Fus3

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About Fus3

Fus3 is a small protein kinase (353 residues) that has 56% amino acid sequence identity with Kss1. Ma et al. 1995 PMID 7579701

Fus3 likely requires phosphorylation at both T180 and Y182 for kinase activity. This has been demonstrated for another MAPK (rat Erk2). Ferrell and Bhatt. 1997 PMID 9228083

Fus3 is localized evenly throughout the cytosol and nucleus both in the presence and absence of pheromone. Choi et al. 1999 PMID 10233162

Activated Fus3-Myc phosphorylates purified Ste12-Flag at the same rate as it phosphorylates His-Far1. The proteins were expressed and purified from insect cells. Breitkreutz et al. 2001 PMID 11525741

In the absence of pheromone, Fus3-GFP (expressed off the Gal promoter) is mainly found in the nucleus though there is some cytoplasmic fraction. This localization is unchanged in ste5Δ, ste7Δ, ste12Δ, dig1Δdig2Δ, and dig1Δdig2Δste12Δ cells, suggesting that none of these proteins function as nuclear or cytosolic anchors for Fus3-GFP. However, note that Fus3 may be in vast excess of these other proteins. van Drogen et al. 2001 PMID 11781566

2 hours after pheromone exposure Fus3-GFP is localized to the shmoo tip. There is still significant nuclear localization as well. van Drogen et al. 2001 PMID 11781566

FRAP (fluorescence return after photobleaching) studies of Fus3-GFP (expressed off the Gal promoter) show that it has a half time of re-entry into the nucleus of 4.2s (+/-0.8s) (half time of recovery to constant nuclear fluorescence). This rate is unaffected by pheromone and cycloheximide. The same rate was seen using catalytically inactive Fus3-GFP (K42R) and unphosphorylatable Fus3-GFP (T180A Y182F). van Drogen et al. 2001 PMID 11781566

Use of temperature sensitive GSP1 alleles (Gsp1 is a homolog of Ran) implicates the Ran-inportin-Β-system in Fus3-GFP nuclear transport. van Drogen et al. 2001 PMID 11781566

The recovery half-time for Fus3-GTP that has been photobleached at the shmoo tip is 0.32s (+/-0.8s). The half-time of recovery of Ste5-GFP at the shmoo tip is significantly longer (8.22s +/-1.3s), suggesting that Fus3 may rapidly dissociate from Ste5. van Drogen et al. 2001 PMID 11781566

Fus3-GFP nuclear localization is increased slightly by pheromone. Blackwell et al. 2003 PMID 12556475

Pathway activity is required for pheromone mediated change in Fus3-GFP localization. Blackwell et al. 2003 PMID 12556475
- Fus3^{T180A Y182A}-GFP fus3Δ kss1Δ cell show no difference in nuclear fluorescence upon pheromone exposure.
- Fus3^{T180A Y182A}-GFP fus3Δ KSS1 cell show increased nuclear fluorescence upon pheromone exposure.
- It has been shown that phosphorylation of one MAPK in a MAPK dimer in mammalian cells is necessary and suffcient for nuclear import.

Kap104 (a karyopherin) is implicated in Fus3 nuclear localization. Blackwell et al. 2003 PMID 12556475

fus3Δ strains exhibit hyperinvasiveness and higher basal and pheromone induced levels of Kss1 phosphorylation. Catalytically inactive Fus3 (K42R) strains also display elevated levels of Kss1 phosphorylation, indicating that this effect is not merely due to Fus3/Kss1 competition for activation. Sabbagh et al., 2001 PMID 11583529
- It is hard to figure out the reason for this. It is possible that this is an artifact of the participation of Fus3 (both through basal activity and pheromone induced activity) in inhibitory feedback loops (such as phosphorylation leading to degradation of Ste11).

Studies with ERK2 show that peptides containing the MAPK-docking site (from MEK1, MEK2, Ste7, Elk-1 and (MKP)-2) compete for binding to Erk2, suggesting that they all bind to the same region on ERK2. Bardwell et al. 2003 PMID 12529172

The consensus MAPK docking motif (found on MAPK targets and binding partners) is (R/K)_1-2X_4-6LXL. Remenyi et al. 2005 PMID 16364919

Deletion of Fus3 causes 4-fold increase in Β-galactosidase expression off of a FRE (Filamentous Response Element) over wild-type cells in the absence of pheromone. Addition of pheromone causes no change in expression Β-gal expression in WT cells, but a further 3-fold increase in Fus3Δ cells. Madhani et al. 1997 PMID 9393860

Fluorescently-tagged Fus3, expressed at wild-type levels, is 3x more concentrated in the nucleus than the cytoplasm in the absence and presence of pheromone. Maeder et al. 2007 PMID 17952059

2.5-3 hours after pheromone treatment, 43% of Fus3 is doubly-phosphorylated, 25% of Fus3 is mono-phosphorylated, and the remainder (32%) is unphosphorylated. Maeder et al. 2007 PMID 17952059

Phospho-Fus3 concentration is highest in the shmoo tip, and drops off in the body of the cell. Maeder et al. 2007 PMID 17952059
- The authors believe that the gradient in Fus3 concentration that they observe is due to phosphorylation of Fus3 at the shmoo tip, and dephosphorylation in the cytosol.
- Closer inspection of the phosphor-Fus3 gradient suggests that there is only a gradient in the shmoo itself, possibly due to a gradient of phopsho-Fus3 targets (binding sites) that is highest right at the tip, and drops off further away in the shmoo.
- If the observed gradient was due entirely to cytosolic dephosphorylation, then the presence of a flat gradient in the cell body would suggest that there is no (or very little) phospho-Fus3 in the cell body.
- However, if the phosph-Fus3 gradient in the shmoo tip was due to decreasing number of phospho-Fus3 targets (binding sites), then the cell body could still contain significant phospho-Fus3, and the gradient in the cell body would be shallow (or not observable) due to rapid diffusion in comparison to dephosphorylation.

Fus3(docking_site, T180~none~PO4, Y182~none~PO4)

Fus3(docking_site, T180~none, Y182~none) Fus3_tot_conc