Specific interactions between gC1qR and alpha1-adrenoceptor subtypes.

The multi-functional protein gC1qR has been reported to interact with an arginine-rich motif in the C-tail of hamster alpha1B-adrenoceptors (ARs), controlling their expression and subcellular localization. Since a similar motif is present in alpha1D-, but not alpha1A-ARs, we studied the specificity of this interaction. Human alpha1-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-alpha1B- or alpha1D-, but not alpha1A-ARs, caused coimmunoprecipitation of HA-gC1qR, while immunoprecipitation of HA-gC1qR caused coimmunoprecipitation of Flag-alpha1B- or alpha1D-, but not alpha1A-ARs, supporting specific interactions between subtypes. C-terminal truncation of Flag-alpha1-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 alpha1B- and alpha1D-, but not alpha1A-ARs, and this interaction depends on the presence of an intact C-tail.