Inhibits signal transduction by increasing the GTPase activity of G protein alpha subunits thereby driving them into their inactive GDP-bound form. Binds selectively to G(z)-alpha and G(alpha)-i2 subunits, accelerates their GTPase activity and regulates their signaling activities. The G(z)-alpha activity is inhibited by the phosphorylation and palmitoylation of the G-protein. Negatively regulates mu-opioid receptor-mediated activation of the G-proteins (By similarity). Forms a complex with G(alpha)z/i2 subunits and mu-opioid receptors; the formation of this complex results in mu-opioid receptor desensitization. Interacts with OPRM1 (By similarity). Shuttles between the cytoplasm/cell membrane and the nucleus. Anchored to the membrane through palmitoylation. Isoform 5 is expressed in brain at high levels in the caudate nucleus and temporal lobe. Fatty acylated. Heavily palmitoylated in the cysteine string motif (By similarity). N- and O-glycosylated in synapsomal membranes. Serine phosphorylated in synapsomal membranes. Sumoylated with SUMO1 and SUMO2 in synaptosomes. The sumoylated forms act as a scaffold for sequestering mu-opioid receptor-activated G(alpha) subunits (By similarity).