THE G-PROTEIN-COUPLED RECEPTOR PHOSPHATASE - A PROTEIN PHOSPHATASE TYPE 2A WITH A DISTINCT SUBCELLULAR-DISTRIBUTION AND SUBSTRATE-SPECIFICITY

Phosphorylation of G-protein-coupled receptors plays an important role in regulating their function, In this study the G-protein-coupled rec eptor phosphatase (GRP) capable of dephosphorylating G-protein coupled receptor kinase-phosphorylated receptors is described. The GRP activi ty of bovine brain is a latent oligomeric form of protein phosphatase type 2A (PP-2A) exclusively associated with the particulate fraction. GRP activity is observed only when assayed in the presence of protamin e or when phosphatase-containing fractions are subjected to freeze/tha w treatment under reducing conditions, Consistent with its identificat ion as a member of the PP-2A family, the GRP is potently inhibited by okadaic acid but not by I-2, the specific inhibitor of protein phospha tase type 1. Solubilization of the membrane-associated GRP followed by gel filtration in the absence of detergent yields a 150-kDa peak of l atent receptor phosphatase activity. Western blot analysis of this pho sphatase reveals a likely subunit composition of AB(alpha)C. PP-2A of this subunit composition has previously been characterized as a solubl e enzyme, yet negligible soluble GRP activity was observed, The subcel lular distribution and substrate specificity of the GRP suggests signi ficant differences between it and previously characterized forms of PP -2A.