Bonne-Andrea, C. | Search bioRxiv

bioRxiv2019

The 20S proteasome activator PA28γ controls the compaction of HP1β-linked heterochromatin

PA28{gamma}, a nuclear activator of the 20S proteasome, is involved in the degradation of several proteins regulating cell growth and proliferation and in the dynamics of various nuclear bodies, but its precise cellular functions remain unclear. Here, using a quantitative FLIM-FRET based microscopy assay monitoring close proximity between nucleosomes in living human cells, we show that PA28{gamma} controls chromatin compaction. We find that its depletion induces a decompaction of pericentromeric heterochromatin, similarly to that observed upon the knockdown of HP1{beta}, a key factor in heterochromatin structure. We show that PA28{gamma} is present at HP1{beta}-containing repetitive-DNA sequences abundant in heterochromatin and importantly, that HP1{beta} on its own is unable to drive chromatin compaction without the presence of PA28{gamma}. At the molecular level, we show that this novel function of PA28{gamma} is independent of its stable interaction with the 20S proteasome, and most likely depends on its ability to maintain appropriate levels of H3K9me3 and H4K20me3, histone modifications that are both involved in heterochromatin formation. Overall, our results implicate PA28{gamma} as a key factor involved in the higher order structuration of chromatin.

cell biology↗

bioRxiv2017

FAM192A/PIP30 regulates Cajal body dynamics by antagonizing PA28γ/coilin interaction

PA28{gamma} is a nuclear activator of the 20S proteasome involved in the regulation of several essential cellular processes, such as cell proliferation, apoptosis, nuclear dynamics and cellular stress response. Unlike the 19S regulator of the proteasome, which specifically recognizes ubiquitylated proteins, PA28{gamma} promotes the degradation of several substrates by the proteasome in an ATP- and ubiquitin-independent manner. However its exact mechanisms of action are unclear and likely to involve additional partners that remain to be identified. Here we report the identification of the first cofactor of PA28{gamma}, PIP30/FAM192A. PIP30 binds directly and specifically via its C-terminal end and in an interaction stabilized by casein kinase 2 phosphorylation to both free and 20S proteasome-associated PA28{gamma}. Its recruitment to proteasome-containing complexes depends on PA28{gamma} and its expression increases the association of PA28{gamma} with the 20S proteasome in cells. Further dissection of its possible roles shows that PIP30 alters PA28{gamma}-dependent activation of peptide degradation by the 20S proteasome in vitro and negatively controls in cells the presence of PA28{gamma} in Cajal Bodies by inhibition of its association with the key Cajal body component coilin. Altogether, our data show that PIP30 deeply affects PA28{gamma} interactions with cellular proteins, including 20S proteasome, demonstrating that it is an important regulator of PA28{gamma} in cells and thus a new player in the control of the multiple functions of the proteasome within the nucleus.\n\nSignificance StatementThe 20S proteasome is a key actor of the control of protein levels and integrity in cells. To perform its multiple functions, it works with a series of regulators, among which a nuclear complex called PA28{gamma}. In particular, PA28{gamma} participates in the regulation of cell proliferation and nuclear dynamics. We describe here the characterization of a novel protein, PIP30/FAM192A, which binds tightly to PA28{gamma} and favors its interaction with the 20S proteasome while inhibiting its association with coilin, a central component of nuclear Cajal bodies. Thus PIP30/FAM192A critically controls the interactome and consequently the functions of PA28{gamma}, and appears to be a new player in the fine regulation of intracellular proteostasis in the cell nucleus.

cell biology↗