Heterotrimeric G Protein-independent Signaling of a G Protein-coupled Receptor: DIRECT BINDING OF ARNO/CYTOHESIN-2 TO THE CARBOXYL TERMINUS OF THE A sub(2A) ADENOSINE RECEPTOR IS NECESSARY FOR SUSTAINED ACTIVATION OF THE ERK/MAP KINASE PATHWAY

The A sub(2A) adenosine receptor is a prototypical G sub(s)-coupled receptor, but it also signals, e.g. to mitogen-activated protein (MAP) kinase, via a pathway that is independent of heterotrimeric G proteins. Truncation of the carboxyl terminus affects the strength of the signal through these alternative pathways. In a yeast two-hybrid interaction hunt, we screened a human brain library for proteins that bound to the juxtamembrane portion of the carboxyl terminus of the A sub(2A) receptor. This approach identified ARNO/cytohesin-2, a nucleotide exchange factor for the small (monomeric) G proteins of the Arf (ADP-ribosylation factor) family, as a potential interaction partner. We confirmed a direct interaction by mutual pull down (of fusion proteins expressed in bacteria) and by immunoprecipitation of the proteins expressed in mammalian cells. To circumvent the long term toxicity associated with overexpression of ARNO/cytohesin-2, we created stable cell lines that stably expressed the A sub(2A) receptor and where ARNO/cytohesin-2 or the dominant negative version E156K-ARNO/cytohesin-2 was inducible by mifepristone. Cyclic AMP accumulation induced by an A sub(2A)-specific agonist was neither altered by ARNO/cytohesin-2 nor by the dominant negative version. This was also true for agonist-induced desensitization. In contrast, expression of dominant negative E156K-ARNO/cytohesin-2 and of dominant negative T27N-Arf6 abrogated the sustained phase of MAP kinase stimulation induced by the A sub(2A) receptor. We therefore conclude that ARNO/cytohesin-2 is required to support the alternative, heterotrimeric G protein-independent, signaling pathway of A sub(2A) receptor, which is stimulation of MAP kinase.