{"uniprot":"Q3KP22","gene":"MAJIN","name":"Membrane-anchored junction protein","organism":"Homo sapiens (Human)","status":"dark","length":176,"sequence":"MSLKPFTYPFPETRFLHAGPNVYKFKIRYGKSIRGEEIENKEVITQELEVPVEKKAVGAVMRKRKHMDEPSSPSRPGLDRAKIGTSSQGPSKKKPPVETRRNRERKTQQGLQETLASDITDVQKQDSEWGHSLPGRIVPPLQHNSPPPKERAATGFFGFLSSLFPFRYFFRKSSHS","function_summary":"A membrane-anchored junction protein family member that uses an N-terminal anchor and a sizable cytosolic interaction domain to scaffold assemblies required for faithful chromosome segregation. By concentrating at centromeric chromatin during mitosis, it organizes kinetochore-associated complexes that secure robust attachment of spindle microtubules. Its soluble tail supports multivalent binding in the cytosol and nucleoplasm, enabling dynamic recruitment to the centromeric region where it stabilizes microtubule–kinetochore interfaces.","plain_language":"MAJIN has barely been studied. The model's best guess: A membrane-anchored junction protein family member that uses an N-terminal anchor and a sizable cytosolic interaction domain to scaffold assemblies required for faithful chromosome segregation. By concentrating at centromeric chromatin during mitosis, it organizes kinetochore-associated complexes that secure robust attachment of spindle microtubules.","uniprot_summary":"Involved in chromosome segregation during mitosis.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0051315","label":"attachment of mitotic spindle microtubules to kinetochore"}],"cellular_component":[{"id":"GO:0005829","label":"cytosol"},{"id":"GO:0005654","label":"nucleoplasm"},{"id":"GO:0000775","label":"chromosome, centromeric region"}]},"interpro":[{"id":"IPR027816","name":"Membrane-anchored junction protein","type":"family","start":1,"end":49},{"id":"IPR027816","name":"Membrane-anchored junction protein","type":"family","start":50,"end":175}],"reasoning":"I begin with the InterPro architecture. The sequence carries IPR027816 (Membrane-anchored junction protein family) in two contiguous blocks: an N-terminal module from residues 1–49 and a larger C-terminal module from residues 50–175. This bipartite arrangement is characteristic of membrane-anchored junction protein families that use an N-terminal membrane-attachment region coupled to an extended cytosolic tail that mediates multivalent interactions. The split across the N- and C-terminal halves implies a single-pass membrane anchor that positions a sizable soluble domain into the cytosol or nucleoplasm, where it can scaffold other factors. Such an architecture causally favors protein–protein interaction surfaces rather than catalytic cores, which supports a primary molecular function of GO:0005515 protein binding.\n\nFrom this scaffold-like foundation, I infer how the protein exerts its role in chromosome dynamics. The membrane-anchored junction protein family is known for junctional and cortical associations, but here the architecture is repurposed to organize kinetochore-proximal assemblies. A membrane-tethered cytosolic domain can transiently accumulate at centromeric chromatin and recruit outer kinetochore components that mediate microtubule attachment. This scaffolding function directly enables the process formalized as GO:0051315 attachment of mitotic spindle microtubules to kinetochore. By stabilizing kinetochore assembly and microtubule capture, the protein ensures faithful chromosome segregation during mitosis.\n\nCellular localization follows from this chain. The presence of a membrane-anchor coupled to a soluble interaction domain explains a distribution across cytosolic and nuclear compartments. The soluble tail rationalizes accumulation in GO:0005829 cytosol and GO:0005654 nucleoplasm, where kinetochore assembly and spindle–kinetochore interfaces form. Its operational focus at centromeric chromatin places it at GO:0000775 chromosome, centromeric region, consistent with a role in mitosis.\n\nMechanistically, I hypothesize that the N-terminal membrane-anchor transiently targets or stabilizes the protein at membrane-rich or cortical sites during interphase, while the C-terminal family domain (residues 50–175) provides a flexible platform that shuttles into the nucleus and concentrates at kinetochores. There, it likely binds outer kinetochore complexes and microtubule-attachment modules to promote end-on attachment fidelity. Candidate partners include the NDC80 complex and associated KMN network for microtubule coupling, as well as checkpoint and cohesion modules that coordinate with centromeric chromatin. This assembly logic explains how a membrane-anchored junction protein family member can drive kinetochore–microtubule attachment and thereby secure accurate chromosome segregation.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q3KP22.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q3KP22"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q3KP22/entry","atlas":"/p/Q3KP22"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}