Disease relevance of FUS3

• FUS3 has a positive role in conjugation, because overexpression of FUS3 increases the pheromone sensitivity of wild-type cells, while the absence of FUS3 causes sterility [1].
• We have isolated dominant gain-of-function (gf) mutations in FUS3, a Saccharomyces cerevisiae mitogen-activated protein (MAP) kinase homolog, that constitutively activate the yeast mating signal transduction pathway and confer hypersensitivity to mating pheromone [2].
• The presence of the interacting N1-Fus3p adapter increased the infectivity of Ste7p-N2-CT phages approximately 1400-fold, which makes SIP a promising technology for the detection and further investigation of interacting proteins [3].

High impact information on FUS3

• Notably, mutation of the phosphoacceptor site in Tec1, deletion of FUS3, or deletion of DIA2 results in a loss of signaling specificity such that pheromone pathway signaling erroneously activates filamentation pathway gene expression and invasive growth [4].
• T273 in a predicted high-affinity Cdc4 binding motif is phosphorylated by Fus3 both in vitro and in vivo [5].
• Degradation requires Fus3 kinase activity and a MAPK phosphorylation site in Tec1 at threonine 273 [4].
• Fus3 but not Kss1 induces Tec1 ubiquination and degradation through the SCFCdc4 ubiquitin ligase [5].
• In the absence of Fus3, there is erroneous crosstalk in which mating pheromone now activates filamentation-specific gene expression using the Kss1 MAPK [6].

Biological context of FUS3

• The suppression of a gpa1 null (G alpha subunit) by a fus3 null also suggests FUS3 is in the signal transduction pathway [1].
• FUS3 encodes a cdc2+/CDC28-related kinase required for the transition from mitosis into conjugation [1].
• This functional redundancy is strain dependent; some standard laboratory strains (S288C) are kss1-. FUS3 has additional functions required for cell cycle arrest and vegetative growth that do not overlap with KSS1 functions [7].
• Yeast homolog of mammalian mitogen-activated protein kinase, FUS3/DAC2 kinase, is required both for cell fusion and for G1 arrest of the cell cycle and morphological changes by the cdc37 mutation [8].
• In contrast, disruption of PTP3 in combination with its homolog PTP2 results in constitutive tyrosine phosphorylation, enhanced kinase activity of Fus3p MAP kinase on stimulation, and delayed recovery from the cell cycle arrest [9].

Anatomical context of FUS3

• After pheromone treatment, Bni1p-GFP and Spa2p failed to localize to the cortex of fus3 mutants, and cell wall growth became completely unpolarized [10].
• Ste5p remains stably bound at the plasma membrane, unlike activated Fus3p, which dissociates from Ste5p and translocates to the nucleus [11].
• In support of this possibility, we find that Fus3 is activated to a greater extent in a "wimp" strain with defective protein kinase A. Finally, BIM1 and BIK1 have been identified as CEN suppressors of far5, suggesting that the microtubule apparatus may regulate the ability of the pheromone response pathway to promote G1 arrest [12].

Associations of FUS3 with chemical compounds

• A FUS3 allele whose gene product is resistant to the HOG-mediated repression of its phosphotyrosine content has been isolated [13].
• These results suggest that the Arabidopsis cDNA encodes a putative serine/threonine kinase that can function in the mating response pathway upstream of FUS3/KSS1 in S. cerevisiae, at the level of STE11 gene [14].
• Ste5 copurifies with Ste11, Fus3, and a hypophosphorylated form of Ste7, and all four proteins cosediment in a glycerol gradient as if in a large complex [15].
• Here we show that expression of the guanine nucleotide exchange factor for Cdc42 SopE2 in Saccharomyces cerevisiae leads to the activation of Fus3 and Kss1 MAPKs, which operate in the mating and filamentation pathways, causing filamentous growth in haploid yeast cells [16].

Physical interactions of FUS3

• We also found that both Ste11p and Ste7p interact with Fus3p and Kss1p [17].
• Relative to Kss1, the MAPK Fus3 binds less strongly to Ste12 and is correspondingly a weaker inhibitor of invasive growth [18].

Enzymatic interactions of FUS3

• Ptp3p directly dephosphorylates and inactivates Fus3p MAP kinase in vitro [9].
• Fourth Ste5 is phosphorylated by Fus3 in purified complexes and copurifies with an additional protein kinase(s) [19].
• FUS3 and presumably KSS1 are phosphorylated and activated during pheromone induction by a STE7-dependent mechanism [20].
• We present evidence that, during mating, Fus3 phosphorylates Tec1 to downregulate this invasive growth-specific transcription factor and its target genes [21].

Regulatory relationships of FUS3

• CLN2 overexpression represses Fus3 kinase activity, independently of the phosphatase Msg5 [22].
• The dac2 mutation could suppress the lethality caused by the disruption of the GPA1 gene (previously shown to encode a protein with similarity to the alpha subunit of mammalian G proteins) [23].
• These observations suggest the possibility that Ste5 promotes signal transduction by tethering Fus3 to its activating protein kinase(s) [19].
• MAP kinase-related FUS3 from S. cerevisiae is activated by STE7 in vitro [24].
• Thus MSG5 might stimulate recovery from pheromone by regulating the phosphorylation state of FUS3 [25].

Other interactions of FUS3

• The predicted FUS3 protein is 35% identical to the cdc2+/CDC28 kinases and 52% identical to the KSS1 predicted kinase [1].
• The predicted Mpk1 protein (Mpk1p) shares 48 to 50% sequence identity with Xenopus MAP kinase and with the yeast mating pheromone response pathway components, Fus3p and Kss1p [26].
• Neither protein altered the ability of the mitogen-activated protein kinase (MAPK) Fus3 to feedback phosphorylate a known substrate, the MAPK kinase Ste7 [27].
• These data support a model in which FUS3 mediates transcription and G1 arrest by direct activation of STE12 and FAR1 and phosphorylates many other proteins involved in the response to pheromone [28].
• We find that Fus3p and Kss1p both control G1 arrest through multiple functions that operate in parallel with Far1p [29].

Analytical, diagnostic and therapeutic context of FUS3

• Based on fractionation and gradient density centrifugation properties, Fus3 exists in a number of complexes, with its activity critically dependent upon association with other proteins [30].
• By indirect immunofluorescence, Fus3 localizes in punctate spots throughout the cytoplasm and nucleus, with slightly enhanced nuclear localization after pheromone stimulation [30].
• Molecular cloning of the DAC2/FUS3 gene essential for pheromone-induced G1-arrest of the cell cycle in Saccharomyces cerevisiae [31].
• In gene targeting experiments, DAC1 and DAC3 appear to be essential whereas DAC2 and DAC4 are not required for viability [32].

References