Heterogeneous processing of a G protein gamma subunit at a site critical for protein and membrane interactions

Negative Feedback Phosphorylation of Gγ Subunit Ste18 and the Ste5 Scaffold Synergistically Regulates MAPK Activation in Yeast

Heterotrimeric G proteins (Gαβγ) are essential transducers in G protein signaling systems in all eukaryotes. In yeast, G protein signaling differentially activates mitogen-activated protein kinases (MAPKs)—Fus3 and Kss1—a phenomenon controlled by plasma membrane (PM) association of the scaffold protein Ste5. Here, we show that phosphorylation of the yeast Gγ subunit (Ste18), together with Fus3 docking on Ste5, controls the rate and stability of Ste5/PM association. Disruption of either element alone by point mutation has mild but reciprocal effects on MAPK activation. Disabling both elements results in ultra-fast and stable bulk Ste5/PM localization and Fus3 activation that is 6 times faster and 4 times more amplified compared to wild-type cells. These results further resolve the mechanism by which MAPK negative feedback phosphorylation controls pathway activation and provides compelling evidence that Gγ subunits can serve as intrinsic regulators of G protein signaling.

Disruption of G-protein γ5 subtype causes embryonic lethality in mice

Heterotrimeric G-proteins modulate many processes essential for embryonic development including cellular proliferation, migration, differentiation, and survival. Although most research has focused on identifying the roles of the various αsubtypes, there is growing recognition that similarly divergent βγ dimers also regulate these processes. In this paper, we show that targeted disruption of the mouse Gng5 gene encoding the γ₅ subtype produces embryonic lethality associated with severe head and heart defects. Collectively, these results add to a growing body of data that identify critical roles for the γ subunits in directing the assembly of functionally distinct G-αβγ trimers that are responsible for regulating diverse biological processes. Specifically, the finding that loss of the G-γ₅ subtype is associated with a reduced number of cardiac precursor cells not only provides a causal basis for the mouse phenotype but also raises the possibility that G-βγ₅ dependent signaling contributes to the pathogenesis of human congenital heart problems.

Structural determinants involved in the formation and activation of G protein betagamma dimers

Heterotrimeric G proteins, composed of an alpha, beta and gamma subunit, represent one of the most important and dynamic families of signaling proteins. As a testament to the significance of G protein signaling, the hundreds of seven-transmembrane-spanning receptors that interact with G proteins are estimated to occupy 1-2% of the human genome. This broad diversity of receptors is echoed in the number of potential heterotrimer combinations that can arise from the 23 alpha subunit, 7 beta subunit and 12 gamma subunit isoforms that have been identified. The potential for such vast complexity implies that the receptor G protein interface is the site of much regulation. The historical model for the activation of a G protein holds that activated receptor catalyzes the exchange of GDP for GTP on the alpha subunit, inducing a conformational change that substantially lowers the affinity of alpha for betagamma. This decreased affinity enables dissociation of betagamma from alpha and receptor. The free form of betagamma is thought to activate effectors, until the hydrolysis of GTP by G alpha (aided by RGS proteins) allows the subunits to re-associate, effectively deactivating the G protein until another interaction with activated receptor.

Sequence Dependence and Differential Expression of Gγ5 Subunit Isoforms of the Heterotrimeric G Proteins Variably Processed after Prenylation in Mammalian Cells

Between 1 and 2% of proteins coded for in the human genome, including all G protein gamma subunits, are predicted to be prenylated. Subsequently, prenylated proteins are proteolytically cleaved at the C terminus and carboxymethylated. These reactions are generally obligatory events required for functional expression of prenylated proteins. The biological role of prenyl substrates has made these reactions significant targets for anticancer drug development. Understanding the enzymology of this pathway will be key to success for this strategy. When Ggamma1, -2, -4, -10, -11, -12, and -13 were expressed in HEK293 cells they were completely processed according to the current understanding of the prenylation reaction. In contrast, Ggamma5 was processed to two forms; a minor one, fully processed as predicted, and a major one that was prenylated without further processing. When the Ca(1)a(2)X motif of Ggamma5, CSFL, was exchanged for that of Ggamma2, CAIL, Ggamma5 was completely processed. Conversely, Ggamma2-SFL was incompletely processed. Differential processing of Ggamma5 was found due to the presence of an aromatic amino acid in its Ca(1)a(2)X motif. Retrieving endogenous Ggamma subunits from HEK293 or Neuro-2a cells with FLAG-Gbeta constructs identified multiple Ggamma subunits by mass spectrometry in either cell, but in both cases the most prominent one was Ggamma5 expressed without C-terminal processing after prenylation. This work indicates that post-prenylation reactions can generate multiple products determined by the C-terminal Ca(1)a(2)X motif. Within the human genome 10% of predicted prenylated proteins have aromatic amino acids in their Ca(1)a(2)X sequence and would likely generate the prenylation pattern described here.

Proteomic analysis of bovine brain G protein gamma subunit processing heterogeneity

We characterized the variable processing of the G protein gamma subunit isoforms associated with bovine brain G proteins, a primary mediator of cellular communication. Ggamma subunits were isolated from purified brain G proteins and characterized by Edman sequencing, by MALDI MS, by chemical and/or enzymatic fragmentation assayed by MALDI MS, and by MS/MS fragmentation and sequencing. Multiple forms of six different Ggamma isoforms were detected. Significant variation in processing was found at both the amino termini and particularly the carboxyl termini of the proteins. All Ggamma isoforms contain a carboxyl-terminal CAAX motif for prenylation, carboxyl-terminal proteolysis, and carboxymethylation. Characterization of these proteins indicates significant variability in the normal processing of all of these steps in the prenylation reaction, including a new variation of prenyl processing resulting from cysteinylation of the carboxyl terminus. These results have multiple implications for intracellular signaling mechanisms by G proteins, for the role of prenyl processing variation in cell signaling, and for the site of action and consequences of drugs that target the prenylation modification.

Genomic analysis of G protein gamma subunits in human and mouse - the relationship between conserved gene structure and G protein betagamma dimer formation

Analysis of the genomic sequences, cDNAs and expressed sequence tags (ESTs) in human and mouse for the 12 genes of the gamma subunits of the heterotrimeric G proteins has allowed us to identify the common versus unique elements of the organization and expression of the members of this important gene family. All of the G protein gamma subunit genes are organized around two coding exons, each containing about 100 nucleotides coding for 30-40 amino acids. These two exons each correspond to a functional domain of the protein, which interestingly appears to impose constraints on both the structure of the protein and the structure of the gene. There is large variation in the intron size between these two coding exons, the number and size of 5' and 3' UTRs, and the overall size of the genes. There is general but not absolute conservation in the size and structure of these genes between humans and mice. Alternative splicing and potential differential promoter usage were detected for several Ggamma subunits, indicating possible differential regulation in expression. Only for Ggamma10, however, did we find an alternative coding transcript. This alternative transcript appears to code for a hybrid protein containing a DnaJ domain in place of its Ggamma exon 1 domain, joined to the Ggamma10 second exon domain. The predicted mRNA is expressed in humans, and the protein coded by it is readily translated in vitro. This protein does not form a functional G protein betagamma dimer, but it could generate a chaperone-like protein related to its DNA-J domain. These studies suggest that alternative splicing is not a prominent mechanism for generating G protein subunit diversity from within the human or mouse genomes. Instead, each of the known 12 gamma subunit genes generate transcripts with one prevalent protein.

Critical determinants of the G protein gamma subunits in the Gbetagamma stimulation of G protein-activated inwardly rectifying potassium (GIRK) channel activity

The betagamma subunits of G proteins modulate inwardly rectifying potassium (GIRK) channels through direct interactions. Although GIRK currents are stimulated by mammalian Gbetagamma subunits, we show that they were inhibited by the yeast Gbetagamma (Ste4/Ste18) subunits. A chimera between the yeast and the mammalian Gbeta1 subunits (ymbeta) stimulated or inhibited GIRK currents, depending on whether it was co-expressed with mammalian or yeast Ggamma subunits, respectively. This result underscores the critical functional influence of the Ggamma subunits on the effectiveness of the Gbetagamma complex. A series of chimeras between Ggamma2 and the yeast Ggamma revealed that the C-terminal half of the Ggamma2 subunit is required for channel activation by the Gbetagamma complex. Point mutations of Ggamma2 to the corresponding yeast Ggamma residues identified several amino acids that reduced significantly the ability of Gbetagamma to stimulate channel activity, an effect that was not due to improper association with Gbeta. Most of the identified critical Ggamma residues clustered together, forming an intricate network of interactions with the Gbeta subunit, defining an interaction surface of the Gbetagamma complex with GIRK channels. These results show for the first time a functional role for Ggamma in the effector role of Gbetagamma.

Gamma 2 subunit of G protein heterotrimer is an N-end rule ubiquitylation substrate

Heterotrimeric G proteins transduce signals from activated transmembrane G protein-coupled receptors to appropriate downstream effectors within cells. Signaling specificity is achieved in part by the specific alpha, beta, and gamma subunits that compose a given heterotrimer. Additional structural and functional diversity in these subunits is generated at the level of posttranslational modification, offering alternate regulatory mechanisms for G protein signaling. Presented here is the identification of a variant of the gamma(2) subunit of G protein heterotrimer purified from bovine brain and the demonstration that this RDTASIA gamma(2) variant, containing unique amino acid sequence at its N terminus, is a substrate for ubiquitylation and degradation via the N-end rule pathway. Although N-end-dependent degradation has been shown to have important functions in peptide import, chromosome segregation, angiogenesis, and cardiovascular development, the identification of cellular substrates in mammalian systems has remained elusive. The isolation of RDTASIA gamma(2) from a native tissue represents identification of a mammalian N-end rule substrate from a physiological source, and elucidates a mechanism for the targeting of G protein gamma subunits for ubiquitylation and degradation.

Identification of a region in G protein gamma subunits conserved across species but hypervariable among subunit isoforms

The heterotrimeric GTP binding proteins, G proteins, consist of three distinct subunits: alpha, beta, and gamma. There are 12 known mammalian gamma subunit genes whose products are the smallest and most variable of the G protein subunits. Sequencing of the bovine brain gamma(10) protein by electrospray mass spectrometry revealed that it differs from the human protein by an Ala to Val substitution near the N-terminus. Comparison of gamma isoform subunit sequences indicated that they vary substantially more at the N-terminus than at other parts of the protein. Thus, species variation of this region might reflect the lack of conservation of a functionally unimportant part of the protein. Analysis of 38 gamma subunit sequences from four different species shows that the N-terminus of a given gamma subunit isoform is as conserved between different species as any other part of the protein, including highly conserved regions. These data suggest that the N-terminus of gamma is a functionally important part of the protein exhibiting substantial isoform-specific variation.

Heterotrimeric G-protein betagamma-dimers in growth and differentiation

Heterotrimeric G-proteins are components of the signal transduction pathways for the soluble and cell-contact signals that regulate normal growth and differentiation. There is now a greater appreciation of the role of the Gbetagamma-dimer in the regulation of a variety of intracellular effectors, including ion channels, adenylyl cyclase, and phospholipase Cbeta. In many cases, Gbetagamma-dimers are required for the activation of mitogen activated protein kinase (MAPK) pathways that promote cellular proliferation, although the underlying mechanisms have yet to be fully elucidated. Activation of phosphotidylinositol-3-kinase (PI3K) is a critical step in the intracellular transduction of survival signals. Gbetagamma-dimers directly activate PI3Kgamma as well as the more widely distributed PI3Kbeta. The activation of PI3Kgamma by Gbetagamma-dimers likely involves direct binding of specific Gbetagamma-dimers to both subunits of PI3Kgamma. Thus, Gbetagamma-dimers transmit signals from numerous receptors to a variety of intracellular effectors in distinct cellular contexts. Five distinct Gbeta-subunits and 12 distinct Ggamma-subunits have been identified. New experimental approaches are needed to elucidate the specific roles of individual Gbetagamma-dimers in the pathways that transduce signals for proliferation and survival.