AAR2

AAR2 (yeast Aar2p / human C20ORF4) is an assembly factor for the U5 small nuclear ribonucleoprotein (snRNP) that controls the timing of spliceosome maturation, first identified through its requirement for pre-mRNA splicing in yeast (PMID:1922071). It is a component of a cytoplasmic precursor U5 snRNP containing Prp8, Snu114, U5 snRNA, and Sm proteins, but is excluded from the tri-snRNP and assembled spliceosome, and its loss impairs snRNP recycling across rounds of splicing (PMID:11720285, PMID:16945917). Mechanistically, Aar2 binds the RNase H domain of Prp8 and, by extending its C terminus to dock the Jab1/MPN domain onto a composite Aar2-RNase H platform, sterically occludes the binding sites for the Brr2/SNRNP200 helicase while also occupying the RNase H RNA-binding surface to block U4/U6 di-snRNA loading, thereby preventing premature spliceosome activation (PMID:21764848, PMID:23442228). Crystal structures of the Aar2-Prp8 assembly establish that Aar2 and Brr2 are mutually exclusive binders of Prp8, so that upon nuclear import Brr2 displaces Aar2 to generate the mature, catalytically competent U5 snRNP (PMID:23442228, PMID:23727230). This handoff is governed by phosphorylation: a phospho-mimetic substitution (S253E in yeast) lowers Aar2 affinity for Prp8 and shifts the equilibrium toward Brr2-Prp8 and U4/U6 binding, and CK2α1 and SGK2 are candidate kinases that abrogate the AAR2-PRPF8 interaction in human cells (PMID:21764848, PMID:23442228, PMID:35225431). Human AAR2 is a conserved ortholog that binds the PRPF8 RNase H domain, but its structure reveals a distinct interaction in which AAR2 locks PRPF8 RH in a conformation compatible only with the first transesterification step, indicating a regulatory role beyond simple placeholder activity for SNRNP200 (PMID:26527271, PMID:36322420).

1. 1991 Medium

   Established that AAR2 functions in pre-mRNA splicing, the first link of the gene to the spliceosomal pathway, by showing its loss selectively blocks intron removal.

   Evidence Genetic analysis of yeast aar2 mutants with Northern hybridization and primer extension showing accumulation of unspliced MATa1 pre-mRNA

   PMID:1922071

   Open questions at the time

   • Molecular function and binding partners not yet identified
   • Basis for apparent substrate specificity (a1 vs ACT1) unexplained
2. 2001 High

   Placed Aar2p physically within a simple U5 snRNP precursor and distinguished it from mature spliceosomal complexes, indicating a role in snRNP biogenesis/recycling rather than catalysis.

   Evidence Biochemical purification, mass spectrometry, depletion experiments, in vitro splicing assays and EM in yeast

   PMID:11720285

   Open questions at the time

   • Direct molecular contacts with Prp8 not defined
   • How Aar2p exits to form mature snRNP unknown
3. 2006 Medium

   Connected Aar2p genetically to Prp8/Prp38 function and a recycling/turnover pathway, reinforcing its role in spliceosome assembly dynamics.

   Evidence Yeast genetic suppressor screen, tandem affinity purification, and two-hybrid analysis recovering Aar2p with mutant Prp8p and Spp382p

   PMID:16945917

   Open questions at the time

   • Mechanism of suppression not resolved at molecular level
   • Functional significance of the Spp382p association unclear
4. 2011 High

   Defined the mechanism of Aar2p as a phosphorylation-controlled assembly factor: it binds the Prp8 RNase H domain and sterically excludes Brr2, with phosphorylation triggering the swap.

   Evidence Yeast two-hybrid, co-IP, in vitro binding, phospho-mimetic S253E mutagenesis, and MS identification of phosphorylation sites

   PMID:21764848

   Open questions at the time

   • Atomic structural basis of the steric block not yet visualized
   • Physiological kinase responsible for phosphorylation not identified
5. 2013 High

   Provided the structural basis for Aar2p-Prp8 regulation, showing how Aar2p docks the Jab1/MPN domain, blocks Brr2 sites and U4/U6 RNA binding, and that Aar2 and Brr2 are mutually exclusive, defining a cytoplasm-to-nucleus maturation pathway.

   Evidence X-ray crystallography of Aar2p-Prp8 RNase H/Jab1-MPN and Brr2-Prp8 Jab1/MPN complexes, with binding, RNA-binding and mutagenesis assays plus fractionation in yeast

   PMID:23354046 PMID:23442228 PMID:23727230

   Open questions at the time

   • In vivo trigger and location of the Aar2-to-Brr2 handoff not fully mapped
   • Conservation of the mechanism to human factors not yet tested
6. 2015 Medium

   Demonstrated that human AAR2 (C20ORF4) is a true functional ortholog by showing conserved binding to the PRPF8 RNase H domain.

   Evidence Western blotting of HeLa proteome, in vitro binding assays, and a 2.35 Å crystal structure of the human AAR2-PRPF8 RH complex

   PMID:26527271

   Open questions at the time

   • Whether human regulation mirrors yeast phosphorylation control not yet shown
   • Identity of human kinases unknown at this stage
7. 2022 High

   Identified candidate human kinases and refined the human structural mechanism, revealing a role beyond simple Brr2 placeholder activity through conformational control of PRPF8.

   Evidence Phospho-yeast two-hybrid kinase array implicating CK2α1 and SGK2; human AAR2-PRPF8 RH crystal structure with SEC and structure-guided mutagenesis

   PMID:35225431 PMID:36322420

   Open questions at the time

   • Kinase activity on AAR2-PRPF8 shown via two-hybrid readout, not direct biochemical phosphorylation in cells
   • Functional consequence of the step-1-locking conformation for spliceosome catalysis not established in vivo

• How AAR2 phosphorylation is regulated in vivo and integrated with nuclear import to time U5 snRNP maturation remains to be fully defined.
• No direct demonstration of physiological kinase phosphorylating AAR2 in human cells
• Spatial/temporal coupling of phosphorylation to nuclear import not mapped
• Functional impact of AAR2 perturbation on human splicing fidelity uncharacterized