{"uniprot":"Q9BSH5","gene":"HDHD3","name":"Haloacid dehalogenase-like hydrolase domain-containing protein 3","organism":"Homo sapiens (Human)","status":"dark","length":251,"sequence":"MAHRLQIRLLTWDVKDTLLRLRHPLGEAYATKARAHGLEVEPSALEQGFRQAYRAQSHSFPNYGLSHGLTSRQWWLDVVLQTFHLAGVQDAQAVAPIAEQLYKDFSHPCTWQVLDGAEDTLRECRTRGLRLAVISNFDRRLEGILGGLGLREHFDFVLTSEAAGWPKPDPRIFQEALRLAHMEPVVAAHVGDNYLCDYQGPRAVGMHSFLVVGPQALDPVVRDSVPKEHILPSLAHLLPALDCLEGSTPGL","function_summary":"A soluble human phosphatase that belongs to a metal-dependent hydrolase fold and uses a cap-domain architecture to hydrolyze phosphate monoesters and related intermediates. By engaging transient protein assemblies, it contributes to nucleotide and nucleoside phosphate turnover and supports pyrimidine nucleotide breakdown. Its soluble architecture and role in nucleotide balance point to nucleolar residency, where it likely associates with ribosome biogenesis hubs to fine-tune local nucleotide pools and maintain metabolic fidelity.","plain_language":"HDHD3 has barely been studied. The model's best guess: A soluble human phosphatase that belongs to a metal-dependent hydrolase fold and uses a cap-domain architecture to hydrolyze phosphate monoesters and related intermediates. By engaging transient protein assemblies, it contributes to nucleotide and nucleoside phosphate turnover and supports pyrimidine nucleotide breakdown.","uniprot_summary":"May function as a phosphatase and have a broad substrate specificity.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0006244","label":"pyrimidine nucleotide catabolic process"}],"cellular_component":[{"id":"GO:0005730","label":"nucleolus"}]},"interpro":[{"id":"IPR036412","name":"HAD-like superfamily","type":"homologous_superfamily","start":7,"end":242},{"id":"IPR023214","name":"HAD superfamily","type":"homologous_superfamily","start":9,"end":222},{"id":"IPR044924","name":"HAD-superfamily hydrolase, subfamily IA, REG-2-like, capping domain superfamily","type":"homologous_superfamily","start":23,"end":106},{"id":"IPR051828","name":"HAD-like hydrolase domain-containing protein","type":"family","start":2,"end":242},{"id":"IPR006439","name":"HAD hydrolase, subfamily IA","type":"family","start":7,"end":18},{"id":"IPR011949","name":"HAD-superfamily hydrolase, subfamily IA, REG-2-like","type":"family","start":8,"end":210},{"id":"IPR006439","name":"HAD hydrolase, subfamily IA","type":"family","start":124,"end":205}],"reasoning":"I begin with the InterPro architecture, which is uniformly diagnostic of haloacid dehalogenase (HAD) superfamily phosphohydrolases. Two broad superfamily signatures, IPR036412 (HAD-like superfamily, residues 7–242) and IPR023214 (HAD superfamily, residues 9–222), span essentially the full length of the polypeptide, establishing a Rossmann-like α/β core that coordinates a divalent cation and catalyzes hydrolysis of phosphorus–nitrogen and phosphorus–oxygen bonds. Within this scaffold, IPR044924 (HAD-superfamily hydrolase, subfamily IA, REG-2-like, capping domain superfamily, residues 23–106) defines a type IA capping module that closes over the active site to tune substrate specificity toward small nucleotide phosphates. Multiple family-level assignments reinforce this subfamily: IPR006439 (HAD hydrolase, subfamily IA, residues 7–18 and 124–205) and IPR011949 (HAD-superfamily hydrolase, subfamily IA, REG-2-like, residues 8–210) together delineate the canonical IA fold and its cap arrangement, while IPR051828 (HAD-like hydrolase domain-containing protein, residues 2–242) covers the full catalytic body. The ordered layout—core catalytic domain flanked by a REG-2-like cap—causes metal-dependent phosphomonoester hydrolysis and supports broad nucleotide phosphate turnover.\n\nThis HAD-IA architecture dictates molecular function. The catalytic core and cap domain confer phosphatase chemistry that can act on diverse phosphate esters; the REG-2-like cap biases the enzyme toward soluble, small-molecule substrates rather than macromolecular phosphatase activity. Such enzymes typically exhibit robust protein-binding surfaces that stabilize transient complexes and guide substrate channeling, consistent with GO:0005515 protein binding as an intrinsic property of this fold.\n\nFrom this chemistry, the biological process follows. A HAD-IA phosphatase that prefers nucleotide phosphates naturally feeds into nucleotide and nucleoside phosphate turnover. The ability to hydrolyze phosphate monoesters and pyrophosphate-like intermediates aligns with participation in pyrimidine nucleotide catabolic routes, matching GO:0006244 pyrimidine nucleotide catabolic process. The enzyme’s broad specificity and soluble nature suggest it helps balance nucleotide pools by clearing or remodeling intermediates.\n\nCellular location is inferred from function and architecture. The absence of transmembrane segments and the soluble HAD fold point to a non-membranous compartment. Coupled with a role in nucleotide metabolism and ribonucleotide handling, the most parsimonious site is the nucleolus, where rRNA biogenesis and nucleotide flux intersect. This supports localization to GO:0005730 nucleolus and explains how a soluble HAD-IA phosphatase can be concentrated in nucleolar assemblies.\n\nMechanistically, the enzyme likely binds Mg2+ (or Mn2+) at the HAD core to activate a nucleophilic water that cleaves phosphomonoesters and possibly phosphoramidates. The REG-2-like cap positions pyrimidine nucleotides and related intermediates for hydrolysis, thereby tuning pyrimidine catabolism and broader nucleotide homeostasis. In the nucleolus, transient protein-binding interfaces would recruit the enzyme to ribonucleoprotein hubs, where it can modulate local nucleotide pools and safeguard ribosome biogenesis fidelity.\n\nThese features suggest interaction partners that organize nucleolar metabolism and RNA processing: nucleolin and nucleophosmin as scaffolds that concentrate RNA and ribosome assembly factors; fibrillarin and nucleolin-containing complexes that manage rRNA modification; and RNA polymerase I-associated assemblies that drive rRNA synthesis. Such associations would position the phosphatase to couple nucleotide turnover with nucleolar function.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q9BSH5.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q9BSH5"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q9BSH5/entry","atlas":"/p/Q9BSH5"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}