Imported:YPM/Pheromone-dependent cell cycle arrest

Category:Yeast Pheromone Response Model Back to main model page

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• Deletion of Far1 eliminates pheromone-dependent cell cycle arrest. Chang and Herskowitz. 1990 PMID 2147873
  • Deletion of Cln2 restores the pheromone-dependent arrest of far1^- cells.

• Overexpression of Far1 does not cause cell cycle arrest in the absence of pheromone but does in the presence of pheromone, suggesting that Far1 by itself (or unmodified) is insufficient to cause arrest. Chang and Herskowitz. 1992 PMID 1498364

• Far1 is phosphorylated in the absence of pheromone, and further phosphorylated in the presence of pheromone. Chang and Herskowitz. 1992 PMID 1498364

• Although Far1 mRNA and protein levels both vary over the cell cycle, their levels are independently regulated. McKinney et al. 1993 PMID 8491380

• Far1 coprecipitates with Cdc28 and Cln2 in a pheromone-dependent manner. Peter et al. 1993 PMID 8500168
  • Deletion of Fus3 greatly reduces the coprecipitation of Far1 with Cln2.
  • Because deletion of Fus3 eliminates pheromone-dependent phosphorylation of Far1, this suggests that Far1 requires phosphorylation by Fus3 for efficient association with Cdc28-Cln2.

• After pheromone treatment, Far1 is phosphorylated by Cdc28. Peter et al. 1993 PMID 8500168
  • Far1 was not detected prior to pheromone treatment, so may be constitutively phosphorylated by Cdc28.

• Far1's ability to cause cell-cycle arrest correlates with its ability to bind Cdc28. Peter et al. 1993 PMID 8500168
  • Different Far1 deletion mutants either displayed no ability to cause pheromone-dependent cell cycle arrest and no ability to bind Cdc28, or caused pheromone-dependent cell cycle arrest and coprecipitated Cdc28.

• Far1 associates weakly with Cln1, Cln2 and Cln3 in the absence of pheromone, and more stably in the presence of pheromone. Tyers and Futcher. 1993 PMID 8395009

• Far1 inhibits Cdc28-Cln1 and Cdc28-Cln2 via its association with Cdc28. Peter and Herskowitz. 1994 PMID 8066461
  • Far1 (purified from cells where the mating pathway is induced by Ste4 overexpreession) inhibits Cdc28-Cln1 and Cdc28-Cln2 in vitro. However, the authors didn't show whether Far1 from vegetative cells was able to inhibit Cdc28-Cln2.
  • Purified Far1-S87P inhibits Cdc28-Cln1 and Cdc28-Cln2 in vitro. The authors didn't show whether WT Far1 inhibits Cdc28-Cln1/2.
  • Purified Far1 does not inhibit Cdc28-Cln3 in vitro. (see Tyers and Futcher. 1993 PMID 8395009)
  • Purified Far1-S87P does not inhibit Cdc28-Clb2 or Cdc28-Clb5 in vitro.
  • A Far1 mutant that binds weakly to Cdc28 only inhibits Cdc28-Cln2 activity in vitro at high concentration.
  • Far1-S87P, a mutant that is constitutively active, inhibits Cdc28-Cln1 and Cdc28-Cln2 in vitro, and causes cell cycle arrest in the absence of pheromone when expressed in vivo.

• The N-terminus of Far1 appears to be important for Cdc28 dependent degradation of Far1. McKinney & Cross. 1995 PMID 7739534
  • Overexpressed Far1 is rapidly degraded in WT cells and in cdc28-4 cells at the permissive temperature, but is much more stable in cdc28-4 cells at the restrictive temperature.
  • N-terminal truncations stabilize Far1.
  • Far1 levels drop rapidly at cell cycle Start. The levels of a Far3 N-terminal deletion mutant, Far1(Δ1-30), is only modestly decreased at cell cycle Start. This change in abundance may be due to a decrease in Far1 mRNA abundance.

• Cell cycle dependent transcription of Far1 is not required for cell cycle arrest in response to pheromone. Oehlen et al. 1996 PMID 8649392
  • Deletion of Far1 eliminates pheromone-dependent arrest, and constitutive expression of Far1 restores pheromone-dependent arrest.

• Far1 is phosphorylated in vitro on S87 by Cdc28-Cln2, and mutation of this residue in vivo eliminates cell cycle dependent degradation of Far1. Henchoz et al. 1997 PMID 9367986
  • In vitro and in vivo analysis demonstrates that Far1 is ubiquitinated and degraded by the G1-S degradation system, involving Cdc34, Cdc53, Cdc4 and Skp1.
  • In vitro, Cdc28-Cln2 is required for Far1 ubiquitination.
  • Mutation of S87 prevents ubiquitination of Far1 in vitro.

• Deletion of Ste7 or Kss1 and Fus3 eliminates coprecipitation of Cln2 with Far1 in the presence of pheromone. Gartner et al. 1998 PMID 9632750

• Fus3 likely phosphorylates Far1 primarily on T306. Gartner et al. 1998 PMID 9632750
  • Deletion of Ste7 or Kss1 and Fus3 eliminates the pheromone-induced mobility shift of Far1.
  • Mutation of T306 to alanine also eliminates the pheromone-induced mobility shift of Far1.

• Evidence suggests that phosphorylation of S87 on Far1 is responsible for rapid degradation of Far1 outside of G1. Gartner et al. 1998 PMID 9632750

• Phosphorylation of Far1 on T306 by Fus3 is required for the cell cycle arrest promoting ability of Far1. Gartner et al. 1998 PMID 9632750
  • The T306A Far1 mutant is as insensitive to pheromone as complete deletion of Far1.
  • Far1(S87A T306A), which also contains the S87A mutation which stabilizes Far1, has the same sensitivity to pheromone as Far1(T306A), suggesting that increased abundance cannot rescue the function of Far1(T306A).
  • Mutation of T63 to alanine increases pheromone resistance, suggesting that phosphorylation at T63 (by Fus3?) may also activate Far1.
  • Far1(T306A) fails to coprecipitate with Cln2, suggesting that phosphorylation at T306 is required for its interaction with Cln2.
  • Far1(T306A) does not accumulate at higher levels than Far1 like Far1(S87A) does, indicating that although Far1(T306A) cannot bind Cdc28-Cln2, it can still be phosphorylated at S87 and be consequently degraded.

• Cdc28-Cln2 activity is not inhibited in vitro by Far1. Gartner et al. 1998 PMID 9632750
  • Cln2 was expressed at endogenous levels from the Cln3 promoter, which unlike the Cln2 promoter is not pheromone-regulated.

• Cln2 expression is repressed by pheromone treatment in a Far1-independent manner. Valdivieso et al. 1993 PMID 8423774
  • cln1Δ cln2Δ cln3Δ, with Cln3 expression off a Gal promoter halted arrest in G1, with some continued expression off of the Cln2 promoter (expressing a reporter gene).
  • Addition of pheromone causes a loss of expression off the Cln2 promoter, regardless of whether the cells contain Far1.
  • This suggests that pheromone-treatment causes repression of Cln2 expression in a Far1-independent manner.

• Hyperactive Ste11-4 restores cell cycle arrest to far1Δ cln2Δ cells. Cherkasova et al. 1999 PMID 10049917
  • This effect is dependent on the kinase activity of Fus3, and to a lesser extent Kss1.
  • Not all cells arrest in G1, based on the presence of budded cells.
  • Restored arrest is due to MAPK-based repression of Cln1 and Cln2 expression.
  • Clb5 expression is also repressed.