SMAX1-LIKE6 (SMXL6; D53-like 2) is a nuclear Clp-ATPase-related (double Clp-N / P-loop NTPase domain) protein that, together with its close paralogs SMXL7 and SMXL8, acts as the transcriptional co-repressor targeted by strigolactone (SL) signalling. In the absence of SL SMXL6 represses target genes, including BRANCHED1 (BRC1), by recruiting TOPLESS-related co-repressors (TPR2) through a C-terminal EAR motif, and promotes PIN1 accumulation and auxin transport in the stem, thereby allowing axillary bud outgrowth. SL perceived by D14 triggers SMXL6 association with D14 and SCF(MAX2), rapid polyubiquitination and proteasomal degradation (an RGKT-region deletion stabilises the protein). SMXL6 can also bind DNA directly, repressing the SMXL6/7/8 promoters as an autoregulated transcription factor, and the SMXL6/7/8 group negatively affects drought tolerance and promotes rounder leaves. SMXL6 acts redundantly with SMXL7 and SMXL8; single mutants have little phenotype.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | REMOVE | Summary: SMXL6 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 SMXL6 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-SMXL6 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:0000044 | 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:0009507 chloroplast | ISM GO_REF:0000122 | REMOVE | Summary: AtSubP prediction of chloroplast. Reason: No transit peptide-based or experimental support; SMXL6 is a nuclear transcriptional co-repressor. |
| 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 | IMP PMID:23893171 SUPPRESSOR OF MORE AXILLARY GROWTH2 1 controls seed germinat... | KEEP AS NON CORE | Summary: SMXL6 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: SMXL6 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: SMXL6 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. |
| GO:0000976 transcription cis-regulatory region binding | IDA PMID:32528176 Transcriptional regulation of strigolactone signalling in Ar... | NEW | Summary: SMXL6 binds the SMXL6/7/8 promoters directly and represses them. Reason: Direct promoter binding, as stated in the abstract, establishes a DNA-binding activity in addition to co-repressor function. The IDA code reflects the authors' direct binding experiments reported in the full text; only the abstract is cached, so the specific assays were not inspected here and the claim rests on the abstract's explicit statement of direct binding. Supporting Evidence: PMID:32528176 represses the transcription of SMXL6, SMXL7 and SMXL8 by binding directly to their promoters, showing that SMXL6 serves as an autoregulated transcription factor |
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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 SMXL6 (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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