AHK2 (ARABIDOPSIS HISTIDINE KINASE 2) is one of three CHASE-domain hybrid sensor histidine kinases that serve as cytokinin receptors in Arabidopsis thaliana, together with AHK3 and CRE1/AHK4. It is an integral membrane protein, located mainly in the endoplasmic reticulum membrane with a smaller plasma-membrane pool, whose extracytosolic CHASE domain binds cytokinins. Ligand binding activates autophosphorylation of a conserved histidine in the cytoplasmic transmitter domain, and the phosphoryl group is relayed through the receiver domain to histidine phosphotransfer proteins (AHPs), which carry it to nuclear type-B response regulators (ARRs) that activate cytokinin-responsive transcription. In replacement experiments AHK2 favours the kinase over the phosphatase mode. Acting partially redundantly with AHK3, AHK2 promotes shoot and leaf growth, cambial activity and secondary growth, and inhibits root growth and branching; it also modulates seed germination and size, chlorophyll retention, and responses to drought, salt, cold and abscisic acid.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000155 phosphorelay sensor kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: AHK2 is a CHASE-domain hybrid sensor histidine kinase that autophosphorylates on its conserved transmitter-domain histidine and relays the phosphoryl group via its receiver domain to AHP phosphotransfer proteins. Reason: Phosphorelay sensor kinase activity is the core catalytic function of the cytokinin receptors and is directly supported by cytokinin-dependent kinase/phosphorelay assays in yeast and E. coli. Supporting Evidence: PMID:16753566 replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0000155 phosphorelay sensor kinase activity | IEA GO_REF:0000002 | ACCEPT | Summary: AHK2 is a CHASE-domain hybrid sensor histidine kinase that autophosphorylates on its conserved transmitter-domain histidine and relays the phosphoryl group via its receiver domain to AHP phosphotransfer proteins. Reason: Phosphorelay sensor kinase activity is the core catalytic function of the cytokinin receptors and is directly supported by cytokinin-dependent kinase/phosphorelay assays in yeast and E. coli. Supporting Evidence: PMID:16753566 replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0000160 phosphorelay signal transduction system | IBA GO_REF:0000033 | ACCEPT | Summary: AHK2 initiates the multistep His-Asp phosphorelay (AHK -> AHP -> ARR) that transduces the cytokinin signal. Reason: Participation in phosphorelay signal transduction is the defining role of the CRE-family receptors. Supporting Evidence: PMID:12068096 A multistep two-component phosphorelay system with distinct AHK, AHP, and ARR proteins plays a central role in cytokinin signal transduction PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0000160 phosphorelay signal transduction system | IEA GO_REF:0000002 | ACCEPT | Summary: AHK2 initiates the multistep His-Asp phosphorelay (AHK -> AHP -> ARR) that transduces the cytokinin signal. Reason: Participation in phosphorelay signal transduction is the defining role of the CRE-family receptors. Supporting Evidence: PMID:12068096 A multistep two-component phosphorelay system with distinct AHK, AHP, and ARR proteins plays a central role in cytokinin signal transduction PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0004673 protein histidine kinase activity | IDA PMID:16753566 Cytokinins regulate a bidirectional phosphorelay network in ... | ACCEPT | Summary: AHK2 has protein histidine kinase activity (autophosphorylation and phosphotransfer to AHPs), stimulated by cytokinin binding. Reason: Histidine kinase activity of the CRE-family receptors is established biochemically (cytokinin-dependent kinase activity in yeast/E. coli and in vitro). Supporting Evidence: PMID:16753566 replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity |
| GO:0004673 protein histidine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: AHK2 has protein histidine kinase activity (autophosphorylation and phosphotransfer to AHPs), stimulated by cytokinin binding. Reason: Histidine kinase activity of the CRE-family receptors is established biochemically (cytokinin-dependent kinase activity in yeast/E. coli and in vitro). Supporting Evidence: PMID:16753566 replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity |
| GO:0004721 phosphoprotein phosphatase activity | IDA NOT PMID:16753566 Cytokinins regulate a bidirectional phosphorelay network in ... | ACCEPT | Summary: NOT annotation: AHK2 was not found to act as a phosphatase toward phospho-AHPs in the absence of cytokinin in the assays of Mahonen et al. 2006, in contrast to CRE1/AHK4. Reason: The negative result is consistent with AHK2 behaving as a kinase-favouring receptor in the same study; the curators read the full paper and the NOT qualifier is retained. Supporting Evidence: PMID:16753566 replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK2 with the phosphotransfer protein AHP4. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK2 with the phosphotransfer protein AHP5. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with ARR14 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | MODIFY | Summary: Interaction of AHK2 with the receptor AHK3 (yeast two-hybrid/coaffinity). Reason: Generic protein binding is uninformative; the partner is a histidine kinase, so the more specific protein histidine kinase binding term captures the reported receptor heteromerization. Proposed replacements: protein histidine kinase binding Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK2 with the phosphotransfer protein AHP3. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK2 with the phosphotransfer protein AHP2. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK2 with the phosphotransfer protein AHP1. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with P15459 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with P42697 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with Q8GW78 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with Q9ASS6 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with Q9C598 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with Q9CAR7 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with Q9FM19 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with Q9FMJ0 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK2 with Q9LDK9 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Interaction of AHK2 with the phosphotransfer protein AHP3. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Interaction of AHK2 with the phosphotransfer protein AHP2. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: AHK2 is an integral membrane receptor; the bulk of cytokinin binding and receptor-fluorescent protein fusions reside in the endoplasmic reticulum membrane. Reason: ER-membrane localization of the CRE-family receptors is supported by cytokinin-binding, fluorescence and fractionation data. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: AHK2 was originally modelled as a plasma-membrane receptor; a minor plasma-membrane pool is reported, but most receptor protein and cytokinin binding are in the ER. Reason: Plasma membrane localization is plausible for a minor fraction but is not the principal site of the receptor; ER membrane is the better-supported location. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0005886 plasma membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: AHK2 was originally modelled as a plasma-membrane receptor; a minor plasma-membrane pool is reported, but most receptor protein and cytokinin binding are in the ER. Reason: Plasma membrane localization is plausible for a minor fraction but is not the principal site of the receptor; ER membrane is the better-supported location. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0005886 plasma membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: AHK2 was originally modelled as a plasma-membrane receptor; a minor plasma-membrane pool is reported, but most receptor protein and cytokinin binding are in the ER. Reason: Plasma membrane localization is plausible for a minor fraction but is not the principal site of the receptor; ER membrane is the better-supported location. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0006970 response to osmotic stress | IMP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK2 contributes to 'response to osmotic stress' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | MODIFY | Summary: Electronic annotation to the generic signal transduction term. Reason: Correct but uninformative; the specific pathway is cytokinin-activated signaling. Proposed replacements: cytokinin-activated signaling pathway Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0009414 response to water deprivation | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'response to water deprivation' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009414 response to water deprivation | IEP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK2 contributes to 'response to water deprivation' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009636 response to toxic substance | IMP PMID:17216481 The floral volatile, methyl benzoate, from snapdragon (Antir... | KEEP AS NON CORE | Summary: Annotation based on reduced root-growth inhibition by the floral volatile methyl benzoate in cytokinin-receptor mutant(s). Reason: The cached abstract names only the cre1 (AHK4) mutant; the full text (Planta, paywalled; no PMC/Europe PMC copy) could not be checked for ahk2 data. Per curation policy we defer to the curator, who read the full paper; a receptor-dependent root-growth response is plausible for AHK2 but is an indirect, non-core output of cytokinin signaling. The cached text contains no AHK2-specific sentence, so no supporting quote is attached; the curator's evidence is in full text not available in the cache. |
| GO:0009651 response to salt stress | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'response to salt stress' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009651 response to salt stress | IEP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK2 contributes to 'response to salt stress' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009736 cytokinin-activated signaling pathway | TAS PMID:12068096 Two-component signal transduction pathways in Arabidopsis. | ACCEPT | Summary: AHK2 is one of three partially redundant cytokinin receptors that initiate cytokinin-activated two-component signaling. Reason: Core biological process of the receptor; triple receptor mutants are completely cytokinin-insensitive. Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:15166290 Plants carrying mutations in all three genes did not show cytokinin responses, including inhibition of root elongation, inhibition of root formation, cell proliferation in and greening of calli, and induction of cytokinin primary-response genes. |
| GO:0009737 response to abscisic acid | IEP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK2 contributes to 'response to abscisic acid' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 the stress-responsive AHK2, AHK3, and CRE1 act as negative regulators in ABA signaling |
| GO:0009884 cytokinin receptor activity | IEA GO_REF:0000117 | ACCEPT | Summary: AHK2 binds cytokinins with its extracytosolic CHASE domain and transduces the signal by histidine autophosphorylation. Reason: Cytokinin receptor activity is the core molecular function of the CRE-family histidine kinases. Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:15155880 Application of exogenous cytokinins to the resultant strains revealed that both AHK2 and AHK3 function as positive regulators for cytokinin signaling similar to AHK4. |
| GO:0009884 cytokinin receptor activity | TAS PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | ACCEPT | Summary: AHK2 binds cytokinins with its extracytosolic CHASE domain and transduces the signal by histidine autophosphorylation. Reason: Cytokinin receptor activity is the core molecular function of the CRE-family histidine kinases. Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:15155880 Application of exogenous cytokinins to the resultant strains revealed that both AHK2 and AHK3 function as positive regulators for cytokinin signaling similar to AHK4. |
| GO:0009909 regulation of flower development | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of flower development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:19913077 Mutation of the cytokinin receptors Arabidopsis histidine kinase 2 (AHK2) and AHK3 nearly abolished the transgene phenotypes and the enhanced CUC2 and CUC3 transcription induced by cytokinin. |
| GO:0009909 regulation of flower development | IMP PMID:19913077 Cytokinin overproduction-caused alteration of flower develop... | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of flower development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:19913077 Mutation of the cytokinin receptors Arabidopsis histidine kinase 2 (AHK2) and AHK3 nearly abolished the transgene phenotypes and the enhanced CUC2 and CUC3 transcription induced by cytokinin. |
| GO:0010029 regulation of seed germination | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of seed germination' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:16361392 Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. |
| GO:0010029 regulation of seed germination | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of seed germination' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:16361392 Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. |
| GO:0010029 regulation of seed germination | IMP PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of seed germination' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:16361392 Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. |
| GO:0010087 phloem or xylem histogenesis | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'phloem or xylem histogenesis' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0010087 phloem or xylem histogenesis | IMP PMID:19622803 The histidine kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS... | KEEP AS NON CORE | Summary: AHK2 contributes to 'phloem or xylem histogenesis' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0010150 leaf senescence | IMP NOT PMID:16407152 Cytokinin-mediated control of leaf longevity by AHK3 through... | ACCEPT | Summary: NOT annotation: loss of AHK2 did not affect cytokinin-dependent delay of leaf senescence, unlike loss of AHK3. Reason: The abstract explicitly states that a loss-of-function mutation in AHK3, but not the other receptors, reduced cytokinin control of leaf senescence. Supporting Evidence: PMID:16407152 A loss-of-function mutation in AHK3, but not of the other cytokinin receptors, conferred a reduced sensitivity to cytokinin in cytokinin-dependent delay of leaf senescence and abolished cytokinin-dependent phosphorylation of ARR2. |
| GO:0010271 regulation of chlorophyll catabolic process | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of chlorophyll catabolic process' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:16361392 lacked the cytokinin-dependent inhibition of dark-induced chlorophyll loss |
| GO:0010271 regulation of chlorophyll catabolic process | IMP PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of chlorophyll catabolic process' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:16361392 lacked the cytokinin-dependent inhibition of dark-induced chlorophyll loss |
| GO:0016772 transferase activity, transferring phosphorus-containing groups | IEA GO_REF:0000002 | MODIFY | Summary: Electronic annotation to a very general transferase term. Reason: Correct but uninformative; the specific activity is phosphorelay sensor kinase activity. Proposed replacements: phosphorelay sensor kinase activity Supporting Evidence: PMID:16753566 replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity |
| GO:0019900 kinase binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | KEEP AS NON CORE | Summary: Yeast two-hybrid interaction of AHK2 with a non-two-component enzyme/kinase identified in a medium-scale interaction screen. Reason: Experimental but from a large-scale Y2H screen without functional follow-up; the interaction is not part of the established receptor mechanism. Retained as non-core pending validation. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0034757 negative regulation of iron ion transport | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Cytokinin repression of root iron-uptake genes (IRT1, FRO2, FIT) in receptor mutants. Reason: The cached abstract names AHK3 and CRE1 as required; the full text (Plant J, no PMC/Europe PMC copy, publisher access blocked) could not be checked for ahk2 data. Deferring to the curator who read the full paper; a contribution of AHK2 is plausible given receptor redundancy. The effect is described as growth-dependent and indirect, so it is non-core. The cached abstract contains no AHK2-specific sentence, so no supporting quote is attached; the curator's evidence is in full text not available in the cache. |
| GO:0034757 negative regulation of iron ion transport | IMP PMID:18397377 Cytokinins negatively regulate the root iron uptake machiner... | KEEP AS NON CORE | Summary: Cytokinin repression of root iron-uptake genes (IRT1, FRO2, FIT) in receptor mutants. Reason: The cached abstract names AHK3 and CRE1 as required; the full text (Plant J, no PMC/Europe PMC copy, publisher access blocked) could not be checked for ahk2 data. Deferring to the curator who read the full paper; a contribution of AHK2 is plausible given receptor redundancy. The effect is described as growth-dependent and indirect, so it is non-core. The cached abstract contains no AHK2-specific sentence, so no supporting quote is attached; the curator's evidence is in full text not available in the cache. |
| GO:0042802 identical protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | KEEP AS NON CORE | Summary: AHK2 self-interaction in yeast two-hybrid / coaffinity assays, consistent with the dimeric architecture of sensor histidine kinases. Reason: Homodimerization is expected for sensor histidine kinases and is experimentally reported, but it is a structural property rather than the core function. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0043424 protein histidine kinase binding | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA-rule electronic annotation for binding to other histidine kinases. Reason: Consistent with experimentally reported heteromeric interactions among CRE-family receptors; retained as non-core. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0043424 protein histidine kinase binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | KEEP AS NON CORE | Summary: AHK2 interacts with other CRE-family receptor histidine kinases. Reason: Receptor homo/heteromerization is experimentally supported (Y2H, coaffinity, in vivo FRET for AHK3-AHK4) but is a structural property rather than the signalling activity itself. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0046686 response to cadmium ion | IEP PMID:40858022 Autophagy and glycolysis synergistically mediate Cd²⁺ transp... | MARK AS OVER ANNOTATED | Summary: IEP annotation from a study of autophagy and glycolysis in cadmium tolerance that reports autophagic degradation of 'AtHK2'. Reason: Probable wrong gene: the abstract places 'AtHK2' in a glycolysis/energy-production context (alongside AtGAPDH and AtENO2), which fits HEXOKINASE2 (HXK2) rather than the cytokinin receptor histidine kinase AHK2. The full text (J Hazard Mater, paywalled, no PMC copy) could not be read to confirm the identifier mapping, so the row is not removed, but it should not be taken as evidence of an AHK2 role in cadmium responses. Supporting Evidence: PMID:40858022 Cd-exposure establishes a link between autophagy and glycolysis through the autophagic degradation of AtHK2 |
| GO:0048509 regulation of meristem development | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of meristem development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20110319 Loss of cytokinin perception leads to a complete failure of meristem establishment and growth arrest after germination. |
| GO:0048509 regulation of meristem development | IMP PMID:20110319 STIMPY mediates cytokinin signaling during shoot meristem es... | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of meristem development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:20110319 Loss of cytokinin perception leads to a complete failure of meristem establishment and growth arrest after germination. |
| GO:0048831 regulation of shoot system development | IMP PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | KEEP AS NON CORE | Summary: AHK2 contributes to 'regulation of shoot system development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:16361392 prominent roles for the AHK2/AHK3 receptor combination in quantitative control of organ growth in plants, with opposite regulatory functions in roots and shoots |
| GO:0070417 cellular response to cold | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: AHK2 contributes to 'cellular response to cold' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20463025 AHK2 and AHK3 were found to be primarily involved in mediating cold to express A-type ARRs despite cytokinin deficiency. |
| GO:0070417 cellular response to cold | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'cellular response to cold' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20463025 AHK2 and AHK3 were found to be primarily involved in mediating cold to express A-type ARRs despite cytokinin deficiency. |
| GO:0070417 cellular response to cold | IMP PMID:20463025 A subset of cytokinin two-component signaling system plays a... | KEEP AS NON CORE | Summary: AHK2 contributes to 'cellular response to cold' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:20463025 AHK2 and AHK3 were found to be primarily involved in mediating cold to express A-type ARRs despite cytokinin deficiency. |
| GO:0071215 cellular response to abscisic acid stimulus | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'cellular response to abscisic acid stimulus' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20463025 The ahk2 ahk3 and arr7 mutants showed hypersensitive response to abscisic acid (ABA) for germination, whereas ARR7 overexpression lines exhibited insensitive response to ABA. |
| GO:0071215 cellular response to abscisic acid stimulus | IMP PMID:20463025 A subset of cytokinin two-component signaling system plays a... | KEEP AS NON CORE | Summary: AHK2 contributes to 'cellular response to abscisic acid stimulus' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:20463025 The ahk2 ahk3 and arr7 mutants showed hypersensitive response to abscisic acid (ABA) for germination, whereas ARR7 overexpression lines exhibited insensitive response to ABA. |
| GO:0080117 secondary growth | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK2 contributes to 'secondary growth' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0080117 secondary growth | IMP PMID:19622803 The histidine kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS... | KEEP AS NON CORE | Summary: AHK2 contributes to 'secondary growth' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0009885 transmembrane histidine kinase cytokinin receptor activity | IMP PMID:15155880 Histidine kinase homologs that act as cytokinin receptors po... | NEW | Summary: AHK2 is a membrane-spanning CHASE-domain histidine kinase that perceives cytokinin and signals by histidine autophosphorylation; the specific receptor-kinase term is more precise than the existing generic cytokinin receptor activity annotations. Reason: Genetic and biochemical evidence establishes the CRE-family receptors as transmembrane histidine kinase cytokinin receptors; AHK4 already carries this term by IDA, and AHK2/AHK3 share the same domain architecture and mechanism. Supporting Evidence: PMID:15155880 Application of exogenous cytokinins to the resultant strains revealed that both AHK2 and AHK3 function as positive regulators for cytokinin signaling similar to AHK4. PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
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Download this section (compressed HTML)Q: What fraction of cytokinin perception occurs at the ER versus the plasma membrane in intact tissues, and does the site of perception differ among AHK2, AHK3 and AHK4?
Q: How are receptor-specific outputs (e.g. AHK3-ARR2 in leaf senescence) achieved given that all three receptors converge on the same AHP pool?
Experiment: Use receptor variants tethered to ER or plasma membrane in the ahk2 ahk3 cre1 triple mutant background to test which pool restores cytokinin-dependent transcription.
Hypothesis: Cytokinin signaling output is initiated predominantly from the ER-resident receptor pool.
Type: genetic complementation
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