Rv0810c: a genus-conserved, structurally ordered small protein of unknown function carrying DUF3073 in Mycobacterium tuberculosis

Small annotated open reading frames are the most neglected part of the functionally uncharacterised M. 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 M. tuberculosis proteomic datasets), with no significant human homologue, no neighbouring-gene overlap, 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 A against 11-12 A 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=5.8x10-8), 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 macrophage-secretory-fraction detection 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.