SMIM18 (small integral membrane protein 18) is a 95-amino-acid (about 11 kDa) protein encoded on chromosome 8p12 within an intron of GTF2E2, on the opposite strand. It has a single predicted helical transmembrane segment (residues 35-55), preceded by a short N-terminal segment and followed by a cysteine cluster and a charged C-terminal tail. It belongs to the SMIM5/18/22 family, whose best studied member, SMIM22 (CASIMO1), is a membrane microprotein that binds squalene epoxidase. The resemblance is largely confined to the transmembrane region, so no function can be inferred from it. Orthologues are found across jawed vertebrates, including teleost fish. The protein has been detected by mass spectrometry, and its RNA is enriched in brain, pituitary gland and retina. Its membrane localisation is predicted from sequence; the specific membrane, topology, interaction partners and biological role are unknown, and no functional study of SMIM18 has been published.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016020 membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping to membrane, based on the single predicted helical transmembrane segment (residues 35-55). Reason: Correct, though general. A 21-residue hydrophobic helix in a 95-residue protein makes SMIM18 a single-pass integral membrane protein. No experiment identifies the specific membrane, so a more precise location cannot be given. This is the only annotation the gene carries and the only one the evidence supports. Supporting Evidence: file:human/SMIM18/SMIM18-uniprot.txt FT TRANSMEM 35..55 file:human/SMIM18/SMIM18-uniprot.txt CC -!- SUBCELLULAR LOCATION: Membrane {ECO:0000305}; Single-pass membrane |
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Download this section (compressed HTML)Q: Which membrane does SMIM18 occupy in neurons and pituitary cells, and is its N-terminus luminal (the N-x-T sequon at Asn6 would then be glycosylated)?
Q: Do SMIM18 and its relative SMIM22/CASIMO1 share any partner or function, given that their similarity is concentrated in the transmembrane helix and juxtamembrane cysteines?
Experiment: Express epitope-tagged SMIM18 (or tag the endogenous locus) in a neuronal or pituitary cell line, co-stain with organelle markers, determine topology by protease protection and PNGase F sensitivity, and identify partners by proximity labelling.
Hypothesis: SMIM18 is a single-pass membrane protein of a defined organelle in brain cells.
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