Heterotrimeric G-protein signaling systems are activated via cell surface receptors possessing the sevenmembrane span motif. Several observations suggest the existence of other modes of stimulus input to heterotrimeric G-proteins. As part of an overall effort to identify such proteins we developed a functional screen based upon the pheromone response pathway in Saccharomyces cerevisiae. We identified two mammalian proteins, AGS2 and AGS3 (activators of G-protein signaling), that activated the pheromone response pathway at the level of heterotrimeric G-proteins in the absence of a typical receptor. ␤-galactosidase reporter assays in yeast strains expressing different G␣ subunits (Gpa1, G s ␣, G i ␣ 2 (Gpa1(1-41)) , G i ␣ 3(Gpa1(1-41)) , G␣ 16(Gpa1(1-41)) ) indicated that AGS proteins selectively activated G-protein heterotrimers. AGS3 was only active in the G i ␣ 2 and G i ␣ 3 genetic backgrounds, whereas AGS2 was active in each of the genetic backgrounds except Gpa1. In protein interaction studies, AGS2 selectively associated with G␤␥, whereas AGS3 bound G␣ and exhibited a preference for G␣GDP versus G␣GTP␥S. Subsequent studies indicated that the mechanisms of G-protein activation by AGS2 and AGS3 were distinct from that of a typical G-proteincoupled receptor. AGS proteins provide unexpected mechanisms for input to heterotrimeric G-protein signaling pathways. AGS2 and AGS3 may also serve as novel binding partners for G␣ and G␤␥ that allow the subunits to subserve functions that do not require initial heterotrimer formation.The seven-membrane span hormone receptor coupled to heterotrimeric G-proteins represents one of the most widely used systems for information transfer across the cell membrane. Signal processing via this system likely operates within the context of a signal transduction complex. Within such a signal transduction complex, there are likely accessory proteins (distinct from receptor, G-protein, and effectors) that participate in the formation of this complex and/or regulate signal transfer from receptor to G-protein. In addition, several reports suggest alternative modes of stimulus input to heterotrimeric G-proteins that do not require direct interaction of the G-protein with the seven-membrane span receptor itself. To identify such entities and to define putative components of such a signal transduction complex we initiated two broad experimental approaches (1-4). One strategy focused on a functional readout involving G-protein activation and was based upon initial observations in our laboratory concerning the transfer of signal from R to G (3, 4). This approach resulted in the partial purification and characterization of the NG10815 G-protein activator that directly increased GTP␥S binding to brain G-protein in the absence of a receptor. To extend this body of work, we developed an expression cloning system in Saccharomyces cerevisiae that was designed to detect mammalian activators of the pheromone response pathway in the absence of a G-proteincoupled receptor (5). The pheromone response pathw...

Sitosterolemia is a rare autosomal recessive disorder characterized by (a) intestinal hyperabsorption of all sterols, including cholesterol and plant and shellfish sterols, and (b) impaired ability to excrete sterols into bile. Patients with this disease have expanded body pools of cholesterol and very elevated plasma plant-sterol species and frequently develop tendon and tuberous xanthomas, accelerated atherosclerosis, and premature coronary artery disease. In previous studies, we have mapped the STSL locus to human chromosome 2p21. Recently, we reported that a novel member of the ABC-transporter family, named "sterolin-1" and encoded by ABCG5, is mutated in 9 unrelated families with sitosterolemia; in the remaining 25 families, no mutations in sterolin-1 could be identified. We identified another ABC transporter, located <400 bp upstream of sterolin-1, in the opposite orientation. Mutational analyses revealed that this highly homologous protein, termed "sterolin-2" and encoded by ABCG8, is mutated in the remaining pedigrees. Thus, two highly homologous genes, located in a head-to-head configuration on chromosome 2p21, are involved as causes of sitosterolemia. These studies indicate that both sterolin-1 and sterolin-2 are indispensable for the regulation of sterol absorption and excretion. Identification of sterolin-1 and sterolin-2 as critical players in the regulation of dietary-sterol absorption and excretion identifies a new pathway of sterol transport.

The G-protein regulatory (GPR) motif in AGS3 was recently identified as a region for protein binding to heterotrimeric G-protein ␣ subunits. To define the properties of this ϳ20-amino acid motif, we designed a GPR consensus peptide and determined its influence on the activation state of G-protein and receptor coupling to G-protein. The GPR peptide sequence (28 amino acids) encompassed the consensus sequence defined by the four GPR motifs conserved in the family of AGS3 proteins. The GPR consensus peptide effectively prevented the binding of AGS3 to Gi␣1,2 in protein interaction assays, inhibited guanosine 5-O-(3-thiotriphosphate) binding to Gi␣, and stabilized the GDP-bound conformation of Gi␣. The GPR peptide had little effect on nucleotide binding to Go␣ and brain G-protein indicating selective regulation of Gi␣. Thus, the GPR peptide functions as a guanine nucleotide dissociation inhibitor for Gi␣. The GPR consensus peptide also blocked receptor coupling to Gi␣␤␥ indicating that although the AGS3-GPR peptide stabilized the GDP-bound conformation of Gi␣, this conformation of Gi␣ GDP was not recognized by a G-protein coupled receptor. The AGS3-GPR motif presents an opportunity for selective control of Gi␣-and G␤␥؊regulated effector systems, and the GPR motif allows for alternative modes of signal input to G-protein signaling systems.The G-protein regulatory (GPR) 1 motif or GoLOCO repeat is a ϳ20-amino acid domain found in several proteins that interact with and/or regulate G-proteins (1, 2). Such proteins include the activator of G-protein signaling AGS3, the AGS3-related protein PINS in Drosophila melanogaster, two members of the RGS family of proteins, and three proteins (LGN, Pcp2, and Rap1GAP) isolated in yeast two-hybrid screens using Gi␣ or Go␣ as bait. Rat AGS3 was isolated in a yeast-based functional screen designed to identify receptorindependent activators of heterotrimeric G-protein signaling (1). The AGS3-related protein PINS is required for asymmetric cell division of neuroblasts in D. melanogaster, where it is found complexed with Gi/Go (3, 4), but neither the signal input nor output for this complex is known. Some insight as to how PINS may regulate Gi/Go is provided by studies with AGS3 (1). In the yeast-based system, AGS3 selectively activated Gi␣2 and Gi␣3. The action of AGS3 as a G-protein activator in the yeast-based system was independent of nucleotide exchange as it was not antagonized by overexpression of RGS4, and it was still observed following replacement of Gi␣2 with Gi␣2-G204A, a mutant that is deficient in making the transition to the GTP-bound state (1, 5). Both of these manipulations effectively prevent receptor-mediated activation of G-protein signaling in the yeast system and block the action of AGS1, which was isolated in the same screen and apparently behaves as a guanine nucleotide exchange factor for heterotrimeric G-proteins (5, 6). These data indicate that the interaction of AGS3 with G-protein influences a unique control mechanism within the activation/deactivation cycle...

CD59 is a membrane glycoprotein that regulates formation of the cytolytic membrane attack complex (MAC or C5b-9) on host cell membranes. It functions by binding to C8 (␣ chain) and C9 after their structural rearrangement during MAC assembly. Previous studies indicated that the CD59 binding site in C9 was located within a 25-residue disulfide-bonded loop, and in C8␣ was located within a 51-residue sequence that overlaps the CD59 binding region of C9. By peptide screens and the use of peptides in binding assays, functional assays, and computer modeling and docking studies, we have identified a 6-residue sequence of human C9, spanning residues 365-371, as the primary CD59 recognition domain involved in CD59-mediated regulation of MAC formation. The data also indicate that both C8␣ and C9 bind to a similar or overlapping site on CD59. Furthermore, data from CD59-peptide docking models are consistent with the C9 binding site on CD59 located at a hydrophobic pocket, putatively identified previously by CD59 mutational and modeling studies.Complement activation leads ultimately to the generation and membrane insertion of a cytolytic protein assembly termed the membrane attack complex (MAC).2 The MAC is formed by the self-assembly of complement proteins C5b, C6, C7, C8, and from 1 to 18 molecules of C9. Host cells are protected from MAC-mediated lysis by CD59, an 18 -21-kDa glycophosphatidylinositol-linked membrane glycoprotein. The MAC appears to play an important role in causing tissue injury following inappropriate complement activation in various ischemic and inflammatory conditions (reviewed in Refs. 1-5), and soluble forms of CD59 have been shown to be therapeutic in rodent models of disease (6, 7). Furthermore, CD59 is sometimes overexpressed on tumor cells, and its expression has been linked to promoting tumor growth and the protection of tumor cells from mAb therapy (8 -10). It is therefore of interest to understand the molecular interaction between CD59 and its complement ligands for the goal of engineering effective soluble CD59-based therapeutics for treating inflammatory conditions, or for designing CD59 inhibitory molecules to enhance tumor cell susceptibility to complement.CD59 functions by binding to the ␣-subunit of C8 in the C5b-8 complex and preventing subsequent binding of C9, and/or by binding to C9 in the C5b-9 complex and preventing recruitment of additional C9 molecules (11-13). CD59 can only bind to C8␣ and C9 in the nascent complex after conformation rearrangements of the two proteins that occur during MAC assembly. The activity of CD59 is species selective, and previous characterization of chimeric human/rabbit C8␣-and C9-identified regions within the primary sequence of these proteins that interact with human CD59 (14, 15). There is considerable sequence homology between C8␣ and C9, and the identified CD59 binding sequence in the two proteins aligned to a region just C-terminal to the proposed membrane binding domain that encompassed residues 320 -415 in human C8␣, and 334 -415 in human C9. Pe...

The G-protein regulatory (GPR) motif, a conserved 25-30 amino acid domain found in multiple mammalian proteins, stabilizes the GDP-bound conformation of G␣ i , inhibits guanosine 5-O-(3-thiotriphosphate) (GTP␥S) binding to G␣ i and competes for G␤␥ binding to G␣. To define the core GPR motif and key amino acid residues within a GPR peptide (TMGEEDFFDLLAKSQSKRMD-DQRVDLAGThese data provide a platform for the development of novel, G-protein-selective therapeutics that inhibit G␣ imediated signaling, selectively activate G␤␥-sensitive effectors, and/or disrupt specific regulatory input to Gproteins mediated by GPR-containing proteins.The activation/deactivation cycle of heterotrimeric G-proteins, key players in cell signaling events, involves guanine nucleotide exchange, GTP hydrolysis, and a number of dynamic, conformationally sensitive protein interactions. In addition to the extensively studied activation of G-proteins by the superfamily of G-protein-coupled receptors, the G-protein activation/deactivation cycle is regulated by nonreceptor proteins that influence subunit interactions, GTPase activity, and guanine nucleotide binding properties of G␣. One signature motif for such regulatory proteins is the regulator of G-protein signaling (RGS) 1 domain, a ϳ120-amino acid motif found in all members of the RGS family (1). Another signature motif (ϳ25-30 amino acids) is defined by the G-protein regulatory (GPR) domain in activator of G-protein signaling (AGS) 3 (2-6), which was discovered in a functional screen for receptor-independent activators of G-protein signaling. The GPR motif was also recognized in RGS12 and RGS14 by general sequence analysis/alignment and termed the GoLOCO motif (7,8).Surprisingly, interaction of the GPR motif with G␣ i stabilizes the GDP bound conformation of G␣ i , competes with G␤␥ for G␣ binding, and inhibits guanine nucleotide exchange (3-6, 9 -12). Thus, the GPR motif acts as a guanine nucleotide dissociation inhibitor of G␣ i . The GPR motif is evolutionarily conserved within individual orthologs and among proteins with apparently diverse functions (see S.M.A.R.T. data base at dylan.embl-heidelberg.de/). Four spatially conserved GPR motifs are found in AGS3 (3) and LGN (13), which were isolated as G␣ i -regulatory/binding proteins. Recombinant AGS3 constructs with more than one GPR motif actually bind more than one G␣ i at the same time (5), suggesting a scaffolding role for such proteins. The AGS3/LGN-related protein PINS, which plays key roles in cell polarity (14 -17), possesses a similar domain structure. The interaction of the PINS protein GPR domains with G␣ is involved in the function of PINS in cell polarity and asymmetric cell division (17). AGS3 is also involved in synaptic adaptation in rat models of addiction (22). Single GPR motifs are found in Rap1GAP, Pcp2, RGS12, and RGS14, which are all implicated as G-protein regulators. Protein interaction studies and/or functional screens in yeast indicate that the AGS3 GPR motif interacts with G␣ i1-3 , but not G␣ s , G␣ q , G␣ z ...