LENG8

Model summary

A nucleoplasmic scaffold that organizes transcription-coupled mRNP maturation and handoff to the nuclear export pathway. Its conserved SAC3/GANP/THP3 core and PCI scaffold domain create multivalent binding surfaces that assemble splicing and export factors, thereby coordinating spliceosome completion with formation of export-competent ribonucleoproteins and their delivery toward nuclear pore–proximal sites.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The polypeptide core is defined by IPR045107 (SAC3/GANP/THP3 family, residues 110–781), which establishes membership in the SAC3/GANP/THP3 scaffold family that organizes TREX-2–type assemblies at the nuclear pore–proximal transcription/export interface. Nested within this family envelope, IPR005062 (SAC3/GANP/THP3 conserved domain, residues 566–780) marks a conserved C-terminal module that typically mediates higher-order assembly and cargo docking. Overlapping these, IPR000717 (PCI domain, residues 636–800) caps the C-terminus with a prototypical PCI scaffold fold. PCI domains are hallmark architectural cores of large nuclear assemblies (e.g., proteasome lid, COP9 signalosome, eIF3), and in the SAC3/GANP/THP3 lineage they create extended helical platforms that bind multiple partners rather than catalyze chemistry. The ordered layout—broad SAC3/GANP/THP3 family region leading into a tightly conserved PCI core—causes multivalent protein–protein interaction capacity and positions the molecule as a non-enzymatic organizer.

2. 02

   This scaffolded architecture dictates the molecular function as protein binding (GO:0005515).

   The absence of catalytic motifs and the dominance of PCI/scaffold signatures argue that activity is mediated by affinity-driven assembly and cargo capture. The SAC3/GANP/THP3 conserved region and PCI fold together generate surfaces that recruit transcription-coupled mRNP maturation factors and nuclear pore–associated adaptors.

3. 03

   From this binding-centric function, the biological process follows.

   TREX-2–type assemblies couple transcription to messenger ribonucleoprotein formation and export. By stabilizing interactions among splicing and export adaptors, this scaffold promotes mRNA maturation and handoff to export pathways, aligning with mRNA splicing via the spliceosome (GO:0000398). The SAC3/GANP/THP3 family is positioned at the crossroads of splicing completion and nuclear export competence; thus, the architecture channels spliced transcripts toward export-competent RNP assembly.

4. 04

   Cellular location is inferred from the assembly’s operational niche.

   PCI-bearing TREX-2 scaffolds function in the nucleoplasm where transcription and RNP assembly occur before nuclear export. This supports nucleoplasm (GO:0005654) as the operative compartment and is consistent with a soluble nuclear scaffold rather than a membrane anchor.

5. 05

   Mechanistically, the SAC3/GANP/THP3 family envelope funnels partner recognition, while the PCI core stabilizes higher-order assemblies that bridge chromatin-proximal transcription complexes to nuclear pore–associated receptors.

   I hypothesize that the protein nucleates a nucleoplasmic platform that captures spliced mRNPs and presents them to nuclear export machinery. Likely partners include nucleoporins and TREX-2 constituents that read phenylalanine-glycine (FG) repeat environments and RNA export adaptors. Through these interactions, the scaffold times mRNP maturation with pore-proximal delivery, ensuring efficient coupling of splicing to export.