RPRD2

Model summary

A nuclear adaptor that uses a helical solenoid scaffold with a CID module to organize protein assemblies controlling late stages of transcript maturation. By forming multivalent interfaces in the nucleoplasm, it binds and stabilizes transcription- and RNA-processing–associated complexes near the polymerase machinery, thereby coordinating mRNA 3′-end formation and associated termination steps through non-enzymatic, high-avidity interactions.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The core of the protein is a contiguous ENTH/VHS-like fold annotated as IPR008942 (ENTH/VHS homologous_superfamily, residues 17–161). Nested within this scaffold are two overlapping cargo-adaptor signatures: IPR006569 (CID domain, residues 19–149) and the lineage-specific IPR047885 (RPRD2, CID domain, residues 27–146). The ordered layout—an ENTH/VHS-type helical solenoid that houses a CID module—creates a compact, non-enzymatic interaction platform. ENTH/VHS folds typically mediate avid, surface-driven binding to partner proteins and sometimes nucleic acids, while CID domains specialize in recognizing C-terminal heptad repeats and low-complexity segments of transcription and RNA-processing factors. This architecture dictates a primary molecular function of multivalent protein binding (GO:0005515), rather than catalysis.

2. 02

   From this binding-centric core, I infer the biological role by considering what assemblies this fold is predisposed to organize.

   CID-bearing adaptors frequently dock onto the C-terminal domain of RNA polymerase II and associated termination/processing machineries. The ENTH/VHS scaffold provides the avidity and geometry to stabilize transient assemblies that finalize transcript maturation. Such assemblies naturally converge on mRNA 3′-end formation, aligning with the process of mRNA 3′-end processing (GO:0031124). The absence of catalytic motifs and the presence of a CID module argue that the protein acts as a regulatory adaptor that times and stabilizes processing steps rather than executing cleavage or polyadenylation chemistry directly.

3. 03

   Cellular localization follows from the assembly context.

   A CID-centered adaptor that coordinates RNA polymerase II–coupled maturation must operate in the soluble nuclear phase where transcription and RNA-processing complexes exchange partners. This places the protein in the nucleoplasm (GO:0005654). Its role in organizing polymerase-associated processing further situates it in proximity to the RNA polymerase II holoenzyme (GO:0016591), where it can bridge transcription with downstream RNA biogenesis.

4. 04

   Mechanistically, the ENTH/VHS–CID tandem forms a bipartite interaction hub that captures CTD-associated factors and 3′-end processing modules, thereby stabilizing the handoff from elongation to cleavage/polyadenylation.

   By concentrating these factors in the nucleoplasm, the protein likely tunes termination efficiency and RNA surveillance. I hypothesize that it transiently binds the RNA polymerase II CTD and recruits cleavage/polyadenylation assemblies (e.g., CPSF and CstF), as well as termination adaptors, to enforce orderly mRNA 3′-end maturation. This model predicts preferential associations with nuclear RNA-processing complexes and explains a nucleoplasmic residency adjacent to the polymerase holoenzyme.