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].

References