Wzm is the polytopic inner-membrane permease (transmembrane domain, TMD) subunit of the Wzm-Wzt ATP-binding cassette (ABC) transporter that exports the A-band common polysaccharide antigen (CPA), a D-rhamnan homopolymer, across the cytoplasmic (inner) membrane during ABC-transporter-dependent lipopolysaccharide O-antigen assembly in Pseudomonas aeruginosa. The polysaccharide is synthesized in the cytoplasm and translocated to the periplasm by this transporter; Wzm provides the transmembrane conduit while the peripheral cytoplasmic Wzt subunit supplies ATP binding and hydrolysis. Wzm has six predicted transmembrane helices and belongs to the ABC-2 integral membrane protein family. Loss of wzm (or wzt) blocks surface expression of A-band polysaccharide and causes it to accumulate in the cytoplasm, without affecting the Wzx/Wzy-dependent B-band O antigen. Wzm assembles as a homotetramer that, together with a Wzt tetramer, forms a hetero-octameric transporter complex.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0015920 lipopolysaccharide transport | IBA GO_REF:0000033 | MODIFY | Summary: Phylogenetically-inferred role in lipopolysaccharide/O-antigen transport, consistent with Wzm's role as the ABC-transporter permease that exports the A-band polysaccharide across the inner membrane. Reason: Wzm-Wzt exports the undecaprenyl-pyrophosphate-linked A-band (D-rhamnan) polysaccharide across the inner membrane (PMID:9244257); the polymer becomes LPS only after WaaL ligates it to lipid A-core in the periplasm, so the transported substrate is a polysaccharide, not LPS. GO has no O-antigen-specific transport term, so the accurate process is GO:0015774 polysaccharide transport. The PAINT node this IBA comes from (PTN002019107, PTHR30413, sole seed Q9HTB8) also carries lipopolysaccharide transport to teichoic-acid and capsule exporters and to Gram-positive and archaeal permeases that have no LPS (recorded as a GO_CENTRAL source defect proposing the same repoint). Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: LINEAGE OR TAXON MISMATCH GRANULARITY MISMATCH Sources checked: PANTHER:PTN002019107 · PTN002019107 SUPPORTS SOURCE BUT NOT TARGET The node spans teichoic-acid, capsule and archaeal polysaccharide permeases, so an LPS-specific term is too narrow for the clade; the substrate-neutral GO:0015774 polysaccharide transport fits the node and, for Wzm, also describes what is transported (a lipid-linked polysaccharide, not LPS). UniProtKB:Q9HTB8 · wzm (self; sole IBD seed, IMP PMID:9244257) SUPPORTS TRANSFER Expected self-seed; the mutant phenotype supports polysaccharide export by Wzm. Proposed replacements: polysaccharide transport Supporting Evidence: PMID:9244257 transport of the polymer to the cell surface was inhibited. The inability of the polymer to cross the inner membrane resulted in the accumulation of cytoplasmic A-band polysaccharide. file:PSEAE/Q9HTB8/Q9HTB8-deep-research-falcon.md This D-rhamnan polymer is assembled on the cytoplasmic face of the inner membrane on a lipid carrier (UndPP), and Wzm–Wzt transports the completed polymer across the membrane to the periplasmic face |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Localization to the cytoplasmic (inner/plasma) membrane, from UniProt subcellular location mapping; Wzm is a multi-pass inner-membrane protein. Reason: Correct and informative cellular component. Wzm is a polytopic inner-membrane protein with six predicted transmembrane helices (UniProt), consistent with its role as the membrane conduit of the ABC transporter. Supporting Evidence: PMID:23223798 an ATP-binding cassette (ABC) transporter consisting of a transmembrane protein, Wzm, and a cytoplasmic nucleotide-binding protein, Wzt. |
| GO:0016020 membrane | IEA GO_REF:0000120 | MODIFY | Summary: Generic membrane localization from InterPro/UniRule electronic mapping. Reason: Correct but more general than the evidence supports. Wzm is a polytopic inner-membrane permease, so the specific location is GO:0005886 plasma membrane, which this review already accepts from the subcellular-location mapping. Proposed replacements: plasma membrane |
| GO:0043190 ATP-binding cassette (ABC) transporter complex | IEA GO_REF:0000120 | ACCEPT | Summary: Wzm is a subunit of the ABC transporter complex; this complex membership is supported experimentally by the demonstration that Wzm and Wzt form a hetero-oligomeric ABC transporter. Reason: Wzm is a bona-fide component of the Wzm-Wzt ABC transporter; the complex-level annotation is correct and is corroborated by direct structural evidence that Wzm forms a homotetramer that assembles with a Wzt tetramer into the functional transporter (PMID:23223798). Supporting Evidence: PMID:23223798 Wzm forms a square-shaped homo-tetramer both in the presence and absence of Wzt |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | MODIFY | Summary: Generic transmembrane transport from InterPro mapping. Reason: Correct but more general than the evidence supports. Wzm moves the undecaprenyl-pyrophosphate-linked A-band polysaccharide across the inner membrane (PMID:9244257). GO has no polysaccharide transmembrane transport term, so the most specific transmembrane process that fits is GO:0034219 carbohydrate transmembrane transport. GO:0015774 polysaccharide transport (the MODIFY target of the GO:0015920 rows) names the substrate but is not a transmembrane-transport term. Proposed replacements: carbohydrate transmembrane transport |
| GO:0140359 ABC-type transporter activity | IEA GO_REF:0000002 | ACCEPT | Summary: ABC-type (ATP-driven) transporter molecular function inferred from the ABC-2 transmembrane domain signature. Reason: Appropriate molecular function for the transporter. Strictly, Wzm is the transmembrane (permease) subunit and contributes to the ABC-type transporter activity of the Wzm-Wzt complex rather than enabling it independently (ATP hydrolysis is provided by Wzt), but at this granularity the term correctly captures Wzm's function as part of the transporter. Captured more precisely in core_functions via contributes_to. Supporting Evidence: PMID:23223798 an ATP-binding cassette (ABC) transporter consisting of a transmembrane protein, Wzm, and a cytoplasmic nucleotide-binding protein, Wzt. |
| GO:0009243 O antigen biosynthetic process | IMP PMID:9244257 Identification and functional characterization of an ABC tra... | ACCEPT | Summary: Experimental (mutant phenotype) support for a role in O-antigen/A-band LPS assembly; wzm mutants fail to export A-band polysaccharide to the cell surface. Reason: Wzm-mediated export across the inner membrane is a required step in ABC-transporter-dependent O-antigen (A-band CPA) assembly. Chromosomal wzm mutants still synthesize the A-band polysaccharide but cannot deliver it to the surface, directly implicating Wzm in the biosynthetic/assembly pathway (PMID:9244257). Supporting Evidence: PMID:9244257 the wzm and wzt mutants were able to synthesize A-band polysaccharide, although transport of the polymer to the cell surface was inhibited. |
| GO:0015920 lipopolysaccharide transport | IMP PMID:9244257 Identification and functional characterization of an ABC tra... | MODIFY | Summary: Direct experimental (gene-replacement mutant) evidence that Wzm is required to transport the A-band polysaccharide across the inner membrane to the cell surface. Reason: The transport role is the core, experimentally supported function of Wzm. Chromosomal wzm mutants accumulate A-band polysaccharide in the cytoplasm because the polymer cannot cross the inner membrane (PMID:9244257). What crosses is the undecaprenyl-pyrophosphate-linked polysaccharide, which becomes LPS only after WaaL ligation, so GO:0015774 polysaccharide transport states the substrate correctly. Proposed replacements: polysaccharide transport Supporting Evidence: PMID:9244257 The inability of the polymer to cross the inner membrane resulted in the accumulation of cytoplasmic A-band polysaccharide. |
| GO:0055085 transmembrane transport | ISS PMID:9244257 Identification and functional characterization of an ABC tra... | MODIFY | Summary: Sequence-similarity-based generic transmembrane transport annotation (from UniProtKB:Q48478). Reason: Correct but more general than the evidence supports; as for the InterPro row, the most specific transmembrane process for export of the A-band polysaccharide across the inner membrane (PMID:9244257) is GO:0034219 carbohydrate transmembrane transport, since GO has no polysaccharide transmembrane transport term. Proposed replacements: carbohydrate transmembrane transport |
| GO:0070258 inner membrane pellicle complex | EXP PMID:23223798 Determination of the quaternary structure of a bacterial ATP... | REMOVE | Summary: Localization to an "inner membrane pellicle complex" — but GO:0070258 is defined as a structure of the apicomplexan-parasite pellicle, which is taxonomically inappropriate for a bacterium. Reason: GO:0070258 is defined as "A membrane structure formed of two closely aligned lipid bilayers that lie beneath the plasma membrane and form part of the pellicle surrounding an apicomplexan parasite cell." This is a taxon-inappropriate term for a Pseudomonas (bacterial) protein and is an erroneous mapping. The cited paper (PMID:23223798) actually characterizes the Wzm-Wzt ABC transporter located in the bacterial inner membrane; the intended concepts (inner-membrane ABC transporter complex) are already correctly captured by GO:0043190 (ABC transporter complex) and GO:0005886 (plasma/inner membrane), both of which this review already carries, so nothing is lost by removing this row rather than replacing it. Recommend removal of this mis-annotation. Supporting Evidence: PMID:23223798 an ATP-binding cassette (ABC) transporter consisting of a transmembrane protein, Wzm, and a cytoplasmic nucleotide-binding protein, Wzt. |
| GO:0015159 polysaccharide transmembrane transporter activity | IMP PMID:9244257 Identification and functional characterization of an ABC tra... | NEW | Summary: Proposed more-specific molecular function for Wzm as the transmembrane conduit of the ABC transporter that moves the A-band polysaccharide across the inner membrane. Reason: The existing GOA molecular-function annotation (GO:0140359, ABC-type transporter activity) is correct but generic. Wzm is the transmembrane (permease) subunit that forms the channel through which the undecaprenyl-pyrophosphate-linked A-band polysaccharide is translocated; loss of wzm blocks passage of the polymer across the inner membrane (PMID:9244257). The substrate becomes LPS only after WaaL ligation, so the substrate-neutral polysaccharide transmembrane transporter activity fits better than lipopolysaccharide transmembrane transporter activity. Recorded as a subunit-level activity that contributes to the ABC-type transporter activity of the Wzm-Wzt complex. Supporting Evidence: PMID:9244257 The inability of the polymer to cross the inner membrane resulted in the accumulation of cytoplasmic A-band polysaccharide. |
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Download this section (compressed HTML)Q: What is the precise substrate specificity and stoichiometry of the Wzm channel during D-rhamnan (A-band CPA) translocation, and does Wzm recognize the polymer directly or only via Wzt?
Q: Does Wzm contribute to substrate recognition/gating, or is its role limited to forming the transmembrane conduit while Wzt provides both energy and polysaccharide binding?
Experiment: Determine a high-resolution structure (cryo-EM) of the intact P. aeruginosa Wzm-Wzt transporter with and without bound D-rhamnan substrate to define the translocation pathway and the Wzm homotetramer architecture in a native lipid environment.
Experiment: Perform site-directed mutagenesis of predicted pore-lining residues in Wzm followed by in vivo A-band CPA surface-export assays (Western blot / immuno-EM with A-band-specific monoclonal antibodies) to map residues required for polysaccharide translocation.
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