The Regulator of G Protein Signaling Domain of Axin Selectively Interacts with G 12 but Not G 13

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 alpha subunit of the heterotrimeric G protein G(12) but not with the closely related G alpha(13) or with several other heterotrimeric G proteins. Axin preferentially binds the activated form of G alpha(12), a behavior consistent with other RGS proteins. However, unlike other RGS proteins, that of axin (axinRGS) does not affect intrinsic GTP hydrolysis by G alpha(12). Despite its inability to act as a GTPase-activating protein, we demonstrate that in cells, axinRGS can compete for G alpha(12) binding with the RGS domain of p115RhoGEF, a known G(12)-interacting protein that links G(12) signaling to activation of the small G protein Rho. Moreover, ectopic expression of axinRGS specifically inhibits G alpha(12)-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 alpha subunit of heterotrimeric G protein G(12) and provide a unique tool to interdict G alpha(12)-mediated signaling processes.