Promyelocytic leukAemia nuclear bodies (PML-NBs) are functionally heterogeneous structures involved in diverse nuclear functions. These functions are enabled by dynamic association of several different proteins with the PML-NB subcompartment — but how does PML-NB structure enable them to act as assembly platforms for the various factors? On page 392, Johann Engelhardt, Karsten Rippe and colleagues investigate this issue by combining high-resolution techniques of 4Pi and correlative electron microscopy. On the basis of imaging PML and Sp100 proteins (both integral to PML-NB structure) and their relative localisation to small ubiquitin-related modifier 1 (SUMO-1), SUMO2/3, heterochromatin protein 1 (HP1) and telomeres, they derive a model to explain how various activities can be concentrated in PML-NBs in a dynamic manner. First, conjugation of PML and Sp100 proteins to SUMO-1 results in their self-assembly into a spherical shell of 50-100 nm thickness and variable diameter. By contrast, SUMO2/3 is found mainly in the interior of a subset of PML-NBs, where it might control the protein composition and therefore function of these PML-NBs. For example, HP1 might be recruited to the interior of PML-NBs through sumoylation of an HP1 interaction partner. Finally, the authors also show that PML-NBs are accessible to diffusing nuclear factors. These data on the dynamic three-dimensional structure of PML-NBs help to explain their biological functions.
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