Ssk22 (YCR073C) is a STE-family serine/threonine MAP kinase kinase kinase (MAP3K) of Saccharomyces cerevisiae and the paralog of Ssk2, with which it forms the functionally redundant MAP3K tier of the Sln1 branch of the high-osmolarity glycerol (HOG) pathway. Under hyperosmotic stress, unphosphorylated Ssk1 released from the Sln1-Ypd1 phosphorelay binds the N-terminal regulatory region of Ssk22 and activates its C-terminal kinase domain; Ssk22 then phosphorylates and activates the MAPKK Pbs2, whose N-terminal docking site binds the Ssk22 kinase domain, leading to activation of the p38-type MAPK Hog1. Ssk22 is a low-abundance protein, is strictly Ssk1-dependent (lacking the Ssk1-independent activation segment of Ssk2), and contributes less than Ssk2 to salt tolerance; it does not share the actin-binding and actin-recovery functions of Ssk2. No Ssk22-specific function or subcellular localization has been experimentally established.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004672 protein kinase activity | HDA PMID:16319894 Global analysis of protein phosphorylation in yeast. | ACCEPT | Summary: Proteome-chip in vitro kinase screen (Ptacek et al. 2005) detected Ssk22 kinase activity on substrates. Reason: Correct though general; high-throughput in vitro evidence that Ssk22 is catalytically active. The specific activity is MAP kinase kinase kinase activity (GO:0004709). Supporting Evidence: PMID:16319894 Here we describe, with the use of proteome chip technology, the in vitro substrates recognized by most yeast protein kinases file:yeast/SSK22/SSK22-uniprot.txt Activates the PBS2 MAP kinase kinase by phosphorylation. |
| GO:0004672 protein kinase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based protein kinase activity from the Ser/Thr kinase active-site signature. Reason: Correct but general; Ssk22 has a canonical STE-family kinase domain and activates Pbs2. Supporting Evidence: file:yeast/SSK22/SSK22-uniprot.txt Activates the PBS2 MAP kinase kinase by phosphorylation. |
| GO:0004674 protein serine/threonine kinase activity | IEA GO_REF:0000003 | ACCEPT | Summary: EC 2.7.11.1 mapping to protein serine/threonine kinase activity. Reason: Correct; Ssk22 is a Ser/Thr kinase. MAP kinase kinase kinase activity (EC 2.7.11.25, as for Ssk2) is the more specific description. Supporting Evidence: file:yeast/SSK22/SSK22-uniprot.txt Activates the PBS2 MAP kinase kinase by phosphorylation. |
| GO:0004709 MAP kinase kinase kinase activity | IEA GO_REF:0000002 | ACCEPT | Summary: MAPKKK activity inferred from IPR017240 (MAPKKK Ssk2/Ssk22). Reason: Core MF. Ssk22 activates Pbs2 redundantly with Ssk2; its kinase domain binds the Pbs2 docking site. Supporting Evidence: PMID:7624781 Pbs2p was activated by MAP kinase kinase kinases (MAPKKKs) Ssk2p and Ssk22p that are under the control of the SLN1-SSK1 two-component osmosensor. PMID:12853477 The Ssk2/Ssk22 MAPKKKs in the SLN1 branch, when activated, exclusively phosphorylate the Pbs2 MAPKK. |
| GO:0004709 MAP kinase kinase kinase activity | ISS PMID:7624781 Activation of yeast PBS2 MAPKK by MAPKKKs or by binding of a... | ACCEPT | Summary: ISS from Ssk2 (SGD:S000005314) supported by Maeda et al. 1995, which showed Pbs2 activation by Ssk2p and Ssk22p. Reason: Core MF. Ssk22-specific genetic evidence exists (SSK22 alone complements Sln1-branch signaling in ssk2 ssk22 backgrounds; the Ssk22 kinase domain binds Pbs2 RSD-I), but direct in vitro Pbs2 phosphorylation by purified Ssk22 has not been reported, so similarity to Ssk2 is an appropriate basis. Supporting Evidence: PMID:7624781 Pbs2p was activated by MAP kinase kinase kinases (MAPKKKs) Ssk2p and Ssk22p that are under the control of the SLN1-SSK1 two-component osmosensor. PMID:9742096 Either SSK2, SSK22, or STE11 MAPKKK can activate PBS2 by phosphorylation. PMID:12853477 The Pbs2 docking site constitutively bound the Ssk2/Ssk22 kinase domain. |
| GO:0005515 protein binding | IPI PMID:11805837 Systematic identification of protein complexes in Saccharomy... | REMOVE | Summary: HTP AP-MS (Ho et al. 2002) detected the Ssk22-Ssk1 interaction. Reason: Bare protein binding is uninformative. Ssk1 is the phosphorelay response regulator whose unphosphorylated form binds the N-terminal regulatory domain of Ssk2/Ssk22 and activates the kinase; the interaction is genuine and repeatedly detected, but GO has no informative MF for binding an upstream activator, and its functional content is captured by the MAP kinase kinase kinase activity annotations and the proposed GO:0007234 osmosensory signaling via phosphorelay pathway annotation. Removing this generic row does not mean the interaction is false. Supporting Evidence: PMID:18573873 Ssk1 binds to an N-terminal regulatory domain of the Ssk2/Ssk22 MAPKKKs, as demonstrated by both two-hybrid analyses and coprecipitation assays |
| GO:0005515 protein binding | IPI PMID:18719252 High-quality binary protein interaction map of the yeast int... | REMOVE | Summary: HTP yeast two-hybrid (Yu et al. 2008) detected the Ssk22-Ssk1 interaction. Reason: Bare protein binding is uninformative. Ssk1 is the phosphorelay response regulator whose unphosphorylated form binds the N-terminal regulatory domain of Ssk2/Ssk22 and activates the kinase; the interaction is genuine and repeatedly detected, but GO has no informative MF for binding an upstream activator, and its functional content is captured by the MAP kinase kinase kinase activity annotations and the proposed GO:0007234 osmosensory signaling via phosphorelay pathway annotation. Removing this generic row does not mean the interaction is false. Supporting Evidence: PMID:18573873 Ssk1 binds to an N-terminal regulatory domain of the Ssk2/Ssk22 MAPKKKs, as demonstrated by both two-hybrid analyses and coprecipitation assays |
| GO:0005515 protein binding | IPI PMID:20489023 A global protein kinase and phosphatase interaction network ... | REMOVE | Summary: HTP kinase/phosphatase AP-MS (Breitkreutz et al. 2010) detected the Ssk22-Ssk1 interaction. Reason: Bare protein binding is uninformative. Ssk1 is the phosphorelay response regulator whose unphosphorylated form binds the N-terminal regulatory domain of Ssk2/Ssk22 and activates the kinase; the interaction is genuine and repeatedly detected, but GO has no informative MF for binding an upstream activator, and its functional content is captured by the MAP kinase kinase kinase activity annotations and the proposed GO:0007234 osmosensory signaling via phosphorelay pathway annotation. Removing this generic row does not mean the interaction is false. Supporting Evidence: PMID:18573873 Ssk1 binds to an N-terminal regulatory domain of the Ssk2/Ssk22 MAPKKKs, as demonstrated by both two-hybrid analyses and coprecipitation assays |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: ATP binding inferred from protein kinase ATP-binding site signatures. Reason: Ssk22 is a protein kinase and ATP is its phosphate donor. Supporting Evidence: file:yeast/SSK22/SSK22-uniprot.txt Activates the PBS2 MAP kinase kinase by phosphorylation. |
| GO:0005575 cellular_component | ND GO_REF:0000015 | ACCEPT | Summary: SGD ND annotation: no experimental localization data for Ssk22. Reason: Accurately records that no Ssk22-specific localization study exists; the IEA cytoplasm annotation is a reasonable inference but not experimental. Supporting Evidence: file:yeast/SSK22/SSK22-deep-research-falcon.md No definitive Ssk22-specific imaging or fractionation study was identified in the retrieved literature. |
| GO:0005737 cytoplasm | IEA GO_REF:0000002 | ACCEPT | Summary: Cytoplasm inferred from IPR017240 membership. Reason: Reasonable: Ssk22 acts with the cytoplasmic Ssk1 and Pbs2, and its paralog Ssk2 is cytosolic; no Ssk22-specific imaging exists. Supporting Evidence: PMID:12853477 The Pbs2 docking site constitutively bound the Ssk2/Ssk22 kinase domain. |
| GO:0038066 p38MAPK cascade | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA placing Ssk2/Ssk22/MEKK4-family MAP3Ks in the p38 (Hog1) MAPK cascade. Reason: GO:0038066 explicitly covers the Hog1 cascade in yeast; Ssk22 is one of the MAP3Ks. Ssk22 appearing among the donors reflects its own experimental annotation and is expected. Supporting Evidence: PMID:7624781 Pbs2p was activated by MAP kinase kinase kinases (MAPKKKs) Ssk2p and Ssk22p that are under the control of the SLN1-SSK1 two-component osmosensor. PMID:23028184 activated Ssk2/Ssk22 exclusively phosphorylates, and thereby activates, Pbs2, and activated Pbs2 phosphorylates only Hog1. |
| GO:0038066 p38MAPK cascade | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA prediction of p38MAPK cascade. Reason: Consistent with experimental evidence. Supporting Evidence: PMID:23028184 activated Ssk2/Ssk22 exclusively phosphorylates, and thereby activates, Pbs2, and activated Pbs2 phosphorylates only Hog1. |
| GO:0038066 p38MAPK cascade | IGI PMID:7624781 Activation of yeast PBS2 MAPKK by MAPKKKs or by binding of a... | ACCEPT | Summary: Genetic interaction with PBS2 places Ssk22 upstream of Pbs2 in the Hog1 cascade. Reason: Core process; SGD experimental annotation consistent with later work. Supporting Evidence: PMID:7624781 Pbs2p was activated by MAP kinase kinase kinases (MAPKKKs) Ssk2p and Ssk22p that are under the control of the SLN1-SSK1 two-component osmosensor. |
| GO:0038066 p38MAPK cascade | IGI PMID:9742096 Requirement of STE50 for osmostress-induced activation of th... | ACCEPT | Summary: Genetic interactions with PBS2 and SSK2: SSK22 alone complements the osmosensitivity and Hog1 activation defect of ssk2 ssk22 ste50 cells. Reason: Core process; full text shows Ssk22 can independently activate Pbs2. Supporting Evidence: PMID:9742096 Either SSK2, SSK22, or STE11 MAPKKK can activate PBS2 by phosphorylation. PMID:9742096 The three MAPKKKs (SSK2, SSK22, and STE11) can in turn independently activate the PBS2 MAPKK, the activator of the HOG1 MAPK. |
| GO:0038066 p38MAPK cascade | IMP PMID:7624781 Activation of yeast PBS2 MAPKK by MAPKKKs or by binding of a... | ACCEPT | Summary: Mutant analysis of Pbs2 activation by Ssk2 and Ssk22. Reason: Core process. Supporting Evidence: PMID:7624781 Pbs2p was activated by MAP kinase kinase kinases (MAPKKKs) Ssk2p and Ssk22p that are under the control of the SLN1-SSK1 two-component osmosensor. |
| GO:0051403 stress-activated MAPK cascade | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based stress-activated MAPK cascade. Reason: Correct: the HOG cascade is a stress-activated MAPK cascade; GO:0038066 is the more specific description. Supporting Evidence: PMID:23028184 activated Ssk2/Ssk22 exclusively phosphorylates, and thereby activates, Pbs2, and activated Pbs2 phosphorylates only Hog1. |
| GO:0106310 protein serine kinase activity | IEA GO_REF:0000116 | ACCEPT | Summary: Rhea-based protein serine kinase activity. Reason: Correct; MAP kinase kinase kinase activity is the more informative term. Supporting Evidence: file:yeast/SSK22/SSK22-uniprot.txt Activates the PBS2 MAP kinase kinase by phosphorylation. |
| GO:0007234 osmosensory signaling via phosphorelay pathway | IGI PMID:23457455 Ssk1p-independent activation of Ssk2p plays an important rol... | NEW | Summary: Ssk22 is activated by the Ssk1 response regulator of the Sln1-Ypd1-Ssk1 phosphorelay; unlike Ssk2 it has no Ssk1-independent activation route. Reason: Ssk22 performs the MAP3K step of the Sln1 phosphorelay osmosensory pathway: unphosphorylated Ssk1 binds its N-terminal regulatory domain and activates it, and Hog1 activation via Ssk22 in ste11 ssk2 backgrounds strictly requires Ssk1. Comparator check: the paralog Ssk2, in the identical role, carries GO:0007234 by SGD IGI/IPI (PMID:9482735), whereas downstream Pbs2 and Hog1 do not, so the convention is that the phosphorelay components plus the directly coupled MAP3Ks carry the term. The term is neither ancestor nor descendant of GO:0038066. Supporting Evidence: PMID:23457455 The activation of Ssk22p, on the other hand, was totally dependent on Ssk1p. PMID:18573873 Ssk1 binds to an N-terminal regulatory domain of the Ssk2/Ssk22 MAPKKKs, as demonstrated by both two-hybrid analyses and coprecipitation assays PMID:7624781 Pbs2p was activated by MAP kinase kinase kinases (MAPKKKs) Ssk2p and Ssk22p that are under the control of the SLN1-SSK1 two-component osmosensor. |
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Download this section (compressed HTML)Q: Does Ssk22 have any function not shared with Ssk2, and why is it retained given its low abundance and weaker contribution to salt tolerance?
Q: Does purified, Ssk1-activated Ssk22 phosphorylate Pbs2 on both Ser514 and Thr518 as the Ssk2/Ssk22 pair does?
Experiment: In vitro kinase assay of immunopurified Ssk22 (wild type vs kinase-dead) on kinase-dead Pbs2 with Ser514/Thr518 phospho-specific detection, to convert the ISS MAP3K annotation to direct evidence.
Experiment: Endogenously tagged Ssk22-GFP imaging before and after osmotic shock to establish its subcellular localization.
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Download this section (compressed HTML)MAP kinase kinase kinase of HOG1 mitogen-activated signaling pathway; functionally redundant with Ssk2p; interacts with and is activated by Ssk1p; phosphorylates Pbs2p; SSK22 has a paralog, SSK2, that arose from the whole genome duplication
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