RBBP5

RBBP5 is a scaffold subunit of the WRAD module that assembles and allosterically activates SET1/KMT2-family histone H3K4 methyltransferases on chromatin (PMID:20716525, PMID:25593305). It bridges the complex through defined interfaces: a conserved RbBP5 motif binds a dedicated surface on WDR5, and these two proteins act cooperatively to stimulate MLL1 catalytic activity (PMID:20716525), while an acidic D/E box in RBBP5 is engaged by the Ash2L SPRY domain to form the catalytically stimulatory RbBP5/Ash2L heterodimer (PMID:25593305). This heterodimer in turn docks onto a non-active-site surface of the MLL1 SET domain, and disease-associated mutations at this interface abolish H3K4 dimethylation (PMID:24680668). Within RBBP5 itself, an internal interaction between the WD40 β-propeller and a C-terminal distal region maintains the compact conformation of the complex, and a vertebrate-specific C-terminal motif mediates nucleosome recognition and methylation (PMID:31544921), with the β-propeller surface additionally contacting nucleic acids to help target the complex to chromatin (PMID:29897600). RBBP5 activity is tuned post-translationally: a phospho-switch stimulates WRAD assembly and methylation rates (PMID:25593305), and phosphorylation at Ser497 by nuclear HKDC1 drives MLL1 complex assembly and H3K4me3 deposition at mitotic genes (PMID:39891906). Through these activities RBBP5 deposits promoter H3K4me3 at specific target genes during EMT and proteostasis responses (PMID:27566588, PMID:39314427) and supports DNA replication licensing at origins (PMID:27744293). RBBP5 is also required for in vivo brain development, and de novo missense variants at its nucleosome interface cause a syndromic neurodevelopmental disorder, with the variants acting as partial loss-of-function alleles that fail to rescue microcephaly in flies (PMID:39036895).

1. 2010 High

   Established the molecular basis by which RBBP5 is recruited into the H3K4 methyltransferase complex and why this matters for enzyme activation, defining a dedicated WDR5–RbBP5 interface.

   Evidence X-ray crystallography with biochemical binding and in vitro methyltransferase assays of the WDR5–RbBP5 interface

   PMID:20716525

   Open questions at the time

   • Did not resolve how the assembled module engages nucleosomes
   • Stoichiometry and dynamics within full WRAD not addressed
2. 2014 High

   Identified the MLL1 SET-domain surface that the RbBP5/Ash2L heterodimer engages and linked it to disease, showing this contact is required for H3K4 dimethylation.

   Evidence In vitro methyltransferase assays, Co-IP/pulldown, and structure-guided mutagenesis of the MLL1 KIS surface

   PMID:24680668

   Open questions at the time

   • Did not define the RBBP5 residues contacting this surface
   • Restricted to MLL1 among KMT2 paralogs
3. 2014 Medium

   Revealed a methylation-independent role for RBBP5 in RNA processing, separating its chromatin-mark function from a direct contribution to pre-mRNA splicing.

   Evidence siRNA knockdown with RT-PCR of unspliced FOS transcripts and ChIP for H3K4me3

   PMID:24715476

   Open questions at the time

   • Mechanism connecting WAR subcomplex to the splicing machinery unknown
   • Tested only on FOS pre-mRNA
4. 2015 High

   Defined the Ash2L–RbBP5 (D/E box) interface and a phospho-switch on RBBP5, explaining how heterodimer formation and phosphorylation stimulate KMT2 methylation rates.

   Evidence X-ray crystallography, mutational analysis, in vitro methyltransferase assays, and erythroid differentiation assays

   PMID:25593305

   Open questions at the time

   • Kinase responsible for the phospho-switch not identified here
   • In vivo relevance of the switch not established
5. 2016 Medium

   Connected RBBP5/WDR5-dependent H3K4 methylation to DNA replication licensing, extending its role beyond transcription.

   Evidence siRNA knockdown, ChIP at replication origins, flow cytometry for polyploidy, and immunofluorescence co-localization with ORC/MCM2-7

   PMID:27744293

   Open questions at the time

   • Direct versus indirect contribution of RBBP5 to MCM2-7 loading unresolved
   • No structural basis for origin targeting
6. 2016 Medium

   Showed signal-dependent recruitment of RBBP5 to a specific target promoter, demonstrating it acts in defined transcriptional programs (EMT) rather than only globally.

   Evidence ChIP, siRNA knockdown, Western blot, and qRT-PCR of the Snail/SNAI1 locus in TGF-β1-treated prostate cancer cells

   PMID:27566588

   Open questions at the time

   • Direct RBBP5–SMAD2/3/CBP contacts not biochemically mapped
   • Generalizability beyond Snail not tested
7. 2018 Medium

   Demonstrated that the RBBP5 β-propeller directly binds nucleic acids, proposing a chromatin-targeting function for this domain.

   Evidence X-ray crystallography of the β-propeller and NMR nucleic-acid binding assays

   PMID:29897600

   Open questions at the time

   • Functional consequence of nucleic-acid binding not validated in cells
   • Sequence specificity of binding undefined
8. 2019 High

   Resolved full-length RBBP5 architecture, showing an intramolecular WD40–C-terminal contact maintains complex compactness and a vertebrate-specific motif mediates nucleosome recognition.

   Evidence Structure determination with biochemical assembly and nucleosome methylation assays plus mutagenesis

   PMID:31544921

   Open questions at the time

   • Dynamics of the compact-to-active transition not captured
   • Role of the vertebrate motif in vivo not tested here
9. 2021 Medium

   Genetic studies in model organisms established conserved roles for RBBP5 orthologs in restricting cell-fate plasticity and in meiotic recombination/segregation.

   Evidence Double RNAi reprogramming assay in C. elegans (rbbp-5) and meiotic deletion/checkpoint epistasis with H3K4 methylation assays in budding yeast (Swd1)

   PMID:33290523 PMID:34849786

   Open questions at the time

   • Whether these roles depend on H3K4 methylation versus methylation-independent functions not fully separated
   • Human ortholog equivalence inferred, not directly tested
10. 2024 Medium

    Linked RBBP5 to human disease by showing de novo nucleosome-interface variants are partial loss-of-function alleles and that Rbbp5 loss causes microcephaly in vivo.

    Evidence Protein structural analysis and transgenic Drosophila rescue/complementation in an Rbbp5 null background for p.T232I and p.E296D

    PMID:39036895

    Open questions at the time

    • Molecular consequence of variants on methyltransferase output not quantified
    • Mammalian neurodevelopmental mechanism not established
11. 2025 High

    Identified a specific upstream kinase (nuclear HKDC1) and phosphosite (Ser497) controlling MLL1 complex assembly, providing a defined signaling input to RBBP5 function.

    Evidence Co-IP, in vitro kinase assay, Ser497 mutagenesis, and ChIP for H3K4me3 with HKDC1 inhibition/knockout in hepatocellular carcinoma cells

    PMID:39891906

    Open questions at the time

    • Whether Ser497 phosphorylation is the same phospho-switch seen structurally not resolved
    • Generality across non-HCC contexts unknown
12. 2025 Medium

    Extended RBBP5 to stress-responsive gene programs, showing it co-activates XBP1s to mark proteostasis genes and that its loss sensitizes mice to proteotoxic stress.

    Evidence RBBP5 knockout mice, H3K4me3 ChIP-seq, co-activator interaction assays, and in vitro stress assays (preprint)

    PMID:39314427

    Open questions at the time

    • Preprint, not peer-reviewed
    • Direct RBBP5–XBP1s contact and recruitment order not biochemically defined
13. 2025 Medium

    Showed lncRNA-directed recruitment of RBBP5 to a specific oncogenic promoter, illustrating RNA-guided targeting of RBBP5-containing complexes.

    Evidence RNA pulldown, mass spectrometry, ChIP, RNAi knockdown, and dual luciferase reporter at the ODC1 promoter in liver cancer cells

    PMID:41022601

    Open questions at the time

    • Whether RBBP5 binds the lncRNA directly or via partners unclear
    • KAT2B/RBBP5 functional cooperation not dissected

• How RBBP5's methylation-independent functions (replication licensing, pre-mRNA processing, XBP1s co-activation) are mechanistically separated from its WRAD scaffolding role remains unresolved.
• No structural model distinguishing methylation-dependent from methylation-independent complexes
• Phosphorylation- and RNA-guided targeting mechanisms not integrated into a unified regulatory model