SMAX1-LIKE8 (SMXL8; D53-like 3) is a nuclear Clp-ATPase-related (double Clp-N / P-loop NTPase domain) protein that acts redundantly with SMXL6 and SMXL7 as a strigolactone (SL)-regulated transcriptional co-repressor. Via its EAR motif it recruits TOPLESS-related co-repressors (TPR2) and represses target genes such as BRANCHED1 (BRC1), allowing axillary bud outgrowth. SL perception by D14 triggers SCF(MAX2)-dependent polyubiquitination and proteasomal degradation of SMXL8. The SMXL6/7/8 group also regulates leaf shape, lateral root density and drought tolerance, and SMXL8 alone has little phenotypic effect.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | REMOVE | Summary: SMXL8 interacts with the SL receptor D14 in a GR24-enhanced manner (Y2H/co-IP). Reason: The interaction is real and central to SL signalling (SMXLs are the receptor's targets), but protein binding conveys no functional information about SMXL8 itself; its activity is represented by transcription corepressor activity. Removal does not imply the interaction is false. Supporting Evidence: PMID:26546446 In response to rac -GR24 treatment, SMXL6 and SMXL7 directly interacted with D14 in yeast cells in a concentration-dependent manner |
| GO:0005515 protein binding | IPI PMID:37759806 SUPPRESSOR OF MAX2 LIKE 6, 7, and 8 Interact with DDB1 BINDI... | REMOVE | Summary: SMXL6/7/8 interact with DWA1, a CUL4-based E3 substrate receptor. Reason: Protein binding is uninformative; the DWA1 association reflects regulation of SMXL stability rather than an activity of the SMXL protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:37759806 physically interacted with SMXL6,7,8 |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro (AAA ATPase core) mapping to ATP binding. Reason: SMXL proteins share Clp-ATPase-like nucleotide-binding domains, but nucleotide binding has not been demonstrated and the conserved P-loop region (RGKT) functions as the SL-triggered degron. The domain-based inference overstates the evidence. |
| GO:0005634 nucleus | EXP PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | ACCEPT | Summary: GFP-SMXL8 co-localises with a nuclear marker in protoplasts. Reason: Direct localisation; SMXL co-repressors act on target promoters in the nucleus. Supporting Evidence: PMID:26546446 the GFP-SMXL6, GFP-SMXL7, and GFP-SMXL8 proteins colocalized with SV40NLS-mCherry |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference of nuclear activity for the SMXL clade. Reason: Consistent with experimental nuclear localisation and nuclear transcriptional function. |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic mapping to nucleus. Reason: Consistent with experimental nuclear localisation. |
| GO:0005737 cytoplasm | ISM GO_REF:0000122 | MARK AS OVER ANNOTATED | Summary: AtSubP prediction of cytoplasm. Reason: Sequence-based prediction only; experimental data place SMXL6/7/8 in the nucleus, and SMXL7 activity requires nuclear localisation. |
| GO:0006457 protein folding | IEA GO_REF:0000108 | REMOVE | Summary: Protein folding inferred (GO_REF:0000108) from the IBA chaperone annotation. Reason: Derived from the protein folding chaperone IBA, which is not supported for the SMXL clade. |
| GO:0009414 response to water deprivation | IEP PMID:32295207 Negative Roles of Strigolactone-Related SMXL6, 7 and 8 Prote... | KEEP AS NON CORE | Summary: smxl6,7,8 triple mutants are more drought resistant than wild type. Reason: A downstream physiological role of SL signalling; the evidence is from the triple mutant, so the contribution of the individual paralog is inferred. Supporting Evidence: PMID:32295207 Our findings together indicate that the SMXL6, 7 and 8 act as negative regulators of drought resistance |
| GO:0016887 ATP hydrolysis activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro (AAA ATPase core) mapping to ATP hydrolysis activity. Reason: No ATPase activity has been demonstrated for SMXL6/7/8; their characterised activity is transcriptional co-repression. Domain similarity to Clp ATPases is insufficient to assert catalytic function. |
| GO:0044183 protein folding chaperone | IBA GO_REF:0000033 | REMOVE | Summary: IBA transfer of protein folding chaperone activity from a deep Clp-ATPase node seeded by bacterial ClpB/ClpC (P9WPC9). Reason: The SMAX1/SMXL clade has diverged from Clp chaperones into hormone-regulated transcriptional co-repressors; there is no evidence of chaperone or disaggregase activity for any SMXL protein, which lack the ClpB/ClpC middle domain architecture. The ancestral chaperone function should not propagate into this clade. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN002461480 · Clp ATPase ancestral node SUPPORTS SOURCE BUT NOT TARGET Chaperone activity is well grounded for bacterial ClpB/ClpC (P9WPC9) but the plant SMXL clade is functionally diverged. Supporting Evidence: PMID:26546446 D53-like SMXLs exhibit TPR2-dependent transcriptional repression activity and repress the expression of BRANCHED1. |
| GO:1902347 response to strigolactone | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA prediction of response to strigolactone. Reason: Correct; SMXL8 is an SL-regulated target of the D14-MAX2 pathway. The specific signalling role is captured as cellular response to strigolactone in core_functions. |
| GO:1902347 response to strigolactone | IMP PMID:23893171 SUPPRESSOR OF MORE AXILLARY GROWTH2 1 controls seed germinat... | KEEP AS NON CORE | Summary: SMXL8 transcripts are induced by GR24 and the gene acts in SL responses. Reason: The abstract reports SL induction of SMXL genes rather than a mutant phenotype, but the genetic role of SMXL6/7/8 in SL responses is well established; the specific signalling role is captured as cellular response to strigolactone in core_functions. Supporting Evidence: PMID:26546447 SMXL6, SMXL7, and SMXL8 are co-orthologs of rice D53 that promote shoot branching |
| GO:0003714 transcription corepressor activity | IDA PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | NEW | Summary: SMXL8 fused to GAL4-BD represses reporter transcription in an EAR-motif- and TPR2-dependent manner. Reason: The characterised molecular activity of the SMXL6/7/8 clade is EAR-motif-dependent co-repression via TOPLESS-related proteins; GOA has no informative MF for this protein. Supporting Evidence: PMID:26546446 Compared with GAL4 alone, the relative luciferase activities were dramatically reduced when GAL4 was fused with SMXL6, SMXL7, or SMXL8, and this was dependent on the EAR motif PMID:26546446 D53-like SMXLs exhibit TPR2-dependent transcriptional repression activity and repress the expression of BRANCHED1. |
| GO:2000032 regulation of secondary shoot formation | IMP PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | NEW | Summary: smxl6/7/8 loss suppresses the bushy max2/max3 phenotypes; SMXL6/7/8 promote axillary bud outgrowth. Reason: SMXL6/7/8 repress BRC1 and are the direct effectors whose SL-triggered removal inhibits bud outgrowth; GOA lacks a branching term for this gene. Supporting Evidence: PMID:26546446 the triple mutant smxl6 / 7 / 8 completely restored max3-9 and displayed a significant decrease in secondary cauline branch number, suggesting that SMXL6 , SMXL7 , and SMXL8 function redundantly in promoting the outgrowth of axillary buds PMID:26546447 SMXL6, SMXL7, and SMXL8 are co-orthologs of rice D53 that promote shoot branching |
| GO:1902348 cellular response to strigolactone | IDA PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | NEW | Summary: SMXL8 is ubiquitinated and degraded within minutes of SL treatment, in a D14/MAX2-dependent manner. Reason: SMXL proteins are the repressor components of the SL signalling pathway, whose regulated degradation is the signalling step. Supporting Evidence: PMID:26546446 Exogenous application of the SL analog rac-GR24 causes ubiquitination and degradation of SMXL6, 7, and 8; this requires D14 and MAX2. |
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Download this section (compressed HTML)Q: Does the Clp-ATPase-like region of SMXL proteins bind or hydrolyse ATP, and does nucleotide state influence D14-SCF(MAX2) recognition?
Q: Which DNA-binding transcription factors (e.g. SPL, BES1, TCP partners) tether SMXL6/7/8 to BRC1 and other targets in axillary buds?
Experiment: In vitro ATPase assays with purified SMXL8 (wild type versus Walker-motif mutants), combined with complementation of smxl6/7/8, to test whether ATPase activity contributes to repression.
Type: biochemistry and genetic complementation
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