studies suggest that gC1qR interacts specifically with α1B‐ and α1D‐, but not α1A‐ARs, and this interaction depends on the presence of an intact C‐tail.