Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

Activator of G-protein signaling 3 (AGS3) andLGN have a similar domain structure and contain four Gprotein regulatory motifs that serve as anchors for the binding of the GDP-bound conformation of specific Gprotein ␣ subunits. As an initial approach to define further the different functional roles of AGS3 and LGN, we determined their expression profile and subcellular distribution. AGS3-and LGN-specific antisera indicated a widespread tissue distribution of LGN, whereas AGS3 is primarily enriched in brain. Brain punch biopsies of 13 discrete brain regions indicated that both AGS3 and LGN are expressed in all areas tested but are differentially regulated during development. LGN is expressed in neuronal, astroglial, and microglial cultures, whereas AGS3 expression is restricted to neurons. In primary neuronal cultures as well as in dividing cultures of PC12 cells, immunocytochemistry indicated distinct subcellular locations of AGS3 and LGN. The subcellular locations of the two proteins were differentially regulated by external stimuli and the cell cycle. In PC12 and COS7 cells, LGN moves from the nucleus to the midbody structure separating daughter cells during the later stages of mitosis, suggesting a role for G-proteins in cytokinesis. Thus, although AGS3 and LGN share a similar overall motif structure and both bind G-proteins, nature has endowed these proteins with different regulatory elements that allow functional diversity by virtue of tissuespecific expression and subcellular positioning.Heterotrimeric G-proteins transduce extracellular stimuli sensed by cell surface receptors into integrated cellular responses. An activated cell surface receptor initiates a change in the conformation of G␣ subunits such that GDP is exchanged for GTP and G␣ appears to dissociate from G␤␥. Both G␣ and G␤␥ then regulate the activity of various effectors with signal termination involving hydrolysis of bound GTP and apparent reassociation of G␣GDP and G␤␥. Recent data indicate that additional proteins influence the G-protein activation cycle with unexpected modes of regulation (1-17). Such proteins have broad implications for signal processing.The family of RGS proteins acts to accelerate the rate of GTP hydrolysis by specific G␣ subunits (12). RGS proteins apparently play important roles in terminating responses initiated by cell surface receptors and maintaining the dynamics of receptor-regulated events by promoting efficient cycling of activated G-proteins (13). A second mode of regulation is represented by proteins such as activators of G-protein signaling (AGS) 1 (1-10), the NG108 -15 G-protein activator (14 -15), phosphatidylethanolamine-binding protein (16), and neuromodulin (17), which regulate the activation state of G-protein independent of a G-protein-coupled receptor. Interestingly, the latter group of proteins may either act in a similar manner as a receptor, promoting nucleotide exchange on G␣ subunits (i.e. AGS1, the NG108 -15 G-protein activator, phosphatidylethanolamine-binding protein), or by influencing subuni...

Appropriate trafficking and targeting of glutamate receptors (GluRs) to the postsynaptic density is crucial for synaptic function. We show that mPins (mammalian homologue of Drosophila melanogaster partner of inscuteable) interacts with SAP102 and PSD-95 (two PDZ proteins present in neurons), and functions in the formation of the NMDAR-MAGUK (N-methyl-D-aspartate receptor-membrane-associated guanylate kinase) complex. mPins enhances trafficking of SAP102 and NMDARs to the plasma membrane in neurons. Expression of dominant-negative constructs and short-interfering RNA (siRNA)-mediated knockdown of mPins decreases SAP102 in dendrites and modifies surface expression of NMDARs. mPins changes the number and morphology of dendritic spines and these effects depend on its Galphai interaction domain, thus implicating G-protein signalling in the regulation of postsynaptic structure and trafficking of GluRs.

Signal processing via heterotrimeric G-proteins in response to cell surface receptors is a central and much investigated aspect of how cells integrate cellular stimuli to produce coordinated biological responses. The system is a target of numerous therapeutic agents, plays an important role in adaptive processes of organs, and aberrant processing of signals through these transducing systems is a component of various disease states. In addition to GPCR-mediated activation of G-protein signaling, nature has evolved creative ways to manipulate and utilize the Gαβγ heterotrimer or Gα and Gαβγ subunits independent of the cell surface receptor stimuli. In such situations, the G-protein subunits (Gα and Gαβγ) may actually be complexed with alternative binding partners independent of the typical heterotrimeric Gαβγ. Such regulatory accessory proteins include the family of RGS proteins that accelerate the GTPase activity of Gα and various entities that influence nucleotide binding properties and/or subunit interaction. The latter group of proteins includes receptor independent activators of G-protein signaling or AGS proteins that play surprising roles in signal processing. This review provides an overview of our current knowledge regarding AGS proteins. AGS proteins are indicative of a growing number of accessory proteins that influence signal propagation, facilitate cross talk between various types of signaling pathways and provide a platform for diverse functions of both the heterotrimeric Gαβγ and the individual Gα and Gαβγ subunits.