Interaction of neuronal nitric oxide synthase with alpha1-adrenergic receptor subtypes in transfected HEK-293 cells

The C-terminal four amino acids (GEEV) of human alpha1A-adrenergic receptors (ARs) have been reported to interact with the PDZ domain of neuronal nitric oxide synthase (nNOS) in a yeast two-hybrid system. The other two alpha1-AR subtypes have no sequence homology in this region, raising the possibility of subtype-specific protein-protein interactions.

We used co-immunoprecipitation and functional approaches with epitope-tagged alpha1-ARs to examine this interaction and the importance of the C-terminal tail. Following co-transfection of HEK-293 cells with hexahistidine/Flag (HF)-tagged alpha1A-ARs and nNOS, membranes were solubilized and immunoprecipitated with anti-FLAG affinity resin or anti-nNOS antibodies. Immunoprecipitation of HFalpha1A-ARs resulted in co-immunoprecipitation of nNOS and vice versa, confirming that these proteins interact. However, nNOS also co-immunoprecipitated with HFalpha1B- and HFalpha1D-ARs, suggesting that the interaction is not specific to the alpha1A subtype. In addition, nNOS co-immunoprecipitated with each of the three HFalpha1-AR subtypes which had been C-terminally truncated, suggesting that this interaction does not require the C-tails; and with Flag-tagged beta1- and beta2-ARs. Treatment of PC12 cells expressing HFalpha1A-ARs with an inhibitor of nitric oxide formation did not alter norepinephrine-mediated activation of mitogen activated protein kinases, suggesting nNOS is not involved in this response.

These results show that nNOS does interact with full-length alpha1A-ARs, but that this interaction is not subtype-specific and does not require the C-terminal tail, raising questions about its functional significance.