Structure of the class D GPCR Ste2 dimer coupled to two G proteins

G protein-coupled receptors (GPCRs) are divided phylogenetically into six classes, A-F(1,2). Over 370 structures of vertebrate GPCRs (classes A, B, C and F) have been determined, leading to a substantial understanding of their function(3). In contrast, there are no structures of Class D GPCRs, which are found exclusively in fungi where they regulate survival and reproduction. We have determined the first structure of a family D GPCR, the Saccharomyces cerevisiae pheromone receptor Ste2, in an active state coupled to the heterotrimeric G protein Gpa1-Ste4-Ste18. Ste2 was purified as a homodimer that coupled to two G proteins. The dimer interface of Ste2 is formed by the N-terminus, transmembrane helices H1, H2, H7 and the first extracellular loop ECL1. We established a Class D1 generic residue numbering system (CD1) to enable comparisons with orthologues and other GPCR classes. The structure of Ste2 bears similarities in overall topology to Class A GPCRs, but H4 is shifted by over 20 Å and the G protein binding site is a shallow groove rather than a cleft. The structure provides a template for the design of novel drugs targeting fungal GPCRs that could be utilised to treat a number of intractable fungal diseases(4).