ACCESSORY PROTEINS FOR G PROTEINS: Partners in Signaling.

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_α subunit, alter subunit interactions within heterotrimeric G_αβγ independent of nucleotide exchange, or form complexes with G_α or G_βγ independent of the typical G_αβγ heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell. [ABSTRACT FROM AUTHOR]