Founding members of the RGS protein superfamily were discovered in 1996 in a wide spectrum of species: “supersensitivity to pheromone-2” (Sst2) in the budding yeast Saccharomyces cerevisiae [5, 19, 20], FlbA in the aspergillus Emericella nidulans [9], EGL-10 in the nematode worm Caenorhabiditis elegans [7], and RGS1 and RGS2 from human B- and T-lymphocytes, respectively [6, 8]. Nearly a decade later, new RGS-box-containing proteins are still being identified in mammalian species (e.g., humans in Fig. 3) and even in plants (Fig. 4C). For example, RGS21 was recently identified as a putative component of taste bud signal transduction that is transacted in lingual epithelium via the T1R2/T1R3 sweet taste and T2R bitter taste 7TM receptors [21].  At a mere 152 amino-acids, RGS21 is the shortest RGS protein known to date, with no obvious structural domain N- or C-terminal to the central RGS-box. The singular nature of the RGS21 domain structure is typical of the A/RZ and B/R4 subfamilies (Figs. 2&3) but is unlike other superfamily members that either contain two or more RGS-boxes in tandem (i.e., the PKA regulatory subunit binding partner D‑AKAP2 [22] and the testes-specific protein PRTD-NY2 [a.k.a. RGS22; Willard & Siderovski, unpublished observations]) or possess one or more functional modules beyond the defining RGS-box (Fig. 2). Several recent findings as to the functions of these multi-domain RGS proteins are described below.

Figure 2. Multidomain architecture of representative members from all subfamilies of the mammalian RGS protein superfamily. Two alternative nomenclatures have been proposed for several RGS protein subfamilies [23, 24]. RZ- or A-subfamily members such as RGS17 [25] are characterized by an N-terminal poly-cysteine region (“Cys”) thought to be reversibly palmitoylated [26]. R4- or B-subfamily members include RGS2 and RGS21 that are described in the text. RGS11 was the first member of the R7- or C-subfamily to be shown to bind Gbeta5 via its Ggamma-like or “GGL” domain [27]. Of the three members of the R12- or D-subfamily (RGS10, RGS12, RGS14), RGS10 is the smallest and comprises little more than an RGS-box [11], whereas both RGS12 and RGS14 have tandem Ras-binding domains (RBDs) and a C-terminal Galpha-i/o-Loco interaction (GoLoco) motif [28], and RGS12 additionally has N-terminal PDZ (PSD95/Dlg/ZO-1 homology) and PTB (phosphotyrosine-binding) domains [29]. Axin and Axin2 (a.k.a. Axil) are negative regulators of the Wnt signaling pathway and comprise the RA- or E-subfamily; neither protein has been shown to interact with Galpha subunits, but rather interact with the tumor suppressor protein adenomatous polyposis coli (APC) using the RGS-box [30]. Axin and Axil also contain other domains that interact with beta-catenin, the kinase GSK3beta, the phosphatase PP2A, and the protein Dishevelled (“DIX” domain) [31]. The GEF- or F-subfamily includes three RhoA-specific guanine nucleotide exchange factors (“GEFs”) with canonical Dbl-homology (DH) and pleckstrin-homology (PH) domains: p115-RhoGEF, PDZ-RhoGEF, and leukemia-associated RhoGEF (LARG); the latter two RhoGEFs each possess an N-terminal PDZ domain, as described in the text. In 1996, we were the first group to identify [8] an N-terminal RGS-box within each member of the G protein-coupled receptor kinase family (known as the GRK- or G-subfamily in the context of the RGS protein superfamily). At least three sorting nexins (SNX13, SNX14, SNX25) have RGS-boxes between phosphatidylinositol-binding (PX) and PX-associated (PXA) domains and thus comprise the SNX- or H-subfamily of RGS proteins. Zheng and colleagues reported that SNX13 (a.k.a. “RGS-PX1”) could act as a GAP for the adenylyl-cyclase-stimulatory isoform of Galpha (Galpha-s) [32]; however, this report has yet to be confirmed in the literature. TM, putative transmembrane regions. The multiple RGS-box family members D-AKAP2 and RGS22 fall outside the eight established subfamilies; the superscript designations of their RGS-boxes match that used in Figure 3.

Figure 3. Relationship between RGS-box sequences of all 37 human RGS proteins identified to date. Unrooted dendrogram was generated by Clustal-W [33] and TreeView [34] using sequences identified by the SMART profile [35] for RGS-boxes as well as those identified by protein-fold recognition algorithms [36]. Subfamily designations and identification of isolated RGS-box sequences from multi-RGS-containing proteins D-AKAP2 and RGS22 are as described for Figure 2. Note that there is no RGS15, contrary to an early report [7].