GTPASE-ACTIVATING SPECIFICITY OF RGS12 AND BINDING-SPECIFICITY OF AN ALTERNATIVELY SPLICED PDZ (PSD-95 DLG/ZO-1) DOMAIN/

Regulator of G-protein signaling (RGS) proteins increase the intrinsic guanosine triphosphatase (GTPase) activity of G-protein cu subunits i n vitro, but how specific G-protein-coupled receptor systems are targe ted for down-regulation by RGS proteins remains uncharacterized. Here, we describe the GTPase specificity of RGS12 and identify four alterna tively spliced forms of human RGS12 mRNA, Two RGS12 isoforms of 6.3 an d 5.7 kilobases (kb), encoding both an N-terminal PDZ ((P) under bar S D-95/ (D) under bar lg/(Z) under bar O-1) domain and the RGS domain, a re expressed in most tissues, with highest levels observed in testis, ovary, spleen, cerebellum, and caudate nucleus. The 5,7-kb isoform has an alternative 3' end encoding a putative C-terminal PDZ domain docki ng site. Two smaller isoforms, of 3.1 and 3.7 kb, which lack the PDZ d omain and encode the RGS domain with and without the alternative 3' en d, respectively, are most abundantly expressed in brain, kidney, thymu s, and prostate. In vitro biochemical assays indicate that RGS12 is a GTPase-activating protein for Gi class a subunits, Biochemical and int eraction trap experiments suggest that the RGS12 N terminus acts as a classical PDZ domain, binding selectively to C-terminal (A/S)-T-X-(L/V ) motifs as found within both the interleukin-8 receptor B (CXCR2) and the alternative 3' exon form of RGS12. The presence of an alternative ly spliced PDZ domain within RGS12 suggests a mechanism by which RGS p roteins may target specific G-protein-coupled receptor systems for des ensitization.