The membrane anchor R7BP controls the proteolytic stability of the striatal specific RGS protein, RGS9-2

A member of the RGS ((r) under bar egulators of (G) under bar protein (s) under bar ignaling) family, RGS9-2 is a critical regulator of G protein signaling pathways that control locomotion and reward signaling in the brain. RGS9-2 is specifically expressed in striatal neurons where it forms complexes with its newly discovered partner, R7BP ((R7) under bar family binding protein). Interaction with R7BP is important for the subcellular targeting of RGS9-2, which in native neurons is found in plasma membrane and its specializations, postsynaptic densities. Here we report that R7BP plays an additional important role in determining proteolytic stability of RGS9-2. We have found that co-expression with R7BP dramatically elevates the levels of RGS9-2 and its constitutive subunit, G beta 5. Measurement of the RGS9-2 degradation kinetics in cells indicates that R7BP markedly reduces the rate of RGS9-2-G beta 5 proteolysis. Lentivirus-mediated RNA interference knockdown of the R7BP expression in native striatal neurons results in the corresponding decrease in RGS9-2 protein levels. Analysis of the molecular determinants that mediate R7BP/RGS9-2 binding to result in proteolytic protection have identified that the binding site for R7BP in RGS proteins is formed by pairing of the DEP ((D) under bar isheveled, (E) under bar GL-10, Pleckstrin) domain with the R7H ((R7 h) under bar omology), a domain of previously unknown function that interacts with four putative a-helices of the R7BP core. These findings provide a mechanism for the regulation of the RGS9 protein stability in the striatal neurons.