Advanced Imaging Reveals Internal Structure of PML Bodies, Nuclear Compartments Involved in DNA Maintenance
Researchers used advanced cryo-electron tomography and correlative microscopy to produce the first detailed in situ molecular architecture of promyelocytic leukemia (PML) bodies, membrane-less nuclear compartments involved in telomere maintenance and DNA damage response. The study found that PML bodies contain a diffuse protein shell surrounding a core with nucleosomes, chaperonin complexes, proteasomes, and unexpectedly, columnar trinucleosome assemblies suggesting discrete chromatin domains inside. The findings suggest PML bodies may select their molecular contents through a size-exclusion-like physical partitioning mechanism, offering new insight into how the nucleus organizes genome function.
Using a cryo-correlative light and electron microscopy (cryo-CLEM) pipeline combined with cryo-electron tomography, scientists have for the first time visualized the internal molecular organization of PML bodies directly within intact cell nuclei. PML bodies are membrane-less nuclear compartments known to regulate telomere maintenance and DNA damage responses, but their internal structure had remained poorly characterized. The study mapped a diffuse PML-protein shell enclosing an inner core containing nucleosomes, TRiC chaperonin complexes in both open and closed conformations, and single-capped PA28 proteasomes. Strikingly, the nucleosomes included density consistent with columnar trinucleosome assemblies, indicating that PML body interiors harbor discrete chromatin domains rather than being purely protein-rich zones. Membrane structures of unknown function were also detected within some PML bodies, and vault complexes were observed in poised positions in the surrounding nucleoplasm. The overall porous architecture of the PML body interior suggests that content selection may operate through a physical partitioning mechanism analogous to size-exclusion chromatography, rather than purely through biochemical affinity. These findings provide a structural foundation for understanding how PML bodies carry out their diverse regulatory functions.
What's missing
As a preprint posted to bioRxiv, this study has not yet undergone formal peer review, so its findings and interpretations should be treated as preliminary. The study does not address whether the observed columnar trinucleosome assemblies are functionally active or represent a specific chromatin state, nor does it clarify the identity or role of the membrane structures detected within some PML bodies. The proposed size-exclusion-like partitioning mechanism remains a hypothesis requiring direct experimental validation.
What different sources said
- bioRxivCenter
In situ molecular architecture of PML bodies reveals open-state columnar trinucleosome assemblies within a porous, chromatin-permissive interior
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