SMIM5 (Q71RC9) review notes

2026-10-03 Tier 2 microprotein review

UniProt Q71RC9 SMIM5_HUMAN, 77 aa, PE1; alias C17orf109. One predicted helical TM segment
(32-52) followed immediately by a cysteine cluster (CSCCCTHCCC, residues 53-62) and a short
basic/proline-rich C terminus. Membrane / single-pass is curator inference (ECO:0000305).
Family: Pfam PF15831 SMIM5_18_22, InterPro IPR031671 (SMIM5/18/22), PANTHER PTHR37344.
SMIM22 (CASIMO1) is the only family member with a published function (SQLE-associated ER
microprotein; see genes/human/SMIM22). Function should not be transferred from SMIM22 to SMIM5.
Expression: HPA "Tissue enhanced (kidney)" (UniProt DR line). HPA JSON (fetched 2026-10-03):
kidney nTPM 32.7, immune-cell enriched in plasmacytoid DC; antibody IF main location "Vesicles",
additional "Golgi apparatus". Not in GOA and not used for annotation.

PubMed SMIM5[tiab] OR C17orf109[tiab] -> 5 hits, all expression/genetic lists:
PMID:42270003 (higher SMIM5 mRNA in diabetic-retinopathy blood), PMID:30275705 (hub gene in oral
squamous cell carcinoma co-expression network), PMID:24318988 (differentially expressed in ccRCC),
PMID:39364777 (chicken GWAS candidate), PMID:33584805 (cattle selection signature). None tests
protein function, so none is cached or cited.

7 x protein binding (IPI) from HuRI PMID:32296183: SGTA (O43765), ARFIP1 (P53367), EHHADH
(Q08426), PRRT2 (Q7Z6L0), RBFA (Q8N0V3), ZFYVE21 (Q9BQ24), SH3GLB1 (Q9Y371).
SGTA (NbExp=9 in UniProt) is a cytosolic quality-control factor that engages exposed hydrophobic
TMDs of mislocalized membrane proteins PMID:23129660. An SGTA hit
for a single-pass TM peptide is expected client recognition.
The others are a mixed set (mitochondrial RBFA, peroxisomal EHHADH, BAR-domain ARFIP1/SH3GLB1,
membrane PRRT2, FYVE ZFYVE21) with no shared pathway; no follow-up exists.
-> REMOVE all seven; no informative MF.
membrane IEA -> ACCEPT.

No function known. No core function, no NEW terms.