COX18 (human) — curation notes

UniProtKB: Q8N8Q8 · HGNC:26801 · Gene: COX18 (syn. OXA1L2) · 333 aa · Chr 4.
Family: OXA1/ALB3/YidC membrane protein insertase family; COX18 sub-branch (as opposed
to OXA1L sub-branch). TCDB 2.A.9.1.3 (YidC/Alb3/Oxa1 insertase family). InterPro
IPR001708 (YidC/ALB3/OXA1/COX18); Pfam PF02096 (60KD_IMP); CDD cd20069 (5TM_Oxa1-like).

Verified function

COX18 is a mitochondrial inner-membrane protein-insertion machinery component required
for cytochrome c oxidase (respiratory chain Complex IV) assembly. It mediates the
membrane insertion / translocation of the C-terminal portion (C-tail, apo-CuA site) of
the newly synthesised core subunit MT-CO2 (COX2) across the inner membrane, acting after
COX20-mediated stabilisation of MT-CO2 and before the SCO1/SCO2/COA6 copper-metallation
step. Its molecular function is a membrane insertase / protein-transmembrane-transporter
(translocase) activity, not a purely structural role.

• OXA1/YidC insertase family membership; acts as COX2 assembly factor:
  PMID:28330871 Oxa1
  family of membrane protein insertases.
• Mechanism — transient interaction with COX2, translocation of C-tail across IMM:
  PMID:28330871
  membrane of the COX2 C-tail that contains the apo-CuA site.
• Yeast Cox18p inserts Cox2p COOH-tail into IMM (conserved function):
  PMID:16212937 mitochondrial
  inner membrane.
• Family members facilitate membrane insertion of hydrophobic proteins; Oxa1 and Cox18
  play successive roles in Cox2 assembly:
  PMID:16911509 in Cox2 assembly;
  PMID:16911509 proteins.
• Human disease / loss-of-function reduces Complex IV:
  PMID:40830826 the mitochondrial inner
  membrane; mutant "impairs Complex IV assembly and activity";
  PMID:40830826;
  PMID:40830826;
  the amphipathic/insertase helix is PMID:40830826.

Localisation

Mitochondrion inner membrane, multi-pass (UniProt SUBCELLULAR LOCATION; 3 TM helices,
matrix/IMS topology). Confirmed IDA PMID:16212937, EXP PMID:28330871.
Broad GO:0005739 mitochondrion (HTP, PMID:34800366) and GO:0016020 membrane (IEA) are
correct but less informative parents of GO:0005743.

Interactions

Complex with TMEM177, COA6, MT-CO2, COX20, SCO1, SCO2 (PMID:29154948); transient with
MT-CO2 and COX20-dependent interaction (PMID:28330871). The IPI GO:0005515 "protein
binding" (with O75880=COX20, P00403=MT-CO2, Q5RI15=TMEM177) is uninformative as an MF —
its real functional meaning is captured by the insertase MF + assembly BP; MARK_AS_OVER_ANNOTATED.

Disease

• MC4DN25 (mitochondrial complex IV deficiency, nuclear type 25): neonatal
  encephalopathy, hypertrophic cardiomyopathy, myopathy, axonal sensory neuropathy
  (PMID:37468577).
• CMT2MM (Charcot-Marie-Tooth, axonal, type 2MM): autosomal recessive axonal
  sensorimotor peripheral neuropathy (PMID:40830826).

Annotation-review summary

• MF core = GO:0032977 membrane insertase activity (multiple lines: IBA, IEA, IMP, EXP,
  IGI, IDA) — ACCEPT.
• BP core = GO:0033617 mitochondrial respiratory chain complex IV assembly (IBA, IMP,
  TAS, IMP) and GO:0032979 protein insertion into mito inner membrane from matrix
  (IBA, IDA) — ACCEPT.
• GO:0008535 respiratory chain complex IV assembly (IGI) is a broader/parent-style term
  of GO:0033617 → MODIFY to GO:0033617. GO:0051204 / GO:0051205 protein insertion into
  (mitochondrial) membrane are broader parents of GO:0032979 → MODIFY / KEEP_AS_NON_CORE.
• CC core = GO:0005743 mitochondrial inner membrane (IBA, IEA, EXP, TAS, IDA) — ACCEPT.
  GO:0005739 mitochondrion (HTP) and GO:0016020 membrane (IEA) broader → KEEP_AS_NON_CORE.
• GO:0005515 protein binding (IPI) → MARK_AS_OVER_ANNOTATED (uninformative bare term).