Specific interactions between gC1qR and α1-adrenoceptor subtypes

The multi-functional protein gC1qR has been reported to interact with an arginine-rich motif in the C-tail of hamster alpha(1B)-adrenoceptors (ARs), controlling their expression and subcellular localization. Since a similar motif is present in alpha(1D-), but not alpha(1A)-ARs, we studied the specificity of this interaction. Human alpha(1)-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-alpha(1B)- or alpha(1D)-, but not alpha(1A)-ARs, caused commumoprecipitation of HA-gC1qR, while immunoprecipitation of HA-gC1qR caused coimmunoprecipitation of Flag-alpha(1B)- alpha(1D)-, but not alpha(1A)-ARs, supporting specific interactions between subtypes. C-terminal truncation of Flag-alpha(1)-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 alpha(1B)- and alpha(1D)-, but not alpha(1A)-ARs, and this interaction depends on the presence of an intact C-tail.