{"uniprot":"O75324","gene":"SNN","name":"Stannin","organism":"Homo sapiens (Human)","status":"dark","length":88,"sequence":"MSIMDHSPTTGVVTVIVILIAIAALGALILGCWCYLRLQRISQSEDEESIVGDGETKEPFLLVQYSAKGPCVERKAKLMTPNGPEVHG","function_summary":"A small single-pass membrane protein that anchors to cellular membranes and presents a cytosolic tail optimized for regulated binding. By coordinating metal ions and assembling transient cytosolic complexes, it likely buffers metal-induced stress and supports detoxification pathways. Its membrane-anchored, cytosol-facing architecture positions it to sense and respond to toxic metal exposure, coupling local metal handling at membrane interfaces to broader stress-adaptation mechanisms in the cytoplasm.","plain_language":"SNN has barely been studied. The model's best guess: A small single-pass membrane protein that anchors to cellular membranes and presents a cytosolic tail optimized for regulated binding. By coordinating metal ions and assembling transient cytosolic complexes, it likely buffers metal-induced stress and supports detoxification pathways.","uniprot_summary":"May be involved in signal transduction.","go_terms":{"molecular_function":[{"id":"GO:0046872","label":"metal ion binding"}],"biological_process":[{"id":"GO:0009636","label":"response to toxic substance"}],"cellular_component":[{"id":"GO:0005737","label":"cytoplasm"},{"id":"GO:0016020","label":"membrane"}]},"interpro":[{"id":"IPR027435","name":"Stannin superfamily","type":"homologous_superfamily","start":1,"end":88},{"id":"IPR015135","name":"Stannin transmembrane","type":"domain","start":2,"end":33},{"id":"IPR015136","name":"Stannin unstructured linker","type":"domain","start":34,"end":59},{"id":"IPR015137","name":"Stannin cytoplasmic","type":"domain","start":61,"end":87},{"id":"IPR038747","name":"Stannin","type":"family","start":1,"end":88}],"reasoning":"I begin with the InterPro architecture. The polypeptide is fully encompassed by IPR027435 (Stannin superfamily, residues 1–88) and IPR038747 (Stannin family, residues 1–88), establishing a compact, conserved stannin scaffold. Within this framework, IPR015135 (Stannin transmembrane, residues 2–33) defines an N-terminal hydrophobic segment that anchors the protein in a membrane. Immediately after insertion, IPR015136 (Stannin unstructured linker, residues 34–59) forms a flexible cytosolic loop that can mediate transient interactions and conformational switching. The C-terminus is captured by IPR015137 (Stannin cytoplasmic, residues 61–87), a soluble tail that typically serves as a regulatory platform. The ordered layout—membrane anchor → unstructured cytosolic linker → cytosolic tail—creates a single-pass membrane architecture optimized for regulated binding rather than catalysis.\n\nThis architecture causally supports metal coordination and binding as the primary molecular function. The transmembrane core of stannin family proteins often harbors residues capable of coordinating divalent cations; the unstructured linker and cytosolic tail expand the binding surface and enable conditional metal interactions. These features justify GO:0046872 metal ion binding as the molecular function: the membrane anchor positions the protein at lipid interfaces where metal ions are abundant, while the cytosolic modules provide adaptable binding chemistry.\n\nFrom this metal-binding foundation, the biological role emerges. Metal coordination at membrane–cytosol interfaces is a common strategy for mitigating toxic metal stress and for buffering reactive species. The stannin scaffold, by sequestering or relaying metal ions and by assembling transient complexes via its unstructured linker and cytosolic tail, equips the cell to sense and respond to toxic metal exposure and oxidative challenges. This mechanistic logic aligns with GO:0009636 response to toxic substance: regulated metal handling and stress-responsive assembly of complexes provide a direct route to detoxification and survival pathways.\n\nCellular location follows directly from the domain architecture and the experimentally supported topology. The N-terminal transmembrane segment (IPR015135) establishes a stable membrane insertion, supporting GO:0016020 membrane. The extended cytosolic linker and tail (IPR015136 and IPR015137) project into the cytoplasm, consistent with GO:0005737 cytoplasm. The combined evidence indicates a single-pass membrane protein with a substantial cytosolic region.\n\nThese structural and functional cues suggest a mechanism: the membrane anchor positions the protein at organellar or plasma membranes; the cytosolic linker and tail recruit or modulate stress-response assemblies, while metal coordination dampens local toxicities. I hypothesize that the protein transiently associates with metal-handling and stress-adaptation modules—such as cytosolic metallochaperones and antioxidant systems—to tune responses to metal-induced damage. Likely partners include cytosolic metal-binding proteins and membrane-associated stress hubs; through these interactions, the protein could buffer metal flux and signal oxidative stress, thereby contributing to cellular resilience under toxic conditions.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-O75324.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/O75324"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/O75324/entry","atlas":"/p/O75324"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}