fimU (PA4550, G3XCZ0) notes

Identity

• UniProt G3XCZ0, unreviewed (TrEMBL), 168 aa; gene fimU, locus PA4550.
  Recommended name "Type II secretion system protein H" is an ARBA-generated
  name (ECO:0000256|ARBA), not a curated function. Class III signal peptide
  (PROPEP 1..9, PROSITE PRU01070), TM helix 12-30 (Phobius), InterPro
  IPR022346 T2SS_GspH, Pfam PF12019 GspH, PF07963 N_methyl. No PANTHER xref in
  the UniProt entry.
• First gene of the fimU-pilVWXY1Y2E minor pilin operon region; P. aeruginosa
  has its own T2SS GspH (XcpU), so FimU is the type IV pilus paralogue, not
  the Xcp component.

Literature

• fimU mutants lack fimbriae and twitching; FimT overexpression can replace it
  [PMID:8682785 "Isogenic mutants constructed by allelic exchange demonstrated
  that the fimU gene was required for fimbrial biogenesis and twitching
  motility"; "overexpression of the fimT gene was found to be able to
  functionally replace the lack of a fimU gene product"].
• Minor pilins are processed by PilD and incorporated into pili
  PMID:20338182; FimU promotes assembly PMID:20338182.
• FimU is incorporated independently and couples the priming complex to the
  major pilin; crystal structure resembles GspH [PMID:25389296 "FimU is
  incorporated independently of the others and is proposed to couple the
  putative minor pilin-PilY1 complex to the major subunit."; "our 1.4 Å
  crystal structure of FimU revealed striking architectural similarity to its
  T2S ortholog GspH, despite minimal sequence identity"]. The paper also shows
  that Xcp minor pseudopilins can prime T4P assembly, i.e. the T2S proteins
  cross over into T4P, not that FimU acts in T2S.

PANTHER node PTN008624060

• IBD of GO:0044096 type IV pilus (2026-03-04), WITH UniProtKB:G3XCZ0 only.
• Leaves (7, 6 taxa): P. aeruginosa FimU, Shewanella FimU (Q8EBJ4),
  Xanthomonas FimT x2 (Q8P7W3, Q8P7X0), Neisseria meningitidis Q9JZV4,
  Vibrio VC_0858 (Q9KTP3), Dictyoglomus turgidum Dtur_1183 (B8E2W2, an
  N-methylation-motif protein). All are FimU/FimT-type minor pilins by name or
  domain; type IV pilus as a location suits minor pilins, which are
  incorporated into the fibre.

PseudoCAP IBA-seed assessment

Decisions summary

• ACCEPT: plasma membrane (IEA); type IV pilus (IBA, IDA); motility (IMP);
  type IV pilus assembly (IMP).
• MODIFY: pilus assembly (IDA) to GO:0043683 type IV pilus assembly.
• REMOVE: type II protein secretion system complex and protein secretion by
  the type II secretion system (IEA from IPR022346 GspH): domain shared with
  T2SS GspH, but FimU is the T4P paralogue; P. aeruginosa has a separate
  Xcp GspH (XcpU).