FNBP4

Model summary

A large human nuclear scaffold that concentrates RNA-processing machinery within nuclear speckles. Through tandem WW modules and extensive low-complexity regions, it assembles and stabilizes macromolecular complexes that coordinate pre-mRNA maturation. By organizing spliceosome-associated factors in speckle compartments, it modulates mRNA splicing efficiency and fidelity, with potential coupling to actin/formin-linked pathways that influence nuclear speck architecture and RNA-processing dynamics.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The sequence is assigned to IPR053076 (Formin-binding protein 4 family) spanning residues 1–1017, marking the entire polypeptide as a member of the FBp4 family that typically functions as a large nuclear scaffold. Two WW modules are present: an N-terminal WW domain IPR001202 (WW domain, residues 215–248) embedded within IPR036020 (WW domain superfamily, residues 212–248), and a C-terminal WW domain IPR001202 (WW domain, residues 595–629) embedded within IPR036020 (WW domain superfamily, residues 591–628). WW domains are compact β-sheet modules that bind proline-rich and PPxY-containing motifs and often mediate assembly of macromolecular complexes. The tandem arrangement—an N-terminal WW domain followed by a C-terminal WW domain—creates a multivalent binding platform separated by a long central region that can organize additional low-complexity or coiled interfaces. This architecture causes broad protein-binding capacity rather than enzymatic catalysis, aligning with GO:0005515 protein binding as the molecular function.

2. 02

   WW-domain scaffolds frequently operate in nuclear RNA-processing assemblies.

   The FBp4 family signature and the dual WW modules together predict avid assembly of ribonucleoprotein complexes that regulate pre-mRNA maturation. Such scaffolding is well-suited to concentrate spliceosomal and splicing-regulatory factors, which mechanistically supports involvement in GO:0000398 mRNA splicing, via spliceosome. The presence of two distinct WW modules enables simultaneous capture of multiple proline-rich ligands, stabilizing spliceosome-associated hubs and tuning splice-site selection.

3. 03

   Cellular localization follows from function and architecture.

   Proteins that scaffold spliceosome-associated assemblies typically reside in nuclear speckles—dynamic, phase-separated compartments that store and exchange splicing factors. The WW-domain-driven multivalency and the FBp4 family context together predict accumulation in nuclear speckles, consistent with GO:0016607 nuclear speck. This nuclear speck residence explains how the protein encounters and coordinates splicing machinery in a spatially organized nuclear environment.

4. 04

   Mechanistically, I hypothesize that the N-terminal WW domain nucleates initial contacts with PPxY/proline-rich partners, while the C-terminal WW domain stabilizes higher-order assemblies, collectively concentrating spliceosomal components and auxiliary regulators within speckles.

   The formin-binding ancestry of the family suggests a regulatory bridge between actin/formin signaling and nuclear RNA processing—potentially via transient interactions with nuclear actin or formin-derived pathways that influence speck architecture and mRNA maturation.

5. 05

   From this model, likely interaction partners include speckle-resident splicing factors and adaptors that present WW-compatible motifs.

   Candidates include serine/arginine-rich splicing regulators (e.g., SRSF family), U2AF and SF3 complexes, and speckle organizers such as SON and SRRM2. Transient contacts with nuclear actin and formin-related adaptors could couple cytoskeletal cues to nuclear speck dynamics, fine-tuning spliceosome assembly and function.