Ste4:Ste18

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About Ste4

Ste4 is multiply phosphorylated in a pheromone dependent manner, but these phosphorylations have no detectable effect on response. Li et al. 1998 PMID 9671029

Ste5 anf Far1 both bind to Ste4 with homologous RING-H2 domains, suggesting that both Far1 and Ste5 would not be able to simultaneoulsy bind a single molecule of Ste4. Sette et al. 2000 PMID 11071925

Sucrose density gradient fractionation suggests that the majority of Ste4 is associated with the plasma membrane and not other internal membranes. Song et al. 1996 PMID 8702760
- The majority of Ste4 that associates with membranes cosediments with Pma1 (an integral plasma membrane protein marker), suggesting that the majority of Ste4 is associated with the plasma membrane.
- The authors did not attempt to measure Ste4 in the soluble fraction (not associated with membranes).

Renografin density gradient fractionation suggests that about 30% of Ste4 is in the soluble fraction, 40% is associated with the plasma membrane, and 30% is associated with other parts of the pellet. Hirschman et al. 1997 PMID 8995254
- Pma1 was used as a marker for the plasma membrane fractions.
- The Renografin concentrations used have an ionic strength equivalent to 0.5 M NaCl. This could disrupt weak protein association with the plasma membrane.

Ste18 is farnesylated (prenylated) at C107, and palmitoylated (thioacylated) at C106. Manahan et al. 2000 PMID 10712512
Farnesylation at C107 is required for palmitoylation at C106. Manahan et al. 2000 PMID 10712512
Defect in palmitoylation results in compromised mating ability, and defect in farnesylation (which causes a defect in palmitoylation as well) results in a sterile phenotype. Manahan et al. 2000 PMID 10712512

Total amount of G protein per cell was measured by quantitative Western blots and fluorescent quantification to be 10000 per cell. Yi et al. 2003 PMID 12960402

Total Ste4:Ste18 concentration of 0.2 nM (which equates to ~7800 moleculse per cell for a 5 μm cell) was used in a model (no source). Hao et al. 2003 PMID 12968019

Ste4 and Ste18 have a half-life of greater than 3 hours. Hirschman et al. 1997 PMID 8995254
- Since Ste4 and Ste18 are so stable, the major form of loss of Ste4:Ste18 is not due to degradation, but rather due to dilution. See Protein dilution/synthesis due to cell growth for more details.

Ste4 and Ste18 are required for eachother's stability. Hirschman et al. 1997 PMID 8995254
- Deletion of Ste18 results in 50% lower Ste4 levels, and in these cells Ste4 has a half-life of about 90 min.
- Deletion of Ste4 causes Ste18 levels to drop such that it is barely detectable.

Ste4(Gpa1_site, Ste5_site, Ste20_site)

Ste4(Gpa1_site, Ste5_site, Ste20_site) Ste4_tot_conc