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Gene Review

STE18  -  Ste18p

Saccharomyces cerevisiae S288c

Synonyms: Guanine nucleotide-binding protein subunit gamma, J1866, YJR086W
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High impact information on STE18

  • Sequencing of the cloned genes revealed that the STE4 polypeptide shows extensive homology to the beta subunits of mammalian G proteins, while the STE18 polypeptide shows weak similarity to the gamma subunit of transducin [1].
  • 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 [2].
  • We conclude that tight membrane attachment of the wild-type Gbetagamma depends on palmitoylation at Cys 106 and prenylation at Cys 107 of Ste18p [3].
  • The Gbetagamma subunit (a complex of Ste4p and Ste18p) is associated with both internal and plasma membranes, and a portion is not stably associated with either membrane fraction [3].
  • The Cys 107 substitution also resulted in reduced steady-state accumulation of Ste18p, suggesting that the stability of Ste18p requires modification at Cys 107 [3].

Biological context of STE18

  • On activation by binding of pheromone to a serpentine receptor, the G(betagamma) (Ste4, Ste18) dimer transmits the signal to a mitogen-activated protein kinase cascade, leading to gene activation, arrest in the G(1) stage of the cell cycle, production of shmoos (mating projections), and cell fusion [4].
  • However, the phenotypes of the STE5Hyp mutations were less pronounced in ste4 and ste18 mutants, suggesting that the STE5Hyp-generated signal partially depends on the proposed G beta gamma complex [5].
  • Cell cycle arrest mediated by STE4 overexpression was prevented in cells that either were overexpressing the SCG1 gene product (the alpha subunit of the G protein) or lacked the STE18 gene product (the gamma subunit of the G protein) [6].
  • The cloned STE18 gene was subjected to a saturation mutagenesis using doped oligonucleotides [7].
  • In contrast, cells mutant for the receptor (ste2) or the beta or gamma subunit (ste4 and ste18) of the G protein were extremely defective in both diploid and prezygote formation and discriminated poorly between signaling and nonsignaling mating partners, implying that these components are important for chemotropism [8].

Anatomical context of STE18

  • We found that the products of the STE4 and STE18 genes are stably associated with plasma membrane as well as with internal membranes and that 30% of the protein pool is not tightly associated with either membrane fraction [9].

Associations of STE18 with chemical compounds

  • Like Ras, Ste18p contains a farnesyl-directing CaaX box motif (C-terminal residues 107 to 110) and a cysteine residue (Cys 106) that is a potential site for palmitoylation [3].
  • Mutant Ste18p containing serine at position 106 (mutation ste18-C106S) migrated more rapidly than wild-type Ste18p during sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) [3].
  • The yeast G protein gamma subunit (Ste18p) is unusual among G(gamma) subunits because it is farnesylated at cysteine 107 and has the potential to be thioacylated at cysteine 106 [10].
  • Deletion of the C-terminal TLM sequence or modification of the ultimate methionine to lysine, arginine or leucine, all changes which do not affect the CAAX box cysteines, have only minor effects on Ste18-dependent mating [11].

Physical interactions of STE18

  • The identification of a ste18 mutant indicated that this screen could identify proteins that interact directly with Ste4p [12].

Regulatory relationships of STE18

  • However, STE18 was essential for the response, since overexpression of STE4 was unable to activate a response in a ste18 null strain [13].
  • 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 [11].

Other interactions of STE18

  • However, inactivation of the prenyltransferase by disruption of DPR1 has only a minor effect on Ste18-dependent mating [11].
  • One of these mutants has been mapped to chromosome X, 31 cM distal to SUP4, and defines a new locus designated STE18 [14].


  1. The STE4 and STE18 genes of yeast encode potential beta and gamma subunits of the mating factor receptor-coupled G protein. Whiteway, M., Hougan, L., Dignard, D., Thomas, D.Y., Bell, L., Saari, G.C., Grant, F.J., O'Hara, P., MacKay, V.L. Cell (1989) [Pubmed]
  2. Beta and gamma subunits of a yeast guanine nucleotide-binding protein are not essential for membrane association of the alpha subunit but are required for receptor coupling. Blumer, K.J., Thorner, J. Proc. Natl. Acad. Sci. U.S.A. (1990) [Pubmed]
  3. Dual lipid modification of the yeast ggamma subunit Ste18p determines membrane localization of Gbetagamma. Hirschman, J.E., Jenness, D.D. Mol. Cell. Biol. (1999) [Pubmed]
  4. Signal transduction by a nondissociable heterotrimeric yeast G protein. Klein, S., Reuveni, H., Levitzki, A. Proc. Natl. Acad. Sci. U.S.A. (2000) [Pubmed]
  5. Mutational activation of the STE5 gene product bypasses the requirement for G protein beta and gamma subunits in the yeast pheromone response pathway. Hasson, M.S., Blinder, D., Thorner, J., Jenness, D.D. Mol. Cell. Biol. (1994) [Pubmed]
  6. Overexpression of the STE4 gene leads to mating response in haploid Saccharomyces cerevisiae. Whiteway, M., Hougan, L., Thomas, D.Y. Mol. Cell. Biol. (1990) [Pubmed]
  7. Mutagenesis of Ste18, a putative G gamma subunit in the Saccharomyces cerevisiae pheromone response pathway. Whiteway, M., Dignard, D., Thomas, D.Y. Biochem. Cell Biol. (1992) [Pubmed]
  8. Mating in Saccharomyces cerevisiae: the role of the pheromone signal transduction pathway in the chemotropic response to pheromone. Schrick, K., Garvik, B., Hartwell, L.H. Genetics (1997) [Pubmed]
  9. The G beta gamma complex of the yeast pheromone response pathway. Subcellular fractionation and protein-protein interactions. Hirschman, J.E., De Zutter, G.S., Simonds, W.F., Jenness, D.D. J. Biol. Chem. (1997) [Pubmed]
  10. Dual lipid modification motifs in G(alpha) and G(gamma) subunits are required for full activity of the pheromone response pathway in Saccharomyces cerevisiae. Manahan, C.L., Patnana, M., Blumer, K.J., Linder, M.E. Mol. Biol. Cell (2000) [Pubmed]
  11. Site-directed mutations altering the CAAX box of Ste18, the yeast pheromone-response pathway G gamma subunit. Whiteway, M.S., Thomas, D.Y. Genetics (1994) [Pubmed]
  12. Genetic relationships between the G protein beta gamma complex, Ste5p, Ste20p and Cdc42p: investigation of effector roles in the yeast pheromone response pathway. Akada, R., Kallal, L., Johnson, D.I., Kurjan, J. Genetics (1996) [Pubmed]
  13. Stoichiometry of G protein subunits affects the Saccharomyces cerevisiae mating pheromone signal transduction pathway. Cole, G.M., Stone, D.E., Reed, S.I. Mol. Cell. Biol. (1990) [Pubmed]
  14. Expression of MF alpha 1 in MATa cells supersensitive to alpha-factor leads to self-arrest. Whiteway, M., Hougan, L., Thomas, D.Y. Mol. Gen. Genet. (1988) [Pubmed]
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