OmcS (locus GSU2504) is a hexaheme c-type cytochrome of Geobacter sulfurreducens that polymerizes outside the cell into helical filaments carrying electrons from the cell to insoluble acceptors. The 432-residue precursor has a cleavable signal peptide and no transmembrane segment or lipid anchor; the mature protein is only loosely held at the outer surface and shears off intact cells readily. Purified OmcS is 47 kDa, carries six low-spin bis-histidinyl hexacoordinated hemes, has a midpoint potential near -212 mV, and is mostly disordered in secondary structure. Cryo-electron microscopy of filaments sheared from cells showed that the conductive extracellular appendages long attributed to type IV pili are in fact micrometre-long polymers of OmcS, with hemes stacked within a few angstroms of each other, inter-subunit heme stacking, and heme iron coordinated axially by a histidine from the neighbouring subunit. Reaching the cell exterior requires the PilA-N/PilA-C filament: without it OmcS remains in the periplasm. Reduced OmcS delivers electrons in vitro to insoluble Fe(III) oxide, Mn(IV) oxide, humic substances, chelated Fe(III) and the humic analogue AQDS. Deleting omcS abolishes Fe(III) oxide reduction while leaving growth on fumarate and soluble Fe(III) citrate intact, and the paralogous hexaheme cytochrome OmcT, encoded immediately downstream and 62.6% identical, cannot substitute. OmcS is also essential for direct interspecies electron transfer to a Geobacter metallireducens partner, in which its transcript rises several hundred-fold. The gene is expressed during growth on Fe(III) oxide and on fumarate but not on soluble Fe(III) citrate.
Definition: A helical, non-membrane-bounded protein polymer that lies outside the cell and is built from repeating subunits of a secreted multiheme c-type cytochrome. The hemes stack along the filament axis, so electrons can be transferred along its length between the cell and extracellular electron acceptors or donors.
Justification: This is the same term proposed in the review of Geobacter sulfurreducens OmcZ, repeated here deliberately rather than cross-referenced, because two independently characterised genes needing the same missing term is the argument for creating it. OmcS is the better-established case of the two: its filament has two deposited structures (PDB 6EF8, 6NEF), and dedicated classifications already exist for it elsewhere, as CDD cd21555 "OmcS-like" and TCDB family 5.B.3.1.1, "the geobacter nanowire electron transfer (g-net) family". GO has no component term for such a filament. The terms available describe either the surface the filament is attached to (GO:0009986 cell surface) or the unrelated pilus (GO:0009289), which would wrongly imply a pilin appendage, and the historical confusion of these filaments with type IV pili is precisely what the 2019 structure corrected. Members differ in heme packing and arose more than once, so the class should be defined by structure and location rather than by homology. The narrative case for the term, the naming question (the field says "extracellular cytochrome nanowire"), and the three further process-term gaps are collected in projects/EXTRACELLULAR_CYTOCHROME_NANOWIRES.md.
Parent term: supramolecular complex
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0009055 electron transfer activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic propagation of the generic electron-carrier term from node PTN001251186 of PANTHER family PTHR30333, seeded by the quinol dehydrogenases TorC, TorY and CymA. OmcS also has direct evidence of its own: six redox-active hemes and measured electron transfer to several acceptors. Reason: The node was examined in this repository's review of PTHR30333 and assessed SOUND. The point there is worth repeating, because the family is heterogeneous and this row could look like over-propagation: the seeds are all inner-membrane quinol dehydrogenases, and OmcS is not one, but what PAINT placed at the node is the bare carrier term rather than a quinol-oxidising activity. That term holds for OmcS on its own evidence, so the propagation delivers something true. The term is also as specific as the evidence allows, since GO has no term for conduction along an extracellular cytochrome filament, so this is ACCEPT rather than MODIFY. Supporting Evidence: file:interpro/panther/PTHR30333/PTHR30333-paint.tsv PTHR30333 PTN001251186 GO:0009055 F IBD false PMID:21236241 Dithionite-reduced OmcS was able to transfer electrons to a variety of substrates of environmental importance including insoluble Fe(III) oxide, Mn(IV) oxide and humic substances. PMID:21236241 Purified OmcS had a molecular mass of 47015 Da, and six low-spin bis-histidinyl hexacoordinated heme groups. |
| GO:0019645 anaerobic electron transport chain | IBA GO_REF:0000033 | ACCEPT | Summary: Propagation of the process term from the same node. OmcS occupies the extracellular end of the anaerobic respiratory chain, where electrons leave the cell for an insoluble acceptor. Reason: OmcS does part of the work of the process rather than merely being required for it: it is itself the carrier over the final stretch, as the filament structure and the in vitro transfer to Fe(III) and Mn(IV) oxides show. The genetic phenotype is specific in the way that places it in the extracellular branch rather than the shared upstream segment, since deleting omcS leaves fumarate and Fe(III) citrate respiration intact and blocks only Fe(III) oxide. The node carrying this term was assessed SOUND in the PTHR30333 family review. Other terminal and envelope cytochromes reviewed here carry the same term (ImcH, CbcL, OmcZ, and Shewanella OmcA), and GO offers nothing narrower for respiration on an extracellular solid acceptor. Supporting Evidence: file:interpro/panther/PTHR30333/PTHR30333-paint.tsv PTHR30333 PTN001251186 GO:0019645 P IBD false PMID:16332857 When either omcS or omcE was deleted, G. sulfurreducens could no longer reduce Fe(III) oxide but could still reduce soluble electron acceptors, including Fe(III) citrate. |
| GO:0009986 cell surface | IEA GO_REF:0000044 | ACCEPT | Summary: Automatic mapping of the UniProt "Cell surface" subcellular location, which rests on the same three experimental papers as the EXP rows below. Reason: The mapping is correct and the underlying location is right. GO:0009986 is defined for products attached either integrally or loosely to the cell envelope, which is exactly what OmcS is: loosely bound to the outer surface, shearable from intact cells, and polymerised into cell-attached filaments. Kept consistent with the three EXP rows for the same term. Supporting Evidence: PMID:21236241 OmcS, an abundant c-type cytochrome that is only loosely bound to the outer surface |
| GO:0009986 cell surface | EXP PMID:16332857 Outer membrane c-type cytochromes required for Fe(III) and M... | ACCEPT | Summary: OmcS was identified among the most abundant proteins sheared from the outer surfaces of intact cells, which is a direct demonstration of surface exposure. Reason: Shearing from intact cells localises the protein to the accessible outer surface without disrupting the envelope, and the result has been reproduced by immunogold labelling and by purification from the surface fraction. This is also where the core activity happens, since an insoluble Fe(III) oxide particle is never brought inside. Supporting Evidence: PMID:16332857 Two of the most abundant proteins that were easily sheared from the outer surfaces of intact cells were c-type cytochromes. |
| GO:0009986 cell surface | EXP PMID:20400557 Alignment of the c-type cytochrome OmcS along pili of Geobac... | ACCEPT | Summary: Immunogold labelling localised OmcS along the pili at the cell surface. Reason: Independent method, same conclusion about surface exposure. Note that the "along the pili" detail has been superseded in its interpretation: the 2019 cryo-EM work showed the conductive filaments to be polymers of OmcS itself rather than pili decorated with OmcS. That revision does not affect the cell-surface claim, which is what this row asserts. Supporting Evidence: PMID:20400557 Immunogold localization revealed that OmcS, a cytochrome that is required for Fe(III) oxide reduction by Geobacter sulfurreducens, was localized along the pili. |
| GO:0009986 cell surface | EXP PMID:21236241 Biochemical characterization of purified OmcS, a c-type cyto... | ACCEPT | Summary: The biochemical characterisation describes OmcS as loosely bound to the outer surface and purifies it from that fraction. Reason: Third independent line for the same location, and the one that states the attachment mode explicitly. Loose attachment is within the scope of GO:0009986 by definition. Supporting Evidence: PMID:21236241 OmcS, an abundant c-type cytochrome that is only loosely bound to the outer surface |
| GO:0009279 cell outer membrane | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: Automatic mapping of the UniProt "Cell outer membrane" subcellular location. In the UniProt record that location is immediately qualified by the note "Loosely associated with the cell surface", but the note does not survive into the GO row, which asserts residence in the outer membrane on its own. Reason: GO:0009279 is the outer lipid bilayer of the Gram-negative envelope, and nothing shows OmcS to be part of it. The entry records no transmembrane segment, no lipid anchor and no lipobox; the protein is described in its own curated note and in the primary literature as loosely associated and easily sheared; and the mature functional form is a polymerised extracellular filament. The term is marked over-annotated rather than removed because OmcS does sit at the outer-membrane surface, so the claim is imprecise rather than false, and because the companion row GO:0009986 cell surface already captures the location correctly. Kept consistent with the three EXP rows for this term. Supporting Evidence: PMID:21236241 OmcS, an abundant c-type cytochrome that is only loosely bound to the outer surface PMID:30951668 G. sulfurreducens nanowires are assembled by micrometer-long polymerization of the hexaheme cytochrome OmcS |
| GO:0009279 cell outer membrane | EXP PMID:16332857 Outer membrane c-type cytochromes required for Fe(III) and M... | MARK AS OVER ANNOTATED | Summary: The 2005 study that named OmcS. It recovered the protein by shearing intact cells and described it as a predicted outer membrane hexaheme cytochrome. Reason: What the experiment shows is surface accessibility, not membrane residence: the proteins were obtained by shearing the outside of intact cells, and the paper's own wording for the outer membrane assignment is "predicted". A prediction stated in a paper whose experiments demonstrate something adjacent does not carry the EXP code's weight for this particular term. The surface localisation the same experiment does establish is accepted above under GO:0009986. Supporting Evidence: PMID:16332857 One, designated OmcS, has a molecular mass of ca. 50 kDa and is predicted to be an outer membrane hexaheme c-type cytochrome. PMID:16332857 Two of the most abundant proteins that were easily sheared from the outer surfaces of intact cells were c-type cytochromes. |
| GO:0009279 cell outer membrane | EXP PMID:20400557 Alignment of the c-type cytochrome OmcS along pili of Geobac... | MARK AS OVER ANNOTATED | Summary: Immunogold localisation along the pili. The signal is at the cell exterior, not within the outer membrane. Reason: This experiment places OmcS outside the outer membrane rather than in it, which is if anything evidence against the term as written. Marked over-annotated for consistency with the other rows for this term and because OmcS is still at the outer-membrane surface; the same experiment supports the cell-surface row, which is accepted. Supporting Evidence: PMID:20400557 Immunogold localization revealed that OmcS, a cytochrome that is required for Fe(III) oxide reduction by Geobacter sulfurreducens, was localized along the pili. |
| GO:0009279 cell outer membrane | EXP PMID:21236241 Biochemical characterization of purified OmcS, a c-type cyto... | MARK AS OVER ANNOTATED | Summary: Purification and characterisation of OmcS from the surface fraction, with the attachment described as loose. Reason: The paper that most directly addresses how OmcS is held states that it is only loosely bound to the outer surface, which is the opposite of integral membrane residence. Kept consistent with the other three rows for this term. Supporting Evidence: PMID:21236241 OmcS, an abundant c-type cytochrome that is only loosely bound to the outer surface |
| GO:0020037 heme binding | IDA PMID:21236241 Biochemical characterization of purified OmcS, a c-type cyto... | NEW | Summary: OmcS carries six covalently bound low-spin c-type hemes, counted in the purified protein by mass and spectroscopy and resolved in two independent cryo-EM structures of the filament. Reason: Heme binding is the cofactor basis of everything OmcS does and is demonstrated directly, yet GOA has no molecular-function row for it beyond the generic carrier term. The purified protein was shown to hold six bis-histidinyl hexacoordinated hemes (PMID:21236241), the sequence carries exactly six CXXCH motifs, and the filament structures resolve the heme chain including inter-subunit stacking and axial coordination by a histidine from the neighbouring subunit (PMID:30951668). UniProt curates the heme cofactor with experimental evidence. Recorded alongside the electron-transfer row rather than as a separate core function, as for the other multiheme cytochromes reviewed here. Supporting Evidence: PMID:21236241 Purified OmcS had a molecular mass of 47015 Da, and six low-spin bis-histidinyl hexacoordinated heme groups. PMID:30951668 The inter-subunit interfaces show unique structural elements such as inter-subunit parallel-stacked hemes and axial coordination of heme by histidines from neighboring subunits. |
| GO:0051258 protein polymerization | IDA PMID:30951668 Structure of Microbial Nanowires Reveals Stacked Hemes that ... | NEW | Summary: The conductive filaments on the cell surface are homopolymers of OmcS itself, resolved by cryo-EM at 3.4 angstroms and extending for micrometres. Reason: OmcS performs the polymerisation rather than merely being its substrate: the filament contains no other protein, and the contacts that hold it together are made by OmcS surfaces, including hemes stacked across the subunit interface and heme iron coordinated by a histidine donated from the adjacent subunit. The polymer is the functional form, so this is not incidental aggregation. The same action was taken for OmcZ, the other polymerising cytochrome of this organism. The row is kept at the generic process level because GO has no term for assembly of an extracellular cytochrome filament; the matching component term is proposed below. Supporting Evidence: PMID:30951668 G. sulfurreducens nanowires are assembled by micrometer-long polymerization of the hexaheme cytochrome OmcS PMID:30951668 The inter-subunit interfaces show unique structural elements such as inter-subunit parallel-stacked hemes and axial coordination of heme by histidines from neighboring subunits. |
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Download this section (compressed HTML)Q: How should GO represent direct interspecies electron transfer? OmcS is described as essential for DIET and is induced several hundred-fold in cocultures, but there is no GO term for the process, and the usual single-organism framing of a biological process does not fit a two-cell electrical connection.
Q: Where does an OmcS filament get its electrons? The filament is released at the cell surface with no known anchor to a specific outer-membrane conduit, so the handover from the porin-cytochrome complexes is unresolved.
Q: Is PANTHER's placement of OmcS in PTHR30333:SF1, the NapC subfamily, defensible? OmcS lacks the NapC/NirT N-terminal domain that defines that family, and both CDD and TCDB give it a dedicated family of its own.
Q: Why does a Geobacter sulfurreducens cell make two unrelated polymerising cytochromes, OmcS and OmcZ, with different acceptor preferences and different conductivities, rather than one?
Experiment: In a background making only OmcS filaments (omcZ, omcE and omcT deleted), delete each outer-membrane conduit in turn (extABCD, ombB-omaB-omcB, ombC-omaC-omcC) and measure Fe(III) oxide reduction rate, with immunogold TEM to confirm the filaments are still made and correctly localised. A conduit whose loss phenocopies omcS deletion while the filaments remain identifies the entry point.
Hypothesis: OmcS filaments are electrically coupled to a specific outer-membrane porin-cytochrome conduit rather than collecting electrons promiscuously.
Type: genetic epistasis with electrochemistry
Experiment: Fractionate G. sulfurreducens into inner membrane, outer membrane, periplasm and sheared surface material, with marker proteins for each compartment, and test whether OmcS partitions with an outer-membrane marker or is quantitatively removed by gentle shearing before fractionation. Carbonate extraction and phase separation would distinguish a peripheral association from integration into the bilayer.
Hypothesis: The outer membrane localisation in UniProt reflects fractionation behaviour rather than membrane residence.
Type: subcellular fractionation with controls
Experiment: Express and purify monomeric OmcS and test whether it self-assembles into filaments under physiological conditions without any accessory factor, following the design used for OmcZ and OzpA. If assembly needs a partner or a modification, identify it by fractionating culture supernatant for the activity that permits polymerisation.
Hypothesis: OmcS polymerisation is regulated, as OmcZ polymerisation is by the protease OzpA.
Type: in vitro reconstitution
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