omcE (GSU0618, UniProt Q74FJ0) — Geobacter sulfurreducens PCA

Starting point

• Q74FJ0 is unreviewed (TrEMBL), SubName "Cytochrome c", 232 aa, SignalP signal
  peptide 1-23, mature chain 24-232. No transmembrane feature, no lipid anchor.
• Four CXXCH motifs, at precursor positions 51, 103, 201 and 226 — a tetraheme
  cytochrome, matching the structure paper's "tetrahaem packing". Pfam PF09699
  ("Doubled CXXCH motif", IPR010177 Paired_CXXCH_1) covers 200-232, i.e. the last two.
• One structure: PDB 7TFS, cryo-EM at 4.30 Å, chain A = 1-232, from PMID:35798889.
• GOA is empty (omcE-goa.tsv is header only, checked 2026-11-10). Every row in the
  review is therefore NEW.
• No provider deep-research file; literature assembled by hand.

Classification — the third variation on the same problem

OmcE's placement is the worst of the three. The family is named for a dissimilatory
sulfite reductase; the subfamily is named for a decaheme lipoprotein, and OmcE is a
tetraheme cytochrome with no lipid anchor. PTHR35038-entries.csv shows SF6 also
holds MtrA (P0DSN3, decaheme, periplasmic) and a hydroxylamine oxidoreductase (Q1PX48,
octaheme enzyme); the wider family mixes in NrfB (SF5), OmcC (SF8), photosynthetic
reaction-centre cytochrome c subunits and a perchlorate reductase gamma subunit. This
family looks at least as heterogeneous as PTHR30333 and is a clear candidate for its own
family review; it is not reviewed here.

PAINT for PTHR35038 carries a single IBD, PTN002858933 / GO:0016491 oxidoreductase
activity, seeded by NrfB (P0ABL1) and Q8EJI6, with no taxon recorded. OmcE receives no
IBA from it — consistent with the empty GOA.

Biology, with provenance

Identity and structure

• OmcE was found alongside OmcS among proteins sheared from the cell surface, at about
  30 kDa, and predicted then to be a tetraheme outer-membrane cytochrome
  PMID:16332857.
• The 2022 cryo-EM structure showed OmcE is a second, thinner filament
  PMID:35798889,
  about 40 Å across versus about 50 Å for OmcS.
• The convergence result is the interesting one: OmcE and OmcS share no sequence or
  backbone similarity, yet their polymerised heme arrangements match, including a heme in
  one subunit coordinated by a histidine from the next
  PMID:35798889.
  The authors call that cross-subunit coordination unique among c-type cytochrome
  structures solved to date, and generalise it into a proposal that conductive bacterial
  appendages form a class defined by heme packing rather than homology
  PMID:35798889.
• Caveat that must not be dropped: the filaments were enriched from omcS-mutant
  strains, not from wild type
  PMID:35798889,
  and the authors read this as compensation
  PMID:35798889.
  So OmcE demonstrably polymerises, but how much filament a wild-type cell makes, and
  whether that matters physiologically, is open.
• Extra surface densities are hypothesised to be glycans, not demonstrated
  PMID:35798889,
  with candidate sites S83, N86, N87, N132, T184 and a 17-gene glycosyltransferase cluster
  downstream of omcE. Note that the sequence contains no N-X-S/T sequon at all, so any
  N-linked modification would be non-canonical; S83 and T184 would be O-linked. No
  glycosylation annotation is made.

Function

• Deleting omcE abolishes Fe(III) oxide reduction while leaving soluble acceptors intact,
  and complementation restores it
  PMID:16332857,
  PMID:16332857.
• The phenotype is specific to insoluble iron. Deleting omcE does not affect current
  production on an anode
  PMID:19461962,
  and a later systematic comparison found the same split
  PMID:37881251.
  So of the three nanowire cytochromes, OmcZ owns the electrode, OmcS owns Fe(III) oxide
  and DIET, and OmcE is Fe(III)-oxide-specific and dispensable elsewhere.
• OmcE is the specific binding partner for the artificial mediator resorufin: the redox
  shift seen in biofilms disappears in an omcE mutant but not in omcS or omcZ mutants
  PMID:39278116,
  and heterologously expressed OmcE monomers bind resorufin with Kd 720 nM
  PMID:39278116,
  apparently at heme II.

What has not been measured

No one has purified OmcE and shown it reducing an acceptor, the way Qian did for OmcS.
Wang et al. did not measure OmcE filament conductivity either; they inferred similar
electron transport from the matching inter-heme distances, and warned that
"measures of conductivity attributed to a specific protein need to be treated with some caution"
PMID:35798889. This is why the molecular-function row below is IC rather than IDA.

Annotation decisions

All rows are NEW (GOA is empty).
- GO:0020037 heme binding — IDA. Four CXXCH motifs, bis-His coordination resolved in
7TFS, UniProt BINDING features from the structure.
- GO:0009055 electron transfer activity — IC. Authored inference, justified by the
stacked bis-His heme chain in the structure, the Fe(III)-oxide phenotype, and the
resorufin binding at a heme. Flagged explicitly because no direct transfer assay exists.
- GO:0019645 anaerobic electron transport chain — IMP. Deletion plus complementation.
- GO:0051258 protein polymerization — IDA, with the omcS-mutant caveat in the reason.
- GO:0009986 cell surface — IDA. Sheared from the outer surface of intact cells;
filaments are extracellular.

Not added:
- GO:0009279 cell outer membrane. Same reasoning as the OmcS review, where the four
existing rows were marked over-annotated: no transmembrane segment, no lipid anchor,
recovered by shearing. Here there is no existing row to judge, so the term is simply not
proposed.
- GO:0016722 oxidoreductase activity, acting on metal ions — nothing has been assayed on
purified OmcE at all.
- Any glycosylation term — the modification is hypothesised, unidentified, and the protein
has no N-glycosylation sequon.