Disease relevance of GPA1

• Suppressors of a gpa1 mutation cause sterility in Saccharomyces cerevisiae [1].
• Identification of a second-site suppressor mutation of the GTPase defect associated with McCune - Albright syndrome: A model using the yeast heterotrimeric G protein, GPA1 [2].
• This ploidy-dependent toxicity was shown to result from enhanced degradation of Gpa1, the alpha subunit (G alpha) of a heterotrimeric guanine nucleotide-binding protein (G protein) that regulates cell differentiation in response to mating pheromones [3].
• As a consequence, virulence of the gpa1 mutant strain was significantly attenuated in an animal model of cryptococcal meningitis [4].

High impact information on GPA1

• We demonstrate that Gpa1 is present at endosomes, where it interacts directly with both Vps34 and Vps15 and stimulates increased production of phosphatidylinositol 3-phosphate [5].
• To identify candidate Gpa1 effectors, we expressed the constitutively active Gpa1(Q323L) mutant in each of nearly 5000 gene-deletion strains and measured mating-specific responses [5].
• Positional cloning and functional assays showed that polymorphisms in GPA1 and AMN1 affect expression of genes involved in pheromone response and daughter cell separation, respectively [6].
• The GPA1 gene of S. cerevisiae encodes a G alpha subunit that plays a positive role in the transduction of signals stimulating recovery from pheromone-induced cell cycle arrest [7].
• The suppression of a gpa1 null (G alpha subunit) by a fus3 null also suggests FUS3 is in the signal transduction pathway [8].

Biological context of GPA1

• GPA1 hyperexpression suppressed the phenotype of STE4 overexpression [9].
• These results suggest that the dac1 mutation is an allele of the GPA1 gene and thus provide genetic evidence that the yeast G protein homolog is directly involved in the mating pheromone signal transduction pathway [10].
• The Saccharomyces cerevisiae GPA1 gene encodes a protein highly homologous to the alpha subunit of mammalian G proteins and is essential for haploid cell growth [1].
• These observations suggest that GPA1 (SGP1) and SGP2 are involved in mating factor-mediated signal transduction, which causes both cell cycle arrest in the late G1 phase and induction of genes necessary for mating such as FUS1 [1].
• On GTP hydrolysis, G(alpha) (GDP) then reassociates with G(betagamma) [Gilman, A. G. (1987) Annu. Rev. Biochem. 56, 615-649] [11].

Anatomical context of GPA1

• Electrophysiological measurements indicated that the membrane conductance of oocytes injected with STE2 mRNA, or with both STE2 and GPA1 (encoding a yeast G protein alpha-subunit) mRNAs, did not change and was not affected by pheromone binding [12].
• Unlike the mammalian G alpha i proteins alpha i and alpha o, nonmyristoylated Gpa1 is able to associate with membranes [13].
• In contrast, wild-type Gpa1p reaches the plasma membrane in cells that do not express Gbetagamma or cell surface receptors [14].
• In this case, Gpa1 accumulates in both the cytoplasm and vacuole [15].

Associations of GPA1 with chemical compounds

• Introduction of the GPA1 gene encoding an S. cerevisiae homolog of the alpha subunit of mammalian guanine nucleotide-binding regulatory proteins (G proteins) into sterile dac1 mutants resulted in restoration of pheromone responsiveness and mating competence to both a and alpha cells [10].
• A mutation of the GPA1 protein, GPA1Val-50, in which Gly-50 was replaced by valine, could complement the growth defect of a GPA1 disruption, gpal::HIS3 [16].
• Upon regulation of expression of GPA1 by the galactose-inducible GAL1 promoter, the loss of GPA1 function was found to lead to cell-cycle arrest at the late G1 phase [17].
• We have isolated seven mutant alleles of GPA1 that confer pheromone resistance: G50D (a glycine-to-aspartate change at position 50), G322E, G322R, E355K, E364K, G470D, and an E364K-G470D double mutant [18].
• A number of genes have been implicated in the regulation of laccase and melanization, including IPC1, GPA1, MET3, and STE12 [19].

Physical interactions of GPA1

• Growth arrest in G1 phase is activated by the Ste4p/Ste18p complex via a downstream pathway and it is negatively controlled by the Scg1p subunit [20].
• Golf complements a GPA1 null mutation in Saccharomyces cerevisiae and functionally couples to the STE2 pheromone receptor [21].
• We also show that signaling by activated Gpa1 requires direct coupling to an RNA binding protein Scp160 [22].
• Mutation of the Q323 residue of Gpa1p resulted in constitutive activation of the pheromone response pathway and eliminated the ability to interact with Ste4p, consistent with a defect in GTPase activity [23].

Regulatory relationships of GPA1

• CLN2 overexpression suppressed the constitutive signaling and division-arrest phenotypes of cells with a disrupted gpa1 gene, indicating that the site of action for repression is downstream of the alpha-subunit (Gpa1p) of the heterotrimeric G protein [24].
• In addition, overexpression of these mutant versions of STE18 causes a dominant negative phenotype and inhibits the constitutive mating response generated by GPA1 deletion in cells which contain a functional STE18 gene [25].
• The dac2 mutation could suppress the lethality caused by the disruption of the GPA1 gene (previously shown to encode a protein with similarity to the alpha subunit of mammalian G proteins) [26].
• The Saccharomyces cerevisiae RGS protein Sst2p is involved in desensitization to pheromone and acts as a GTPase-activating protein for the Galpha subunit Gpa1p [27].
• The stability of Galpha/Gpa1 is influenced by the ubiquitin-dependent N-end rule pathway, suggesting that the regulation of G alpha levels may be important for effective mating response and recovery [28].

Other interactions of GPA1

• Wild-type STE4 and STE18 gene products were not essential for membrane localization of the GPA1 gene product, as indicated by cell fractionation and immunological analyses, suggesting that G beta and G gamma subunits interact with the receptor or make the G alpha subunit competent to associate correctly with the receptor, or both [29].
• These results suggest that the GPA1 protein functions downstream of the STE2 receptor [30].
• Furthermore, the steady-state level of the FUS1 transcript, which normally increases in response to mating factors, was also elevated when the GPA1 function was impaired [30].
• Cells from which AKR1 had been deleted were alive but misshapen at 30 degrees C and inviable at 37 degrees C. During a screen for mutants that required one or more copies of wild-type AKR1 for survival at 30 degrees C, we isolated mutations in GPA1, which encodes the G alpha subunit of the pheromone receptor-coupled G protein [31].
• Of the two yeast G-protein alpha-subunits (GPA1 and GPA2), only GPA1 has been well studied and shown to negatively regulate the mitogen-activated protein kinase pathway upon pheromone stimulation [32].

Analytical, diagnostic and therapeutic context of GPA1

• After treatment of MATa cells with alpha factor, the myristoylated form of Gpa1p increases dramatically, and the unmyristoylated form decreases concomitantly [33].
• We have isolated a gene, GPA1, from Cryptococcus neoformans by the PCR technique [34].
• Our results provide genetic validation of DAC1 and DAC3 as potential chemotherapy targets and demonstrate that T. brucei expresses at least three probable histone deacetylases with distinct function [35].
• In gene targeting experiments, DAC1 and DAC3 appear to be essential whereas DAC2 and DAC4 are not required for viability [35].
• Sequence analysis of a 4-kb genomic clone containing NAG1 indicates that this gene is part of a cluster containing two other genes of the GlcNAc catabolic pathway, i.e., DAC1, GlcNAc-6-phosphate deacetylase, and HXK1, hexokinase [36].

References