Hendzel, M. J. | Search bioRxiv

bioRxiv2020

Matrix metalloproteinase-2 mediates ribosomal RNA transcription by cleaving nucleolar histones

Cell proliferation and survival require continuous ribosome biogenesis and protein synthesis. Genes encoding ribosomal RNA (rRNA) are physically located in a specialized substructure within the nucleus known as the nucleolus, which has a central role in the biogenesis of ribosomes. Matrix metalloproteinase-2 (MMP-2) was previously detected in the nucleus. However, its role there is elusive. Herein we report that MMP-2 resides within the nucleolus to regulate rRNA transcription. MMP-2 is enriched at the promoter region of rRNA gene repeats and its inhibition downregulates pre-rRNA transcription. The N-terminal tail of histone H3 is clipped by MMP-2 in the nucleolus and is associated with increased rRNA transcription. Knocking down/out MMP-2 or inhibiting its activity prevents histone H3 cleavage and reduces both rRNA transcription and cell proliferation. In addition to the known extracellular roles of MMP-2 in tumor growth, our data reveal an epigenetic mechanism whereby intranucleolar MMP-2 regulates cell proliferation through histone proteolysis and facilitation of rRNA transcription.

cell biology↗

bioRxiv2019

KMT5C displays robust retention and liquid-like behavior in phase separated heterochromatin

The pericentromere exists as a distinct chromatin compartment that is thought to form by a process of phase separation. This reflects the ability of the heterochromatin protein CBX5 (aka HP1) to form liquid condensates that encapsulate pericentromeres.1,2 In general, phase separation compartmentalizes specific activities within the cell, but unlike membrane-bound organelles, their contents rapidly exchange with their surroundings.3 Here, we describe a novel state for the lysine methyltransferase KMT5C where it diffuses within condensates of pericentromeric heterochromatin but undergoes strikingly limited nucleoplasmic exchange, revealing a barrier to exit similar to that of biological membranes. This liquid-like behavior maps to a discrete protein segment with a small number of conserved sequence features and containing separable determinants for localization and retention that cooperate to confer strict spatial control. Accordingly, loss of KMT5C retention led to aberrant spreading of its catalytic product (H4K20me3) throughout the nucleus. We further found that KMT5C retention was reversible in response to chromatin state, which differed markedly for CBX5 and the methyl-CpG binding protein MeCP2, revealing considerable plasticity in the control of these phase separated assemblies. Our results establish that KMT5C represents a precedent in the biological phase separation4 continuum that confers robust spatial constraint of a protein and its catalytic activity without progression to a gel or solid.

cell biology↗