Specifically phosphorylates the activated forms of G protein- coupled receptors (PubMed:19423652, PubMed:20048153). Such receptor phosphorylation initiates beta-arrestin-mediated receptor internalization, and signaling events leading to their desensitization (PubMed:19423652, PubMed:20048153). Seems to be involved in the desensitization of D2-like dopamine receptors in striatum and chemokine receptor CXCR4 which is critical for CXCL12-induced cell chemotaxis (PubMed:20048153). Phosphorylates rhodopsin (RHO) (in vitro) and a non G protein-coupled receptor: LRP6 during Wnt signaling (in vitro) (PubMed:19801552). {ECO:0000269|PubMed:19423652, ECO:0000269|PubMed:19801552, ECO:0000269|PubMed:20048153}. This is the function of GRK6 (G protein-coupled receptor kinase 6, Ensembl gene identifier ENSG00000198055).