G protein-coupled receptors (GPCRs) are involved in the response of eukaryotic cells to a wide variety of stimuli, traditionally mediating their effects through heterotrimeric G proteins comprised of G alpha, G beta and G gamma subunits. The fission yeast Schizosaccharomyces pombe is an established tool for GPCR research, possessing two G alpha-dependent signalling cascades. A complete G alpha beta gamma complex has been characterised for the glucose-sensing pathway, but only the G alpha subunit, Gpa1p, has been identified in the pheromone-response pathway. Here, we report the use of the yeast two-hybrid system to identify a novel protein, Gnr1p, which interacts with Gpa1p. Gnr1p is predicted to contain seven WD repeats and to adopt a structure similar to typical G beta subunits. Disruption and overexpression studies reveal that Gnr1p negatively regulates the pheromone-response pathway but is not required for signalling. Human G beta subunits complement the loss of Gnr1p, functioning as negative regulators of G alpha signalling in fission yeast.
