{"uniprot":"Q5JS54","gene":"PSMG4","name":"Proteasome assembly chaperone 4","organism":"Homo sapiens (Human)","status":"dark","length":123,"sequence":"MEGLVVAAGGDVSLHNFSARLWEQLVHFHVMRLTDSLFLWVGATPHLRNLAVAMCSRYDSIPVSTSLLGDTSDTTSTGLAQRLARKTNKQVFVSYNLQNTDSNFALLVENRIKEEMEAFPEKF","function_summary":"A soluble cytoplasmic assembly chaperone that organizes early steps of proteasome biogenesis. It uses selective, non-enzymatic binding to stabilize and guide intermediate assemblies of the core particle, coordinating with partner chaperones to align α-subunit ring formation and prevent off-pathway oligomerization. By scaffolding early proteasome precursors in the cytoplasm, it channels assembly toward a mature proteolytic machine and preserves proteostasis.","plain_language":"PSMG4 has barely been studied. The model's best guess: A soluble cytoplasmic assembly chaperone that organizes early steps of proteasome biogenesis. It uses selective, non-enzymatic binding to stabilize and guide intermediate assemblies of the core particle, coordinating with partner chaperones to align α-subunit ring formation and prevent off-pathway oligomerization.","uniprot_summary":"Involved in regulation of the proteasome complex assembly.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0051131","label":"chaperone-mediated protein complex assembly"}],"cellular_component":[{"id":"GO:0032991","label":"protein-containing complex"}]},"interpro":[{"id":"IPR032157","name":"Proteasome assembly chaperone 4","type":"family","start":7,"end":123}],"reasoning":"I begin with the domain architecture: a single, centrally placed InterPro signature IPR032157 (Proteasome assembly chaperone 4 family) spanning residues 7–123. This compact, dedicated assembly-factor family signature, without catalytic motifs or transmembrane segments, defines a soluble, non-enzymatic chaperone specialized for proteasome biogenesis. The solitary PAC4-family module dictates that the polypeptide’s primary role is to bind specific partner chaperones and immature core-particle subunits during assembly rather than to catalyze proteolysis or ATP-driven remodeling.\n\nThis architecture causes a molecular function centered on multivalent protein binding (GO:0005515). PAC4-family chaperones form obligate heteromeric assemblies—most notably a trimeric PAC1–PAC4 scaffold and transient contacts with α-ring precursors—so the sequence-encoded role is to stabilize folding intermediates and enforce assembly fidelity through selective, non-catalytic interactions.\n\nFrom this binding-driven mechanism, the biological process follows directly: chaperone-mediated protein complex assembly (GO:0051131). The PAC4 module is tailored to guide construction of the 20S core particle and the 26S holocomplex by shepherding α-subunit ring formation and preventing off-pathway oligomerization. By constraining assembly routes, it ensures accurate stoichiometry and timing that ultimately yield a functional proteasome.\n\nCellular localization is inferred from the soluble, assembly-focused nature of the PAC4 family and the absence of targeting signals for membranes or organelles. Such assembly chaperones operate in the cytoplasm, where proteasome precursors and assembly lines reside; thus a cytoplasmic localization is expected. Functionally, the protein acts within and alongside protein-containing complexes (GO:0032991), including hetero-oligomeric chaperone assemblies and proteasome precursors.\n\nMechanistically, I hypothesize that the protein forms a PAC1–PAC4 chaperone scaffold that nucleates and stabilizes early 20S assembly intermediates. It likely binds immature α-ring subcomplexes, aligning them for correct ring closure and shielding hydrophobic surfaces. Transient interactions with additional assembly factors and α/β-type proteasome subunits would be favored, with the PAC1 partner providing cooperative structural support. This assembly-centric chaperoning in the cytoplasm channels intermediates toward productive proteasome biogenesis and away from nonproductive aggregates.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q5JS54.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q5JS54"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q5JS54/entry","atlas":"/p/Q5JS54"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}