Activation of Wnt signaling pathways causes release and stabilization of the transcription regulator -catenin from a destruction complex composed of axin and the adenomatous polyposis coli (APC) protein (canonical signaling pathway). Assembly of this complex is facilitated by a protein-protein interaction between APC and an RGS domain in axin. Because GRK2 has a RGS domain that is closely related to the RGS domain in axin, we determined if GRK2 regulated canonical signaling. We found that GRK2 inhibited Wnt1-induced activation of a reporter construct as well as reduced Wnt3a-dependent stabilization and nuclear translocation of -catenin. GRK2 enzymatic activity was required for this negative regulatory effect, and depletion of endogenous GRK2 using siRNA enhanced canonical signaling. GRK2-dependent inhibition of canonical signaling is relevant to osteoblast (OB) biology because overexpression of GRK2 attenuated Wnt/-catenin signaling in calvarial OBs. Co-immunoprecipitation studies found that: 1. GRK2 bound APC, 2. The GRK2-APC interaction was promoted by GRK2 enzymatic activity, and 3. Deletion of the RGS domain in GRK2 prevented both the GRK2-APC interaction and GRK2-dependent inhibition of canonical signaling. These data suggest that: 1. GRK2 negatively regulates Wnt signaling, 2. GRK2-dependent inhibition of canonical signaling requires a protein-protein interaction between the RGS domain in GRK2 and APC, and 3. Enzymatic activity promotes the GRK2-APC interaction and is required for the negative regulatory effect on canonical signaling. We speculate that inhibiting GRK2 activity in bone forming OBs might be a useful therapeutic strategy for increasing bone mass.