Abstract

Axin, a negative regulator of the Wnt signaling pathway, contains a canonical regulator of G protein signaling (RGS) core domain. Herein, we demonstrate both in vitro and in cells that this domain interacts with the α subunit of the heterotrimeric G protein G₁₂ but not with the closely related Gα₁₃ or with several other heterotrimeric G proteins. Axin preferentially binds the activated form of Gα₁₂, a behavior consistent with other RGS proteins. However, unlike other RGS proteins, that of axin (axinRGS) does not affect intrinsic GTP hydrolysis by Gα₁₂. Despite its inability to act as a GTPase-activating protein, we demonstrate that in cells, axinRGS can compete for Gα₁₂ binding with the RGS domain of p115RhoGEF, a known G₁₂-interacting protein that links G₁₂ signaling to activation of the small G protein Rho. Moreover, ectopic expression of axinRGS specifically inhibits Gα₁₂-directed activation of the Rho pathway in MDA-MB 231 breast cancer cells. These findings establish that the RGS domain of axin is able to directly interact with the α subunit of heterotrimeric G protein G₁₂ and provide a unique tool to interdict Gα₁₂-mediated signaling processes.