Rv0810c: a genus-conserved, structurally ordered small protein of unknown function carrying DUF3073 in Mycobacterium tuberculosis
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Small annotated open reading frames are the most neglected part of the functionally un-characterised Mycobacterium tuberculosis genome. We revisit Rv0810c, a 60-residue protein carrying the unknown-function domain DUF3073 (Pfam PF11273), flagged as an Actinobacteria-signature protein in 2006 but never studied since. Rv0810c is genuinely translated (detected in 11 of 16 Mycobacterium tuberculosis proteomic datasets), with no significant human homo-logue, no overlap with a neighbouring gene, and no CRISPR-interference polar effect on either flank. Residue-resolved confidence reveals a bipartite architecture: a rigid 33-residue module (pLDDT 91.9) followed by an extended, acidic, intrinsically disordered tail (radius of gyration 22.8 Å against 11–12 Å expected for a globular protein). The gene is under strong purifying selection (non-synonymous/synonymous ratio 0.86 against 1.93 among 74 size-matched controls, p = 5.8 × 10 −8 ), and its two commonest missense variants are each confined to one sub-lineage, indicating clonal expansion rather than relaxed constraint. DUF3073 is present without a single confirmed loss across 260 well-supported Actinomycetia genera. Despite this conservation, eight independent computational strategies, spanning sequence, structure, electrostatic-patch, embedding-similarity and homo-oligomerisation searches, converge on the same negative: no assignable fold, binding site, or functional neighbour in curated or uncurated sequence space. A phosphosite (Thr24), reproducibly reported by three laboratories, cannot be attributed to a kinase by chemical-genetic or sequence-motif evidence. The contradiction between predicted cytoplasmic topology and reported detection in the macrophage secretory fraction is narrowed, not resolved: ESX secretion, an immunodominant-epitope confound and host-induced transcription are excluded. Rv0810c exemplifies a class of genuinely uncharacterisable small proteins for which negative reporting, not a manufactured function, is the honest outcome.
Importance
Many genes of the tuberculosis bacillus still have no known function, and the smallest are the least studied. Rv0810c is one of them: a tiny protein, conserved in essentially every actinobacterium sequenced to date, demonstrably made by the cell, yet without any assignable role after twenty years. We applied eight independent computational approaches and report that all of them fail. Rather than propose a speculative function, we document what is known, what is excluded, and which experiments would settle the question. Publishing such negative dossiers stops other groups from repeating the same dead ends, and identifies the proteins for which laboratory work, not prediction, is now the only way forward.
