Sln1 (YIL147C) is the plasma membrane hybrid sensor histidine kinase that acts as the osmosensor/turgor sensor of the SLN1 branch of the high-osmolarity glycerol (HOG) pathway in Saccharomyces cerevisiae. It spans the membrane twice, with an extracellular domain between the two transmembrane helices that promotes dimerization and activity, and a cytoplasmic HAMP region, HisKA/HATPase transmitter and C-terminal CheY-like receiver domain. At normal osmolarity Sln1 is active: it autophosphorylates His576 using ATP, transfers the phosphoryl group to its own receiver Asp1144, and from there to His64 of the histidine phosphotransfer protein Ypd1, which phosphorylates the response regulators Ssk1 (Asp554) and Skn7 (Asp427). Phosphorylated Ssk1 cannot activate the MAPKKKs Ssk2/Ssk22, so the Pbs2-Hog1 MAPK cascade is kept off. Hyperosmotic stress, or any reduction of turgor, inactivates Sln1; Ssk1 then becomes dephosphorylated and activates Ssk2/Ssk22, Pbs2 and Hog1. Via Ypd1 and the nuclear transcription factor Skn7, Sln1 also controls transcription of cell-wall-related genes such as OCH1, which increases under hypo-osmotic conditions. Loss of SLN1 is lethal because of constitutive HOG pathway activation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000155 phosphorelay sensor kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference that Sln1 is a two-component sensor histidine kinase; Sln1 autophosphorylates His576 in response to (the absence of) osmotic stress and relays the phosphate onward. Reason: This is the most precise MF for Sln1 and the module-level function. Sln1 is the founding eukaryotic example of a phosphorelay sensor kinase: ATP-dependent autophosphorylation of His576 is regulated by turgor/osmolarity and initiates the Sln1-Ypd1-Ssk1/Skn7 relay. The phylogenetic placement is sound; Sln1 is squarely inside the hybrid sensor kinase clade. Supporting Evidence: PMID:8808622 This phosphorelay system is initiated by the autophosphorylation of Sln1p at His576. PMID:12821642 Loss of turgor inactivates the Sln1 histidine kinase activity, leading to subsequent phosphorylation and activation of the HOG MAPK pathway. file:yeast/SLN1/SLN1-uniprot.txt Histidine kinase that acts as an osmosensor at the plasma membrane. |
| GO:0000155 phosphorelay sensor kinase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro (HisKA dimerisation/phosphoacceptor domain) mapping to phosphorelay sensor kinase activity. Reason: Consistent with experimental and IBA evidence; Sln1 has the HisKA/HATPase transmitter with the conserved phosphoacceptor His576. Supporting Evidence: PMID:8808622 This phosphorelay system is initiated by the autophosphorylation of Sln1p at His576. file:yeast/SLN1/SLN1-uniprot.txt Histidine kinase that acts as an osmosensor at the plasma membrane. |
| GO:0000160 phosphorelay signal transduction system | IBA GO_REF:0000033 | ACCEPT | Summary: Sln1 participates in a phosphorelay (two-component) signal transduction system. Reason: Correct; Sln1 initiates the His576->Asp1144->Ypd1 His64->Ssk1 Asp554/Skn7 Asp427 phosphorelay. The more specific experimentally supported child is GO:0007234 osmosensory signaling via phosphorelay pathway. Supporting Evidence: PMID:8808622 This phosphate is then sequentially transferred to Sln1p-Asp-1144, then to Ypd1p-His64, and finally to Ssk1p-Asp554. PMID:9843501 the Sln1 kinase and Ypd1 phosphorelay intermediate regulate the activity of two distinct response regulators, Ssk1p and Skn7p. |
| GO:0000160 phosphorelay signal transduction system | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro receiver-domain mapping to phosphorelay signal transduction system. Reason: Correct; the C-terminal receiver domain (Asp1144) is an obligate intermediate in the relay. Supporting Evidence: PMID:12455952 Sln1R is an obligate intermediate in the phosphorelay reaction between Sln1p and Ypd1p PMID:8808622 This phosphate is then sequentially transferred to Sln1p-Asp-1144, then to Ypd1p-His64, and finally to Ssk1p-Asp554. |
| GO:0004673 protein histidine kinase activity | IDA PMID:8808622 Yeast HOG1 MAP kinase cascade is regulated by a multistep ph... | ACCEPT | Summary: Posas et al. demonstrated ATP-dependent autophosphorylation of Sln1 on His576 (abolished by H576Q) and relay onward. Reason: Direct biochemical evidence of protein histidine kinase activity (autophosphorylation). The more specific GO:0000155 phosphorelay sensor kinase activity better captures the sensory context, but this parent term is fully correct. Supporting Evidence: PMID:8808622 This phosphorelay system is initiated by the autophosphorylation of Sln1p at His576. PMID:12455952 Sln1p consists of a sensor kinase module that undergoes histidine autophosphorylation and a receiver module that autocatalytically transfers the phosphoryl group from histidine to aspartate. |
| GO:0004673 protein histidine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: EC 2.7.13.3 / ARBA mapping to protein histidine kinase activity. Reason: Correct; consistent with experimental data. Supporting Evidence: PMID:8808622 This phosphorelay system is initiated by the autophosphorylation of Sln1p at His576. |
| GO:0005034 osmosensor activity | IDA PMID:12821642 Yeast osmosensor Sln1 and plant cytokinin receptor Cre1 resp... | ACCEPT | Summary: Reiser et al. showed that Sln1 kinase activity is inhibited by reductions in turgor (hyperosmotic stress, nystatin, cell wall removal), and that the periplasmic domain is essential for sensing. Reason: Sln1 is a transmembrane protein whose kinase output is set by extracellular osmolarity/turgor; this matches the GO definition (sensing extracellular osmolarity, spanning the membrane). Together with GO:0000155 it defines the core sensory function. Supporting Evidence: PMID:12821642 Loss of turgor inactivates the Sln1 histidine kinase activity, leading to subsequent phosphorylation and activation of the HOG MAPK pathway. PMID:10198019 These results demonstrate that only full-length Sln1p is capable of altering kinase activity in response to differences in medium osmolarity. |
| GO:0005515 protein binding | IPI PMID:14656441 The yeast YPD1/SLN1 complex: insights into molecular recogni... | REMOVE | Summary: Crystal structure of Ypd1 in complex with the Sln1 receiver domain (Sln1 residues 1087-1220). Reason: Bare protein binding is uninformative. The Sln1-Ypd1 interaction is real and well established (structure, and Ypd1 is the phosphoacceptor of Sln1-Asp1144), but its functional meaning, phosphotransfer from the Sln1 receiver to Ypd1 His64, is already captured by GO:0000155 phosphorelay sensor kinase activity and the phosphorelay process terms; there is no informative binding MF that adds to this. Removal does not imply the interaction is false. Supporting Evidence: PMID:14656441 Here we report the first crystal structure of a prototypical monomeric histidine-containing phosphotransfer (HPt) protein YPD1 in complex with its upstream phosphodonor, the response regulator domain associated with SLN1. PMID:8808622 Ypd1p binds to both Sln1p and Ssk1p and mediates the multistep phosphotransfer reaction (phosphorelay). |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Sln1 acts at the plasma membrane. Reason: Well supported by fractionation and GFP localization; Sln1 is a two-TM integral plasma membrane sensor. Supporting Evidence: PMID:10198019 Wild-type cells and strains expressing the truncation constructs from low-copy-number vectors show a higher percentage (∼80%) of Sln1p in the plasma membrane fraction PMID:14665464 Sln1p-GFP localized to the rim of the cell, consistent with the expected plasma membrane localization of the sensor kinase |
| GO:0005886 plasma membrane | IDA PMID:10198019 The extracellular domain of the Saccharomyces cerevisiae Sln... | ACCEPT | Summary: Subcellular fractionation shows ~80% of wild-type Sln1 in the plasma membrane fraction. Reason: Direct experimental localization, consistent with GFP imaging and topology. Supporting Evidence: PMID:10198019 Wild-type cells and strains expressing the truncation constructs from low-copy-number vectors show a higher percentage (∼80%) of Sln1p in the plasma membrane fraction PMID:10198019 The SLN1 gene encodes an enzyme with histidine kinase and aspartate phosphotransferase activities and functions as a plasma membrane sensor. |
| GO:0005886 plasma membrane | IDA PMID:14665464 Saccharomyces cerevisiae histidine phosphotransferase Ypd1p ... | ACCEPT | Summary: Functional Sln1-GFP localizes to the cell rim, consistent with plasma membrane localization. Reason: Direct experimental localization with a complementing fusion. Supporting Evidence: PMID:14665464 Sln1p-GFP localized to the rim of the cell, consistent with the expected plasma membrane localization of the sensor kinase |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: UniProt subcellular location (cell membrane) mapping. Reason: Correct and consistent with experimental data. Supporting Evidence: PMID:10198019 Wild-type cells and strains expressing the truncation constructs from low-copy-number vectors show a higher percentage (∼80%) of Sln1p in the plasma membrane fraction |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro mapping to the generic signal transduction term. Reason: Correct but very general; the specific process is GO:0007234 osmosensory signaling via phosphorelay pathway. Supporting Evidence: PMID:8808622 This phosphate is then sequentially transferred to Sln1p-Asp-1144, then to Ypd1p-His64, and finally to Ssk1p-Asp554. |
| GO:0007234 osmosensory signaling via phosphorelay pathway | IDA PMID:8808622 Yeast HOG1 MAP kinase cascade is regulated by a multistep ph... | ACCEPT | Summary: Posas et al. established the Sln1-Ypd1-Ssk1 multistep phosphorelay that controls the Hog1 MAPK cascade in response to osmolarity. Reason: Core process. Sln1 performs the first steps of the relay (His576 autophosphorylation, transfer to Asp1144 and to Ypd1), and its inactivation by hyperosmotic stress/loss of turgor is the input to the SLN1 branch of the HOG pathway. Supporting Evidence: PMID:8808622 This phosphate is then sequentially transferred to Sln1p-Asp-1144, then to Ypd1p-His64, and finally to Ssk1p-Asp554. PMID:12821642 Loss of turgor inactivates the Sln1 histidine kinase activity, leading to subsequent phosphorylation and activation of the HOG MAPK pathway. |
| GO:0007234 osmosensory signaling via phosphorelay pathway | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA mapping to osmosensory signaling via phosphorelay pathway. Reason: Correct; consistent with the experimental annotation. Supporting Evidence: PMID:8808622 This phosphate is then sequentially transferred to Sln1p-Asp-1144, then to Ypd1p-His64, and finally to Ssk1p-Asp554. |
| GO:0009927 histidine phosphotransfer kinase activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT propagation of histidine phosphotransfer kinase activity within the hybrid histidine kinase family. Reason: The current GO definition of GO:0009927 ('Serves as a phospho-His intermediate enabling the transfer of phospho group between a hybrid kinase and a response regulator') describes the histidine phosphotransfer (HPt) intermediate role, which in the yeast relay is played by Ypd1; Sln1 is the hybrid kinase that donates the phosphate and has no HPt domain. The Ault et al. data show Sln1 receiver-to-Ypd1 phosphotransfer, i.e. the Sln1 side of the reaction. However, this term is applied to hybrid kinases as well as HPt proteins across GO (SGD Sln1, EcoliWiki BarA, TAIR AHK1, and the PAINT PTHR43047 node), so this is a community convention rather than an isolated error, and the experimental evidence is sound. Kept as non-core; the core Sln1 activity is GO:0000155. Whether hybrid kinases should carry GO:0009927 is raised as a question. Supporting Evidence: PMID:12455952 The Sln1p aspartyl phosphate is then transferred to Ypd1p, which in turn transfers the phosphoryl group to a conserved aspartate on one of two response regulators, Ssk1p and Skn7p. file:yeast/SLN1/SLN1-deep-research-falcon.md Direct transfer from Sln1-His576 to Ypd1 is not supported—the intervening Sln1-Asp1144 step is required |
| GO:0009927 histidine phosphotransfer kinase activity | IDA PMID:12455952 Altered phosphotransfer in an activated mutant of the Saccha... | KEEP AS NON CORE | Summary: Ault et al. analysed phosphotransfer between the Sln1 receiver (Sln1R) and Ypd1 in vitro, showing the activated sln1-22 receiver shifts the equilibrium toward Ypd1. Reason: The current GO definition of GO:0009927 ('Serves as a phospho-His intermediate enabling the transfer of phospho group between a hybrid kinase and a response regulator') describes the histidine phosphotransfer (HPt) intermediate role, which in the yeast relay is played by Ypd1; Sln1 is the hybrid kinase that donates the phosphate and has no HPt domain. The Ault et al. data show Sln1 receiver-to-Ypd1 phosphotransfer, i.e. the Sln1 side of the reaction. However, this term is applied to hybrid kinases as well as HPt proteins across GO (SGD Sln1, EcoliWiki BarA, TAIR AHK1, and the PAINT PTHR43047 node), so this is a community convention rather than an isolated error, and the experimental evidence is sound. Kept as non-core; the core Sln1 activity is GO:0000155. Whether hybrid kinases should carry GO:0009927 is raised as a question. Supporting Evidence: PMID:12455952 Sln1R is an obligate intermediate in the phosphorelay reaction between Sln1p and Ypd1p PMID:12455952 The Sln1p aspartyl phosphate is then transferred to Ypd1p, which in turn transfers the phosphoryl group to a conserved aspartate on one of two response regulators, Ssk1p and Skn7p. |
| GO:0009927 histidine phosphotransfer kinase activity | IMP PMID:12455952 Altered phosphotransfer in an activated mutant of the Saccha... | KEEP AS NON CORE | Summary: Ault et al. used the activated sln1-22 (P1148S) receiver mutant to show altered Sln1-to-Ypd1 phosphotransfer. Reason: The current GO definition of GO:0009927 ('Serves as a phospho-His intermediate enabling the transfer of phospho group between a hybrid kinase and a response regulator') describes the histidine phosphotransfer (HPt) intermediate role, which in the yeast relay is played by Ypd1; Sln1 is the hybrid kinase that donates the phosphate and has no HPt domain. The Ault et al. data show Sln1 receiver-to-Ypd1 phosphotransfer, i.e. the Sln1 side of the reaction. However, this term is applied to hybrid kinases as well as HPt proteins across GO (SGD Sln1, EcoliWiki BarA, TAIR AHK1, and the PAINT PTHR43047 node), so this is a community convention rather than an isolated error, and the experimental evidence is sound. Kept as non-core; the core Sln1 activity is GO:0000155. Whether hybrid kinases should carry GO:0009927 is raised as a question. Supporting Evidence: PMID:12455952 Sln1R is an obligate intermediate in the phosphorelay reaction between Sln1p and Ypd1p |
| GO:0016020 membrane | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro (HAMP domain) mapping to membrane. Reason: Correct but general; plasma membrane is the specific location. Supporting Evidence: PMID:10198019 Wild-type cells and strains expressing the truncation constructs from low-copy-number vectors show a higher percentage (∼80%) of Sln1p in the plasma membrane fraction |
| GO:0016772 transferase activity, transferring phosphorus-containing groups | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro mapping to transferase activity, transferring phosphorus-containing groups. Reason: Correct but very general parent of the kinase activities experimentally established for Sln1. Supporting Evidence: PMID:8808622 This phosphorelay system is initiated by the autophosphorylation of Sln1p at His576. |
| GO:0071944 cell periphery | HDA PMID:26928762 One library to make them all: streamlining the creation of y... | ACCEPT | Summary: High-throughput SWAT-GFP imaging placed Sln1 at the cell periphery. Reason: Consistent with the plasma membrane localization established in low-throughput studies; less specific. Supporting Evidence: PMID:26928762 we constructed and investigated a library of ∼1,800 strains carrying SWAT-GFP modules at the amino termini of endomembrane proteins PMID:14665464 Sln1p-GFP localized to the rim of the cell, consistent with the expected plasma membrane localization of the sensor kinase |
| GO:0140299 molecular sensor activity | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA mapping to molecular sensor activity. Reason: General parent of both GO:0000155 phosphorelay sensor kinase activity and GO:0005034 osmosensor activity, which are experimentally supported. Sln1 senses turgor/osmolarity rather than a defined ligand, so the specific children are preferred, but the parent is consistent with the ontology structure. Supporting Evidence: PMID:12821642 Loss of turgor inactivates the Sln1 histidine kinase activity, leading to subsequent phosphorylation and activation of the HOG MAPK pathway. |
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Download this section (compressed HTML)Q: GO:0009927 histidine phosphotransfer kinase activity is now defined as the phospho-His intermediate (HPt) role between a hybrid kinase and a response regulator, which in yeast is Ypd1. Should hybrid kinases such as Sln1 (and BarA, AHK1, and the PAINT PTHR43047 node) continue to carry it, or should the Sln1 receiver-to-Ypd1 phosphotransfer be represented only by GO:0000155 and has_input Ypd1 in a GO-CAM?
Q: What physical variable does Sln1 sense (turgor, membrane tension, plasma membrane-cell wall contact), and how is the signal transmitted from the periplasmic domain and TMD1 to the HAMP/transmitter?
Q: Should the Sln1-Ypd1-Skn7 branch (hypo-osmotic/cell-wall transcription, e.g. OCH1) be represented for Sln1, or only for Ypd1 and Skn7?
Experiment: Reconstitute full-length Sln1 in proteoliposomes or nanodiscs and measure His576 autophosphorylation and phosphotransfer to Ypd1 as a function of membrane tension/osmotic gradient, to establish whether the sensing mechanism is intrinsic to Sln1.
Experiment: Cryo-EM structure of full-length dimeric Sln1 in active (low osmolarity) and inactive states to define the conformational change linking the extracellular domain to the kinase.
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Download this section (compressed HTML)Transmembrane histidine phosphotransfer kinase and osmosensor; regulates MAP kinase cascade; transmembrane protein with an intracellular kinase domain that signals to Ypd1p and Ssk1p, thereby forming a phosphorelay system similar to bacterial two-component regulators
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