{"uniprot":"Q9H853","gene":"TUBA4B","name":"Putative tubulin-like protein alpha-4B","organism":"Homo sapiens (Human)","status":"dark","length":241,"sequence":"MRHQQTERQDPSQPLSRQHGTYRQIFHPEQLITGKEDAANNYAWGHYTIGKEFIDLLLDRIRKLADQCTGLQGFLVFHSLGRGTGSDVTSFLMEWLSVNYGKKSKLGFSIYPAPQVSTAMVQPYNSILTTHTTLEHSDCAFMVDNKAIYDICHCNLDIERPTYTNLNRLISQIVSSITASLRFDGALNVDLTEFQTNLVSYLTSTSPWPPMHQSSLQKRYTTSSCWWQRLPMPALSLPTRW","function_summary":"A cytoplasmic cytoskeletal subunit that assembles with a partner tubulin to form the building block of microtubules. Through a conserved tubulin fold and alpha-tubulin determinants, it binds other proteins to drive heterodimer formation, filament assembly, and lattice stabilization. This scaffolding role underpins microtubule organization that supports cell division and intracellular transport, with a soluble pool equilibrating into filamentous arrays within the cytoskeleton.","plain_language":"TUBA4B has barely been studied. The model's best guess: A cytoplasmic cytoskeletal subunit that assembles with a partner tubulin to form the building block of microtubules. Through a conserved tubulin fold and alpha-tubulin determinants, it binds other proteins to drive heterodimer formation, filament assembly, and lattice stabilization.","uniprot_summary":"Tubulin is the major constituent of microtubules.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0000226","label":"microtubule cytoskeleton organization"}],"cellular_component":[{"id":"GO:0015630","label":"microtubule cytoskeleton"}]},"interpro":[{"id":"IPR036525","name":"Tubulin/FtsZ, GTPase domain superfamily","type":"homologous_superfamily","start":10,"end":204},{"id":"IPR003008","name":"Tubulin/FtsZ, GTPase domain","type":"domain","start":8,"end":185},{"id":"IPR000217","name":"Tubulin","type":"family","start":20,"end":213},{"id":"IPR002452","name":"Alpha tubulin","type":"family","start":27,"end":40},{"id":"IPR002452","name":"Alpha tubulin","type":"family","start":59,"end":67},{"id":"IPR002452","name":"Alpha tubulin","type":"family","start":87,"end":99},{"id":"IPR002452","name":"Alpha tubulin","type":"family","start":100,"end":111},{"id":"IPR002452","name":"Alpha tubulin","type":"family","start":153,"end":165}],"reasoning":"I begin with the InterPro architecture. The core of the protein is defined by IPR003008 (Tubulin/FtsZ, GTPase domain, residues 8–185) nested within IPR036525 (Tubulin/FtsZ, GTPase domain superfamily, residues 10–204). This NTPase-like fold is the hallmark of tubulin/FtsZ modules that assemble into protofilaments and microtubules. The broader IPR000217 (Tubulin family, residues 20–213) situates the sequence within canonical tubulin, while multiple alpha-tubulin-specific signatures—IPR002452 (Alpha tubulin family, residues 27–40, 59–67, 87–99, 100–111, 153–165)—pinpoint the subfamily and imply the characteristic N-terminal acidic loop and central helical elements that stabilize the α/β heterodimer and govern lattice assembly. The ordered layout—an N-terminal alpha-tubulin determinant followed by the conserved tubulin GTPase fold—creates a rigid dimerization and polymerization scaffold rather than a soluble enzyme.\n\nThis architecture causes a molecular function centered on multivalent protein binding (GO:0005515). The tubulin/FtsZ GTPase fold provides the structural platform for obligate α/β heterodimer formation and for lateral and longitudinal contacts in microtubules. The alpha-tubulin family determinants tune binding specificity toward β-tubulin and a spectrum of microtubule-associated proteins (MAPs), motors, and nucleation factors. The absence of catalytic specializations and the dominance of assembly modules indicate that binding and scaffolding—not enzymatic turnover—define activity.\n\nFrom this binding-centric mechanism, the biological process follows: the protein organizes the microtubule cytoskeleton (GO:0000226). The α/β heterodimer is the fundamental unit that polymerizes into protofilaments and cylindrical microtubules, thereby constructing tracks for intracellular transport, shaping cell division structures, and directing spindle and phragmoplast dynamics. The alpha-tubulin signatures ensure proper lattice geometry and post-translational modification interfaces that regulate assembly/disassembly cycles.\n\nCellular localization is dictated by function and architecture. The polymerization-competent tubulin fold and alpha-tubulin determinants place the protein in the microtubule cytoskeleton (GO:0015630). This aligns with a soluble cytoplasmic pool that equilibrates with insoluble filamentous assemblies, consistent with a cytoplasm/cytoskeleton distribution.\n\nMechanistically, the protein operates as an α-tubulin that forms a tight heterodimer with β-tubulin, binds GTP at the heterodimer interface, and contributes to the structural core of growing microtubules. It likely nucleates with γ-tubulin ring complexes and elongates via addition of β-tubulin GTP sites. Post-translational modifications on alpha-tubulin surfaces modulate lattice stability and MAP affinity, tuning dynamics. I therefore hypothesize interactions with β-tubulin as the obligate partner; with γ-tubulin ring complexes for nucleation; with MAPs such as MAP2 and tau that stabilize or regulate filaments; and with motors including kinesins and cytoplasmic dynein that translate microtubule tracks into directed transport and cell division machinery.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q9H853.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q9H853"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q9H853/entry","atlas":"/p/Q9H853"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}