Regulates G protein-coupled receptor signaling cascades. Inhibits signal transduction by increasing the GTPase activity of G protein alpha subunits, thereby driving them into their inactive GDP- bound form (PubMed:11602604, PubMed:18434541). Plays an important role in the phototransduction cascade by regulating the lifetime and effective concentration of activated transducin alpha. May regulate extra and intracellular mitogenic signals (By similarity). {ECO:0000250|UniProtKB:P97428, ECO:0000269|PubMed:11602604, ECO:0000269|PubMed:18434541}. This is the function of RGS16 (regulator of G protein signaling 16, Ensembl gene identifier ENSG00000143333).