{"uniprot":"Q4VC39","gene":"HIGD2B","name":"Putative HIG1 domain family member 2B","organism":"Homo sapiens (Human)","status":"dark","length":106,"sequence":"MATLGFVTPEAPFESSKPPIFEGLSPTVYSNPEGFKEKFLRKTRENPVVPIGFLCTAAVLTNGLYCFHQGNSQCSRLMMHTQIAAQGFTIAAILLGLAATAMKSPP","function_summary":"A mitochondrial assembly factor that uses a hypoxia-induced helical module to bind and organize respiratory-chain components into higher-order supercomplexes. By stabilizing cytochrome c oxidase-containing assemblies, it preserves electron transfer efficiency, limits oxidative stress, and thereby suppresses apoptosis. Its scaffolding role also influences mitochondrial morphology and motility. While primarily mitochondrial, a minor nucleoplasmic pool is consistent with stress-responsive shuttling that coordinates assembly with gene-regulatory programs.","plain_language":"HIGD2B has barely been studied. The model's best guess: A mitochondrial assembly factor that uses a hypoxia-induced helical module to bind and organize respiratory-chain components into higher-order supercomplexes. By stabilizing cytochrome c oxidase-containing assemblies, it preserves electron transfer efficiency, limits oxidative stress, and thereby suppresses apoptosis.","uniprot_summary":"Involved in regulation of mitochondrial morphology and motility (By similarity). Involved in regulation of apoptosis.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0043066","label":"negative regulation of apoptotic process"}],"cellular_component":[{"id":"GO:0005739","label":"mitochondrion"},{"id":"GO:0005654","label":"nucleoplasm"}]},"interpro":[{"id":"IPR007667","name":"Hypoxia induced protein, domain","type":"domain","start":20,"end":106},{"id":"IPR050355","name":"Mitochondrial Respiratory Supercomplex Factor 1","type":"family","start":14,"end":104}],"reasoning":"I begin with the InterPro architecture. The core of the protein is defined by IPR007667 (Hypoxia induced protein, domain; residues 20–106), a compact membrane-associated HIG1-type domain that typically forms a small helical bundle suited for scaffolding and assembly of respiratory chain components. This HIG1 core is encompassed by IPR050355 (Mitochondrial Respiratory Supercomplex Factor 1 family; residues 14–104), which specifies a lineage of assembly factors that stabilize and organize higher-order assemblies of the mitochondrial respiratory chain. The tight overlap and N-terminal positioning of these domains indicate a single functional module that binds partner proteins and organizes membrane-proximal complexes rather than catalyzing chemistry. This architecture causally dictates a non-enzymatic role centered on multivalent contacts, supporting GO:0005515 protein binding as the molecular function.\n\nFrom this binding-centric module, the biological role follows. HIG1-family assembly factors promote the formation and stability of respiratory supercomplexes by docking cytochrome c oxidase and complex IV-associated subunits with adjacent complexes. Such assembly stabilizes electron transfer pathways and reduces electron leak, which lowers reactive oxygen species. By constraining oxidative stress and preserving mitochondrial integrity, this assembly role suppresses pro-apoptotic signaling, aligning with GO:0043066 negative regulation of apoptotic process. The same assembly logic explains how the protein can influence mitochondrial morphology and motility: stabilized supercomplexes tune membrane potential and curvature cues that govern mitochondrial dynamics.\n\nCellular localization is dictated by the domain’s membrane-proximal nature and the experimentally supported compartment. The HIG1 domain and RCF1-family signature point to a mitochondrial role, consistent with GO:0005739 mitochondrion and the stated mitochondrial localization. A nucleoplasmic pool (GO:0005654 nucleoplasm) is also plausible for stress-responsive shuttling or transcription-coupled assembly control; assembly factors of this class can transiently localize to the nucleus under hypoxia or apoptotic signaling regimes, coordinating expression of mitochondrial genes and apoptotic regulators.\n\nMechanistically, I hypothesize that the protein acts as a membrane-anchored assembly scaffold that binds respiratory-chain subunits and their chaperones to nucleate supercomplex formation. By stabilizing cytochrome c oxidase-containing assemblies and tuning their coupling to other complexes, it curbs ROS production and dampens apoptosis. Likely interaction partners include cytochrome c oxidase subunits and assembly factors, as well as cytosolic and mitochondrial chaperones that deliver and position components. This cooperative network explains how a binding-centric HIG1 module can simultaneously shape mitochondrial dynamics and cell survival pathways.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q4VC39.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q4VC39"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q4VC39/entry","atlas":"/p/Q4VC39"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}