Signal processing via heterotrimeric G-protein proteins generally involves an initial input sensed by a cell surface GPCR leading to conformational changes in receptor subdomains then transfer this signal to a G-protein, promoting exchange of GTP for GDP and subunit dissociation or rearrangement allowing Gα and Gαβγ to regulate a number of downstream signaling molecules. Multiple discoveries over the last several years have forced us to broaden our perspective on the role of G-proteins as “signaling switches” recognizing 1) that these entities are regulating intracellular events independent of their role as transducers for GPCRs, 2) the Gα and Gαβγ subunits may function independently of each other and 3) the existence of accessory proteins that provide unexpected modes of signal input in this context.
The discovery of alternative modes of regulation of G-proteins and unexpected functional roles for these proteins resulted from a confluence of several independent lines of investigation. A biochemical approach built upon data suggesting cell-specific differences in signal transfer from R to G, the partial purification of a putative non-receptor G-protein activator from extracts of NG108-15 cells and the identification of other non-receptor proteins that could influence the activation state of G-proteins (see Cismowski and Lanier, 2005
). An extension of this line of investigation led to the development of a functional yeast-based screen for mammalian entities that activated G-protein signaling in the absence of a receptor (Cismowski et al., 1999
; Takesono et al., 1999
). In parallel with these studies was the initiative of several labs to search for Gα and Gαβγ binding partners in yeast two hybrid screens, (see in Sato et al., 2006a
) and the realization that G-protein subunits were associated with intracellular organelles (Stow et al., 1991
; Wilson et al., 1994
). Interspersed with these biochemical approaches was the realization that there were changes in signal processing through G-protein signaling systems that occurred independent of any obvious changes in receptor number or G-protein expression levels suggesting additional undefined regulatory mechanisms.
Proteins other than Gα that bind AGS3
Another line of investigation evolved out of the study of asymmetric cell division in D. melanogaster
neuroblasts and sensory organ precursor cells in parallel with the C. elegans
embryo. Gotta and Arhinger reported that Gβγ regulated the orientation of the mitotic spindle in C. elegans
in the one-cell embryo (Gotta and Ahringer, 2001
). In addition, Gα and a protein containing a G-protein regulatory (GPR) motif (a signature feature of Group II AGS proteins discussed in detail later in this review) interacting with Gα, were identified as regulators of asymmetric cell division in a large scale RNAi-based functional screen (Kamath et al., 2003
). Parallel studies in D. melanogaster
also led to the identification of key players involved in this biological process (Betschinger and Knoblich, 2004
). One of these key players was Pins, which contains four GPR motifs and is an ortholog of the Group II AGS proteins AGS3 and LGN discussed later in this review. Pins and its binding partner Insc influence the positioning of cell fate determinants to generate asymmetry and are important for the stability and targeting of protein complexes that transmit polarity information at the apical cortex of the neuroblast to orient the mitotic spindle. Notably, Pins was identified as a binding partner of Gα (Bellaiche et al., 2001
; Parmentier et al., 2000
; Schaefer et al., 2001
; Schaefer et al., 2000
; Yu et al., 2000
Thus we had the confluence of biochemical data indicating unexpected modes of regulation for heterotrimeric G-proteins and data in model organisms implicating Gα and Gβγ in control of asymmetric cell division. One of the many interesting aspects of the signaling role played by G-proteins in the asymmetric cell division in the model organisms was that the process was apparently an intrinsically regulated event independent of a cell surface receptor. This initiated a lot of discussion in the literature about the implications of such a functional role for G-proteins, how this process is regulated and what G-proteins might be involved with in the cell independent of their well-characterized role as transducers from cell-surface GPCRs.
The investigations alluded to above revealed four major concepts that have altered our basic concepts of G-protein signaling: 1) Gα and Gαβγ are processing signals within the cell distinct from their role as transducers for cell surface receptors; 2) such signals involve previously unrecognized functional roles for heterotrimeric G-protein subunits; 3) Gα and Gβγ may exist complexed with alternative binding partners independent of the classical Gαβγ heterotrimer; and 4) the G-protein activation/deactivation cycle may be regulated independent of nucleotide exchange.
This review focuses on the group of proteins defined in a yeast-based functional screen as receptor-independent activators of G-protein signaling or AGS proteins. The goal of the review is to highlight concepts evolving from the discovery of alternative modes of G-protein regulation via AGS proteins, to discuss various unresolved issues in the field and to provide information on the current status of our knowledge regarding functional roles of AGS proteins. The reader is referred to other reviews for a broader discussion of additional G-protein regulators and a more detailed discussion of the discovery of AGS proteins and their initial characterization along with a more extensive listing of citations (Blumer et al., 2005
; Cismowski and Lanier, 2005
; Sato et al., 2006a