Atypical activation of the G protein Gα by the oncogenic mutation Q209P.

The causative role of G protein-coupled receptor (GPCR) pathway mutations in uveal melanoma (UM) has been well-established. Nearly all UMs bear an activating mutation in a GPCR pathway mediated by G proteins of the G family, driving tumor initiation and possibly metastatic progression. Thus, targeting this pathway holds therapeutic promise for managing UM. However, direct targeting of oncogenic Gα mutants, present in ∼90% of UMs, is complicated by the belief that these mutants structurally resemble active Gα WT. This notion is solidly founded on previous studies characterizing Gα mutants in which a conserved catalytic glutamine (Gln-209 in Gα) is replaced by leucine, which leads to GTPase function deficiency and constitutive activation. Whereas Q209L accounts for approximately half of GNAQ mutations in UM, Q209P is as frequent as Q209L and also promotes oncogenesis, but has not been characterized at the molecular level. Here, we characterized the biochemical and signaling properties of Gα Q209P and found that it is also GTPase-deficient and activates downstream signaling as efficiently as Gα Q209L. However, Gα Q209P had distinct molecular and functional features, including in the switch II region of Gα Q209P, which adopted a conformation different from that of Gα Q209L or active WT Gα, resulting in altered binding to effectors, Gβγ, and regulators of G-protein signaling (RGS) proteins. Our findings reveal that the molecular properties of Gα Q209P are fundamentally different from those in other active Gα proteins and could be leveraged as a specific vulnerability for the ∼20% of UMs bearing this mutation.