C5 (Complement C5; UniProtKB:P01031) — Curation Notes

1. Gene Product Overview & Proteolytic Maturation into Functional Isoforms (C5a and C5b)

• Gene / Protein: C5 (HGNC:1331, UniProtKB:P01031), encoding human complement component C5 (1676 aa precursor).
• PANTHER Classification: Member of PANTHER:PTHR11412 (MACROGLOBULIN / COMPLEMENT), specifically subfamily PANTHER:PTHR11412:SF83 (COMPLEMENT C5).
• Two-Stage Proteolytic Processing:
• Intracellular pro-C5 processing: Following removal of the signal peptide (residues 1–18) and a tetra-basic linker (RPRR, residues 674–677), mature plasma C5 circulates as a ~190 kDa disulfide-linked two-chain glycoprotein composed of the N-terminal beta chain (PRO_0000005985, residues 19–673) and C-terminal alpha chain (PRO_0000005987, residues 678–1676) [PMID:106884, PMID:690134].
• Convertase-mediated activation cleavage (Arg751–Leu752): Upon activation of the classical/lectin (C4b2b3b), alternative (C3bBb3b), or granzyme K (GZMK-initiated C4b2b3b) complement cascades, surface-assembled C5 convertases cleave the C5 alpha chain between Arg751 and Leu752 [PMID:12878586, PMID:21217642, PMID:30643019, PMID:39914456], generating two structurally and functionally distinct mature products (functional_isoforms):
  • C5a (PRO_0000005988, C5a anaphylatoxin, residues 678–751, 74 aa): A soluble four-helix bundle glycoprotein (N-glycosylated at Asn741, position 64 of C5a) released into the extracellular fluid PMID:690134. C5a acts as a high-affinity agonist of the complement anaphylatoxin receptors C5AR1 (CD88, UniProtKB:P21730, coupled to $G\alpha_i$/$G\alpha_o$/$G\alpha_{16}$ and $\beta$-arrestin) and C5AR2 (C5L2, UniProtKB:Q9P296, an atypical $\beta$-arrestin-biased receptor lacking G protein coupling) [PMID:8182049, PMID:9553099, PMID:36806352, PMID:37169960, PMID:37852260]. Plasma carboxypeptidases N and B2 rapidly cleave the C-terminal Arg751 (Arg74 of C5a) to yield C5a des-Arg (residues 678–750), a naturally occurring partial/biased agonist with ~10–100-fold lower potency at C5AR1 that dampens acute inflammatory signaling while retaining leukocyte chemotactic activity at higher concentrations [PMID:690134, PMID:37852260].
  • C5b (PRO_0000005989 alpha' chain residues 752–1676 disulfide-linked to the beta chain PRO_0000005985): Undergoes a dramatic conformational rearrangement of its MG3, MG7, MG8, CUB, and thioester-like domain (TED/C5d) upon release of C5a PMID:22832194. Unlike C3 and C4, C5 lacks an internal reactive thioester bond in its TED domain (C5 is not a covalent opsonin and is not an endopeptidase inhibitor). Instead, nascent C5b exposes a transient (~2 min half-life) binding site captured by C6 to form the stable C5b6 subcomplex, which sequentially recruits C7, heterotrimeric C8 (C8A/C8B/C8G), and ~18 copies of C9 to assemble the cytolytic membrane attack complex (MAC, GO:0005579) pore in target bacterial and cellular membranes [PMID:22832194, PMID:30552328, PMID:30643019, PMID:34667172].

2. Ligand-Side Signaling Pathway Audit (LIGANDS_IN_SIGNALING)

• Missing Pathway Annotation (NEW):
• GO:0038178 (complement component C5a signaling pathway, defined as "The series of molecular signals generated as a consequence of the complement component C5a binding to a cell surface receptor", is_a GO:0007186 G protein-coupled receptor signaling pathway) is annotated in human GOA to the receptors C5AR1 (P21730) and C5AR2 (Q9P296), but not to the ligand C5 (P01031).
• Meanwhile, C5 carries legacy ProtInc TAS annotations for the broader parent terms GO:0007166 (cell surface receptor signaling pathway) and GO:0007186 (G protein-coupled receptor signaling pathway) citing PMID:8759757 (Nilsson et al. 1996, showing pertussis toxin-sensitive C5a-induced calcium mobilization and chemotaxis in HMC-1 human mast cells) and PMID:8898085 (Ames et al. 1996, functional expression of human C5aR and C3aR in Xenopus oocytes).
• We modify GO:0007166 and GO:0007186 to GO:0038178 (complement component C5a signaling pathway) and propose GO:0038178 as a NEW core biological process annotation on C5 (specifically executed by the C5a isoform PRO_0000005988), supported by cryo-EM structures of the human C5a–C5AR1–$G_i$ signaling complex [PMID:36806352, PMID:37169960, PMID:37852260] and receptor-binding/signaling studies [PMID:8182049, PMID:8759757, PMID:9553099].
• Receptor Binding Specificity:
• C5a binds C5AR1 via a two-site/three-site mechanism: the helical bundle core of C5a engages the sulfated/acidic N-terminal extracellular segment (residues 2–30, especially 21–30) of C5AR1, while the C-terminal octapeptide tail of C5a (HKDMQLGR, terminating in Arg751/Arg74) inserts into the transmembrane orthosteric pocket of C5AR1 to trigger TM6 outward movement and $G_i$ coupling [PMID:8182049, PMID:9553099, PMID:36806352, PMID:37169960, PMID:37852260].
• Existing GO:0001664 (G protein-coupled receptor binding, IBA) and GO:0005102 (signaling receptor binding, TAS from PMID:9218605 and PMID:9590258, IPI from PMID:16452172) are well supported; GO:0005102 is refined (MODIFY) to GO:0001664 (G protein-coupled receptor binding).

3. Erroneous Annotations on C5: Chemokine Activity (GO:0008009) and Endopeptidase Inhibitor Activity (GO:0004866)

1. GO:0008009 (chemokine activity) (TAS, PMID:10820279) and GO:0060326 (cell chemotaxis) (IEA, GO:0008009 -> GO:0060326) — REMOVE:
2. C5 carries a legacy ProtInc TAS annotation for GO:0008009 (chemokine activity) citing PMID:10820279 (Luu et al. 2000, "Differential ability of exogenous chemotactic agents to disrupt transendothelial migration of flowing neutrophils"), which compared fMLP, IL-8, ENA-78, GRO-$\alpha$, and zymosan-activated plasma/C5a as "exogenous chemotactic agents".
3. Worse, PAINT node PTN008967461 in PANTHER:PTHR11412 used UniProtKB:P01031 (C5) and UniProtKB:P01024 (C3) as IBD seeds to propagate GO:0008009 (chemokine activity) across the entire Eumetazoan complement C3/C4/C5 clade!
4. Why this is false: C5a and C3a are complement anaphylatoxins (IPR001840 / PF01821 four-helix bundle cleavage fragments of $\alpha_2$-macroglobulin/complement family precursors PTHR11412) that signal through anaphylatoxin GPCRs (C5AR1, C5AR2, C3AR1 in PTHR24225 CHEMOTACTIC RECEPTOR). They are not chemokines (which are small Greek-key $\beta$-sheet cytokines of families PTHR12015 and PTHR10179 with conserved CC/CXC/C/CX3C disulfide motifs that activate chemokine receptors in PTHR10489). Indeed, PMID:37169960 explicitly notes that cryo-EM structures "reveal a conserved recognition pattern of anaphylatoxins to the complement receptors that is different from chemokine receptors".
5. While C5a genuinely mediates leukocyte/neutrophil/mast cell chemotaxis (GO:0006935 chemotaxis, GO:0030593 neutrophil chemotaxis), it is a non-chemokine chemoattractant; both GO:0008009 (TAS) and the GO_REF:0000116 GOC-OWL IEA for GO:0060326 (cell chemotaxis, inferred solely from GO:0008009) must be marked REMOVE.
6. GO:0004866 (endopeptidase inhibitor activity) (IEA, GO_REF:0000002 via IPR001599/IPR002890) — REMOVE:
7. InterPro2GO maps the broad $\alpha_2$-macroglobulin domains (IPR001599 Macroglobln_a2, IPR002890 MG2) to GO:0004866 (endopeptidase inhibitor activity).
8. While $\alpha_2$-macroglobulin (A2M, P01023, PANTHER:PTHR11412:SF165) and pregnancy zone protein (PZP) are genuine bait-region/thioester-trapping endopeptidase inhibitors (and PAINT correctly restricts GO:0004866 to the A2M clade PTN000892454), complement C5 (P01031, PANTHER:PTHR11412:SF83) lacks both the protease-trapping bait region and the internal reactive thioester (C5 is cleaved by C5 convertase endopeptidases at Arg751–Leu752 to generate C5a and C5b) [PMID:21217642, PMID:22832194]. Thus GO:0004866 on C5 is a classic domain-level InterPro2GO over-annotation and is marked REMOVE.