STR (STUNTED ARBUSCULE) is a half-size ATP-binding cassette transporter of the ABCG subfamily from Medicago truncatula, with one nucleotide-binding domain and one six-helix transmembrane domain. It belongs to a conserved STR clade found in plants that form arbuscular mycorrhizal (AM) symbioses and absent from non-host species such as Arabidopsis. STR heterodimerises with the related half-transporter STR2 to form a complete ABC transporter that is located in the periarbuscular membrane, the host-derived membrane surrounding the branches of fungal arbuscules in root cortical cells. STR is expressed at low levels in root vascular tissue and is strongly induced in arbuscule-containing cortical cells under the control of the AM transcriptional program (RAM1, WRI5a, ERM1). Loss of STR or STR2 arrests arbuscules at a small, stunted stage and AM symbiosis fails, whereas root-nodule symbiosis with rhizobia is unaffected; rice STR1/STR2 mutants show the same phenotype. STR-STR2 is proposed to export host-synthesised lipids, most likely sn-2 (beta) monoacylglycerols such as 2-monopalmitin produced by FatM and RAM2, across the periarbuscular membrane for use by the lipid-auxotrophic fungus, but its transported substrate has not been demonstrated biochemically.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:20453115 Two Medicago truncatula half-ABC transporters are essential ... | MODIFY | Summary: STR physically interacts with its partner half-transporter STR2 (UniProt A9YWR6); split-YFP (BiFC) with functional, native-promoter constructs showed STR-STR2 interaction at the periarbuscular membrane, whereas STR-STR controls gave no signal. The informative activity is heterodimer formation between two half-size ABCG transporters to assemble a complete ABC transporter. Reason: Protein binding is uninformative. The evidence (PMID:20453115) is specifically for heterodimerisation of STR with the non-identical half-transporter STR2, which is the structural prerequisite for a functional ABC transporter (one NBD and one TMD per half). Protein heterodimerization activity captures this precisely. Proposed replacements: protein heterodimerization activity Supporting Evidence: PMID:20453115 STR heterodimerizes with STR2, and the resulting transporter is located in the peri-arbuscular membrane file:MEDTR/STR/STR-deep-research-falcon.md Half-transporters require a partner to form the architecture of a complete ABC transporter. |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: STR carries a single ABC nucleotide-binding domain with conserved ATP-binding motifs (InterPro IPR003439/IPR017871), so ATP binding is mechanistically expected. Reason: Correct, domain-based inference; ATP binding is a sub-step of the ABC transport cycle rather than the informative function, which is captured by ABC-type transporter activity. Retained as non-core. Supporting Evidence: file:MEDTR/STR/STR-deep-research-falcon.md STR has one nucleotide-binding domain, containing conserved ABC ATP-binding motifs, and one membrane-spanning domain with approximately six helices. |
| GO:0005886 plasma membrane | EXP PMID:20453115 Two Medicago truncatula half-ABC transporters are essential ... | ACCEPT | Summary: Plasma membrane annotation from PMID:20453115. The STR-STR2 complex was imaged in the periarbuscular membrane (PAM), a specialised domain of the host cell plasma membrane that envelops arbuscule branches; signal was not detected at the peripheral plasma membrane. Reason: The PAM is continuous with, and a specialised domain of, the cortical cell plasma membrane, so plasma membrane is correct though less specific than periarbuscular membrane (also annotated, IDA, same paper). Note that in GO the periarbuscular membrane term is classified under host cell membrane / host cellular component rather than under plasma membrane, so the PM row is not redundant in the graph and is worth retaining. Deferring to the curator, who read the full text. Supporting Evidence: PMID:20453115 the resulting transporter is located in the peri-arbuscular membrane file:MEDTR/STR/STR-deep-research-falcon.md undetectable at the ordinary peripheral plasma membrane under the reported imaging conditions |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: IBA (PTN000443664) transferring plasma membrane localisation across the ABCG half-transporter clade; STR itself and STR2 (A9YWR6) appear among the descendant evidence. Reason: Consistent with the experimental PAM/plasma membrane localisation of STR. STR appearing in its own WITH/FROM reflects its experimental grounding and is expected for an IBA. Supporting Evidence: PMID:20453115 STR heterodimerizes with STR2, and the resulting transporter is located in the peri-arbuscular membrane |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: IEA plasma membrane from the UniProt subcellular location vocabulary (Cell membrane), itself based on PMID:20453115. Reason: Correct; redundant with the EXP and IBA rows but consistent with the experimentally determined PAM localisation. Supporting Evidence: PMID:20453115 the resulting transporter is located in the peri-arbuscular membrane |
| GO:0009610 response to symbiotic fungus | IEP PMID:20453115 Two Medicago truncatula half-ABC transporters are essential ... | KEEP AS NON CORE | Summary: IEP from PMID:20453115. STR expression is induced in cortical cells containing arbuscules during AM symbiosis with Glomus versiforme, in addition to constitutive vascular expression. Reason: The expression induction is well documented, but response to symbiotic fungus reflects transcriptional regulation of STR rather than its function; the functional role is captured by arbuscular mycorrhizal association (IMP). Retained as non-core. Supporting Evidence: PMID:20453115 STR and STR2 are coexpressed constitutively in the vascular tissue, and expression is induced in cortical cells containing arbuscules. |
| GO:0009610 response to symbiotic fungus | IEP PMID:26511916 Hyphal Branching during Arbuscule Development Requires Reduc... | KEEP AS NON CORE | Summary: IEP from PMID:26511916. STR expression in colonised roots depends on the GRAS transcription factor RAM1; later work shows the AP2 factors WRI5a and ERM1 directly bind AW-box elements in the STR promoter. Reason: Supports AM-induced, RAM1-dependent expression of STR, i.e. STR is a target of the symbiotic transcriptional program. This is a response/expression annotation and not the function of STR; non-core. Supporting Evidence: PMID:26511916 RAM1 regulates expression of EXO70I and Stunted Arbuscule, two genes whose loss of function impacts arbuscule branching PMID:37717076 ERM1 functions as a transcriptional activator that regulates lipid metabolism and transport by binding directly to promoters of downstream genes such as STR and STR2 via AW-box and AW-box-like motifs |
| GO:0009877 nodulation | IMP NOT PMID:20453115 Two Medicago truncatula half-ABC transporters are essential ... | ACCEPT | Summary: NOT nodulation. The str mutant nodulates normally with Sinorhizobium meliloti, distinguishing STR from common symbiosis signalling genes required for both AM and root-nodule symbioses. Reason: Well supported negative annotation from the mutant phenotype; STR is specific to the arbuscule stage of AM symbiosis. Supporting Evidence: PMID:20453115 In contrast with legume symbiosis mutants reported previously, str shows a wild-type nodulation phenotype. |
| GO:0016020 membrane | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Generic membrane localisation from the ABC-2 type transmembrane domain (IPR013525). Reason: Correct but uninformative; the experimentally supported location is the periarbuscular membrane. Supporting Evidence: file:MEDTR/STR/STR-deep-research-falcon.md STR has one nucleotide-binding domain, containing conserved ABC ATP-binding motifs, and one membrane-spanning domain with approximately six helices. |
| GO:0016887 ATP hydrolysis activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: ATP hydrolysis activity inferred from the ABC ATPase domain. Reason: Expected from the NBD architecture, but no ATPase activity has been measured for STR or the STR-STR2 heterodimer, and ATP hydrolysis is a component of the transport cycle better represented by ABC-type transporter activity. Non-core. Supporting Evidence: file:MEDTR/STR/STR-deep-research-falcon.md substrate-coupled ATPase activity and transport kinetics have |
| GO:0036377 arbuscular mycorrhizal association | IMP PMID:20453115 Two Medicago truncatula half-ABC transporters are essential ... | ACCEPT | Summary: IMP from PMID:20453115. The str mutant (Gln635-stop) and STR2 RNAi lines are blocked at a stunted arbuscule stage and AM symbiosis fails; wild-type STR complements. The rice orthologues STR1/STR2 give the same phenotype. Reason: Core biological process. STR acts in the host cortical cell at the plant-fungus interface (PAM) to support arbuscule development; its PAM-localised transport activity is thought to supply lipid to the fungus, so this is participation in the symbiotic exchange rather than a remote requirement. This is the plant-side term; the symbiont-side formation of arbuscule for nutrient acquisition (GO:0075328) would not be appropriate. Supporting Evidence: PMID:20453115 we report a Medicago truncatula mutant, stunted arbuscule (str), in which arbuscule development is impaired and AM symbiosis fails PMID:22077667 Mutation of either of the Oryza sativa (rice) ABCG transporters blocked arbuscule growth of different AM fungi at a small and stunted stage |
| GO:0042626 ATPase-coupled transmembrane transporter activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA (PTN008283605) ATPase-coupled transmembrane transporter activity for the ABCG half-transporter clade. Reason: Appropriate, substrate-agnostic level for STR. STR is a half-size ABCG transporter that functions as an STR-STR2 heterodimer; the generic ATPase-coupled transporter activity is supported by architecture and phylogeny, and correctly avoids asserting a specific substrate, which has not been biochemically demonstrated. Supporting Evidence: PMID:20453115 encodes a half-size ATP binding cassette (ABC) transporter of a subfamily (ABCG) PMID:34618047 It is tempting to speculate that sn2-MAG compounds or MAG derivatives are STR/STR2 substrates; however, direct proof has not yet been provided |
| GO:0042803 protein homodimerization activity | IDA NOT PMID:20453115 Two Medicago truncatula half-ABC transporters are essential ... | ACCEPT | Summary: NOT protein homodimerization activity. In BiFC assays STR-STR combinations gave no detectable signal while STR-STR2 did. Reason: Consistent with STR functioning obligately as a heterodimer with STR2 (and with STR2 silencing phenocopying str). Negative BiFC results are not absolute proof, but the curator's NOT annotation is reasonable and informative. Supporting Evidence: file:MEDTR/STR/STR-deep-research-falcon.md whereas STR–STR controls gave no detectable interaction signal |
| GO:0055085 transmembrane transport | IBA GO_REF:0000033 | ACCEPT | Summary: IBA transmembrane transport for the ABCG clade. Reason: Correct generic process for an ABC transporter; appropriately avoids specifying a substrate (lipid export is proposed but not directly demonstrated). Supporting Evidence: PMID:20453115 the resulting transporter is located in the peri-arbuscular membrane where its activity is required for arbuscule development |
| GO:0085042 periarbuscular membrane | IDA PMID:20453115 Two Medicago truncatula half-ABC transporters are essential ... | ACCEPT | Summary: IDA from PMID:20453115. BiFC fluorescence of the STR-STR2 heterodimer surrounds arbuscule branches in the periarbuscular membrane, strongest around fine branches. Reason: Core location; direct experimental evidence with functional (complementing) constructs. Supporting Evidence: PMID:20453115 the resulting transporter is located in the peri-arbuscular membrane where its activity is required for arbuscule development |
| GO:0140359 ABC-type transporter activity | IEA GO_REF:0000002 | ACCEPT | Summary: ABC-type transporter activity from the ABC-2 transmembrane domain (IPR013525). Reason: STR, as one half of the STR-STR2 heterodimer, contributes one NBD and one TMD to a complete ABC transporter. ABC-type transporter activity is the most specific defensible MF; a substrate-specific child (e.g. ATPase-coupled lipid transmembrane transporter activity) is not justified because the substrate (2-monoacylglycerol has been proposed) has not been shown with purified or reconstituted protein. Strictly the activity belongs to the heterodimer, so a contributes_to framing would be more precise. Supporting Evidence: file:MEDTR/STR/STR-deep-research-falcon.md Half-transporters require a partner to form the architecture of a complete ABC transporter. PMID:34618047 The nature of the molecules transported by STR/STR2 remains one of the most exciting questions in AMS research. |
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Download this section (compressed HTML)Q: What molecule does the STR-STR2 heterodimer transport across the periarbuscular membrane? Genetic and lipid-profiling data point to sn-2 monoacylglycerols (2-monopalmitin) produced by FatM/RAM2, but no binding or transport assay has been done with purified protein; until then, should STR be annotated only with substrate-agnostic ABC transporter terms, with lipid export captured via has_input on a GO-CAM rather than a lipid-transporter MF?
Q: The GO term periarbuscular membrane (GO:0085042) sits under host cell membrane / host cellular component (a symbiont-perspective branch) and not under plasma membrane, yet it is used here for a host protein in its own membrane. Should GO:0085042 also be part_of plasma membrane so that plant-side annotations propagate correctly?
Q: Should the transporter activity be annotated to STR with a contributes_to qualifier, given that the functional unit is the STR-STR2 heterodimer?
Experiment: Reconstitute purified Medicago STR-STR2 into proteoliposomes or nanodiscs and measure substrate-stimulated ATPase activity and ATP-dependent transport of 2-monopalmitin, other sn-2 MAGs and cutin-type monomers, with Walker-motif mutants as controls.
Experiment: Cryo-EM structure of STR-STR2 with candidate lipid substrates bound to identify the substrate cavity, followed by in planta complementation of str with cavity mutants.
Experiment: Isotope tracing of plant-derived 16:0 into fungal lipids in str and str2 mutants versus wild type, to test whether STR-STR2 is required for lipid transfer across the periarbuscular membrane specifically.
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