Imported:YPM/Ste12 synthesis/degradation

Category:Reactions - Yeast Pheromone Response Model Back to main model page

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Ste12 Degradation

• Ste12 is ubiquitinated and degraded in response to high amounts of pheromone. Esch et al. 2006 PMID 17041188
  • Ste12 levels decrease about 1 hour after pheromone treatment (saturating amounts of pheromone), and drop to less than 10% of the initial amount by 4 hours after pheromone treatment.
  • Using cycloheximide, the half-life of Ste12 is ~230 min in the absence of pheromone, and ~25 min in the presence of pheromone.
  • In erg6Δ cells (defective for proteasome activity in the presence of inhibitor MG132), both pheromone and the inhibitor MG132 are required for the detection of poly-ubiquitinated Ste12. This shows that Ste12 is poly-ubiquitinated in response to pheromone, making it likely that this is the means of Ste12 degradation.
  • Deletion of Kss1 has a minor effect on the degradation of Ste12, whereas deletion of Fus3 or Far1 greatly affects Ste12 degradation.
  • Because Far1 affects Ste12 degradation, it is likely that Fus3 doesn't directly cause Ste12 degradation, but likely causes it indirectly through Far1.
  • Far1 inhibits Cdc28, and deactivation of Cdc28 (via at temperature sensitive Cdc28 allele) does not affect Ste12 degradation in response to pheromone, suggesting that Cdc28 is not responsible for pheromone-mediated Ste12 degradation.

• Contrary to the results of Ecsh et al. (2006, PMID 17041188), it was earlier found that Ste12 is not degraded in response to pheromone. Chou et al. 2004 PMID 15620356
  • However, the results are suspect because the authors found that Tec1 was degraded when cells were exposed to 5 μM pheromone, but not 2 μM pheromone.
  • Anything above 100 nM pheromone should be saturating, suggesting that the authors are not exposing their cells to the concentrations that they report, potentially due to adsorption of pheromone to the plasticware.
  • Esch et al. (2006 PMID 17041188) found that the amount of cellular Ste12 was not decreased when cells were exposed to low levels of pheromone, supporting the idea that Chou et al. were inadvertently exposing their cells to low, rather than high, amounts of pheromone.

Reaction Definition

The mechanism by which Fus3 and Far1 regulate Ste12 degradation is unclear. Because Fus3 (and not Kss1) phosphorylates Far1 to stabilize it, and is thus downstream of Fus3 in the mating pathway, it appears that it is likely not Fus3 itself but rather Far1, or some effector of Far1, that is directly responsible for Ste12 degradation. Because Far1 may not be included in all models, we will model the degradation of Ste12 as a saturable function of active Fus3. Because Kss1 is a much weaker activator of Far1, it is reasonable to assume that the contribution to Ste12 degradation from Kss1 is negligible.

Assumptions:

• Dig1, Dig2 or either MAPK is able to protect Ste12 from degradation (so only free, unbound Ste12 is degraded)
• Active Kss1 is also able to trigger degradation of Ste12, but at a much reduced rate.
• The MAPK/target interaction properties apply to the kdeg's (presumably caused by Fus3 and Kss1's phosphorylation activity) and Km's (caused by Fus3 and Kss1's ability to bind their targets).

<modelRxnFull><modelRxnRule>

Fus3(docking_site, T180~pT, Y182~none) + Ste12(Dig1_site, Dig2_site, MAPK_site) ->
Fus3(docking_site, T180~pT, Y182~none)

</modelRxnRule>

• Rate law: <modelRxnParam>Sat(kdeg_Fus3pT_Ste12, Km_deg_Fus3pT_Ste12)</modelRxnParam></modelRxnFull>

<modelRxnFull><modelRxnRule>

Fus3(docking_site, T180~none, Y182~pY) + Ste12(Dig1_site, Dig2_site, MAPK_site) ->
Fus3(docking_site, T180~none, Y182~pY)

</modelRxnRule>

• Rate law: <modelRxnParam>Sat(kdeg_Fus3pY_Ste12, Km_deg_Fus3pY_Ste12)</modelRxnParam></modelRxnFull>

<modelRxnFull><modelRxnRule>

Fus3(docking_site, T180~pT, Y182~pY) + Ste12(Dig1_site, Dig2_site, MAPK_site) ->
Fus3(docking_site, T180~pT, Y182~pY)

</modelRxnRule>

• Rate law: <modelRxnParam>Sat(kdeg_Fus3pTpY_Ste12, Km_deg_Fus3pTpY_Ste12)</modelRxnParam></modelRxnFull>

Ste12 Synthesis

• Ste12 is upregulated in response to pheromone treatment. Dolan and Fields. 1990 PMID 2193847

• Ste12 mRNA is weakly upregulated in response to pheromone treatment. Roberts et al. 2000 PMID 10657304; Ren et al. 2000 PMID 11125145

Reaction Definition

See Ste12 mediated protein synthesis