MSL3

MSL3 is a chromatin-reading scaffold subunit of the MSL dosage compensation complex that couples histone-mark recognition to transcriptional upregulation of X-linked genes. Its N-terminal chromo-barrel domain forms an aromatic cage that binds H4K20me1/me2 and co-recognizes DNA, while on nucleosomes it reads H3K36me3 to direct complex spreading from chromatin entry sites to active gene bodies; H4K16 acetylation antagonizes this binding, creating a feedback mechanism (PMID:20657587, PMID:20943666, PMID:19029895). The C-terminal MRG domain mediates interaction with MSL1, which is required for both X-chromosome targeting and activation of the MOF acetyltransferase on nucleosomal substrates; MOF in turn acetylates MSL3 near its chromodomain to regulate roX RNA binding, and this modification is reversed by the deacetylase RPD3 (PMID:21217699, PMID:15988010, PMID:12769850). Loss-of-function MSL3 variants in humans disrupt MSL complex assembly and global H4K16 acetylation, causing a neurodevelopmental syndrome that is partially rescued by HDAC inhibitor treatment (PMID:30224647).

1. 2003 High

   The question of how MSL3 activity is post-translationally regulated was answered by showing that MOF acetylates MSL3 at a single lysine near the chromodomain, modulating its roX2 RNA binding and X-chromosome localization, with RPD3 acting as the opposing deacetylase.

   Evidence RNAi in Schneider cells, mass spectrometry of acetylation site, Co-IP of RPD3, RNA binding assays

   PMID:12769850

   Open questions at the time

   • In vivo dynamics of the acetylation–deacetylation cycle are not resolved
   • Whether acetylation affects chromodomain histone-mark binding was not tested
   • Structural basis of how acetylation alters RNA binding is unknown
2. 2005 High

   The domain architecture of MSL3 was functionally mapped, establishing that the C-terminal MRG domain mediates MSL1 interaction and is required for MOF nucleosomal HAT activation, while nucleic acid binding maps to the N-terminal region.

   Evidence Domain deletion/truncation analysis; in vitro HAT assay; immunofluorescence in Drosophila

   PMID:15988010

   Open questions at the time

   • Whether the MRG–MSL1 interaction is direct or bridged was not yet resolved at atomic resolution
   • Functional contribution of the polar region between domains was unclear
3. 2006 High

   The question of whether the chromo-barrel domain is needed for X-chromosome targeting versus transcriptional output was resolved: the MRG domain targets MSL3 to the X, whereas the chromo-barrel domain is dispensable for targeting but essential for male viability and X-linked gene upregulation.

   Evidence Domain deletion mutants in Drosophila with immunofluorescence, qRT-PCR, and survival assays

   PMID:16547465

   Open questions at the time

   • The histone mark recognized by the chromo-barrel domain was not yet identified
   • How the polar region cooperates with the chromo-barrel domain mechanistically remained unclear
4. 2008 High

   The molecular basis of MSL complex spreading was established: the MSL3 chromodomain reads H3K36me3 on nucleosomes, enabling spreading from entry sites to active gene bodies, and chromodomain mutations abolish spreading without affecting initial recruitment.

   Evidence ChIP-chip of MSL3 chromodomain mutants in Drosophila; in vitro nucleosome binding assays

   PMID:19029895

   Open questions at the time

   • Whether H3K36me3 binding and H4K20me binding are independent or cooperative events was unclear
   • How spreading terminates at gene boundaries was not addressed
5. 2010 High

   Structural determination of the MSL3 chromo-barrel domain revealed the aromatic cage that binds H4K20me1/me2 and showed that co-recognition of the DNA minor groove creates a composite binding surface; H4K16ac antagonizes binding, establishing a feedback link between MOF product and MSL3 chromatin retention.

   Evidence X-ray crystallography of human MSL3 chromo-barrel domain and ternary MSL3–DNA–H4K20me1 complex; peptide binding assays; Drosophila Y31A mutagenesis

   PMID:20657587 PMID:20943666

   Open questions at the time

   • Whether H3K36me3 and H4K20me1 are read simultaneously on the same nucleosome or sequentially is unresolved
   • Quantitative contribution of DNA co-recognition versus histone-mark recognition in vivo is unmeasured
6. 2011 High

   The atomic basis of MSL complex assembly was resolved: MSL1 recruits MSL3 through an extended hydrophobic interface and MOF through electrostatic contacts, and disruption of either interface impairs targeting to dosage-compensated gene bodies.

   Evidence X-ray crystallography of MSL3–MSL1 and MOF–MSL1 binary complexes; site-directed mutagenesis; ChIP in Drosophila

   PMID:21217699

   Open questions at the time

   • Structure of the full pentameric MSL complex including roX RNA was not determined
   • Whether MSL1-mediated assembly is regulated by MSL3 acetylation status is unknown
7. 2018 High

   The relevance of MSL3 to human disease was established: pathogenic MSL3 variants disrupt MSL complex assembly and cause global loss of H4K16ac, defining a neurodevelopmental syndrome that is partially rescuable by HDAC inhibitors.

   Evidence Patient-derived cell lines; western blot for H4K16ac; Co-IP for complex assembly; transcriptomics; HDAC inhibitor rescue

   PMID:30224647

   Open questions at the time

   • Which MSL3 domains are disrupted by specific patient variants was not systematically mapped
   • Long-term in vivo efficacy of HDAC inhibitor rescue is untested
   • Whether transcriptomic changes reflect direct MSL complex targets or secondary effects is unresolved
8. 2022 Medium

   An MSL-complex-independent role for Msl3 was uncovered in female Drosophila germline stem cells, where Msl3 reads H3K36me3 and cooperates with the ATAC acetyltransferase complex to promote transcription of RpS19b, linking it to translational control of Rbfox1 and meiotic entry.

   Evidence Drosophila msl3 null mutants; RNA-seq; polysome fractionation; genetic interactions with Set2 and ATAC complex

   PMID:34878097

   Open questions at the time

   • Single study; independent confirmation is needed
   • Whether Msl3–ATAC interaction is direct or chromatin-mediated is unknown
   • Whether mammalian MSL3 has analogous complex-independent functions is untested

• Key unresolved questions include the structural basis of the full MSL holocomplex with roX RNA, whether H3K36me3 and H4K20me marks are read on the same or adjacent nucleosomes, and how MSL3 acetylation integrates with chromo-barrel-domain histone-mark recognition to regulate complex spreading dynamics.
• No structure of the complete MSL holocomplex with roX RNA exists
• Simultaneous versus sequential reading of H3K36me3 and H4K20me1 is unresolved
• In vivo kinetics of MSL3 acetylation–deacetylation cycle and its impact on spreading are unknown