Biology subjects

Paloni, M.

Publications and source records attributed to Paloni, M..

5 recordsLinked to original sources

bioRxiv2022

SLX4 assembles nuclear condensates that compartmentalize the SUMO-RNF4 pathway and drive DNA repair.

SLX4, disabled in Fanconi anemia group P, is a scaffolding protein that coordinates the action of structure-specific endonucleases and other proteins involved in replication-coupled repair of DNA interstrand crosslinks (ICLs). Here we show that SLX4 dimerization and SUMO-SIM interactions drive the assembly of SLX4 membraneless compartments in the nucleus called condensates. Super-resolution microscopy reveals that SLX4 forms chromatin-bound clusters of nanocondensates. We report that SLX4 compartmentalizes the SUMO-RNF4 signaling pathway. SENP6 and RNF4 regulate the assembly and disassembly of SLX4 condensates, respectively. SLX4 condensation per se triggers the selective modification of proteins by SUMO and ubiquitin. Specifically, SLX4 condensation induces ubiquitylation and chromatin extraction of topoisomerase 1 DNA-protein cross-links. SLX4 condensation also induces the nucleolytic degradation of newly replicated DNA. We propose that the compartmentalization of proteins by SLX4 through site-specific interactions ensures the spatiotemporal control of protein modifications and nucleolytic reactions during DNA repair.

biochemistry↗

bioRxiv2022

Cryo-EM structure of the agonist-bound Hsp90-XAP2-AHR complex

SummaryLiving organisms have developed protein sensors helping them to adapt to their environment1. The aryl hydrocarbon receptor (AHR) is an emblematic member of this class of proteins, and a ligand-dependent transcription factor that mediates a broad spectrum of (patho)physiological processes in response to numerous substances including pollutants, natural products and metabolites2. However, in the absence of high-resolution structural data, a molecular understanding of how AHR is activated by such diverse compounds is lacking. Here we present a 2.85 [A] cryo-electron microscopy structure of the cytosolic complex comprising AHR bound to the ligand indirubin, the chaperone Hsp90 and the co-chaperone XAP2. The structure reveals a closed Hsp90 dimer with AHR threaded through its lumen. XAP2 directly interacts with Hsp90 and the AHR ligand-binding domain, thereby acting as a brace stabilizing the entire complex. Importantly, we provide the first experimental visualization of the AHR PAS-B domain bound to a ligand, revealing a unique organization of the ligand-binding pocket and the structural determinants of ligand-binding specificity and promiscuity of the receptor. By providing unprecedented structural details of the molecular initiating event leading to AHR activation, our study rationalizes prior biochemical data and provides a framework for future mechanistic studies and structure-guided drug design.

biochemistry↗

bioRxiv2021

Functional control of a 0.5 MDa TET aminopeptidase by a flexible loop revealed by MAS NMR

Large oligomeric enzymes control a myriad of cellular processes, from protein synthesis and degradation to metabolism. The 0.5 MDa large TET2 aminopeptidase, a prototypical protease important for cellular homeostasis, degrades peptides within a ca. 60 [A] wide tetrahedral chamber with four lateral openings. The mechanisms of substrate trafficking and processing remain debated. Here, we integrate magic-angle spinning (MAS) NMR, mutagenesis, co-evolution analysis and molecular dynamics simulations and reveal that a loop in the catalytic chamber is a key element for enzymatic function. The loop is able to stabilize ligands in the active site and may additionally have a direct role in activating the catalytic water molecule whereby a conserved histidine plays a key role. Our data provide a strong case for the functional importance of highly dynamic - and often overlooked - parts of an enzyme, and the potential of MAS NMR to investigate their dynamics at atomic resolution.

biophysics↗

bioRxiv2021

Arginine multivalency stabilizes protein/RNA condensates

Biomolecular condensates assembled through liquid-liquid phase separation (LLPS) of proteins and RNAs are currently recognized to play an important role in cellular organization. Their assembly depends on the formation of a network of transient, multivalent interactions between flexible scaffold biomolecules. Understanding how protein and RNA sequences determine these interactions and ultimately regulate the phase separation is an open key challenge. Recent in vitro studies have revealed that arginine and lysine residues, which are enriched in most cellular condensates, have markedly distinct propensities to drive the LLPS of protein/RNA mixtures. Here, we employ explicit-solvent atomistic Molecular Dynamics (MD) simulations to shed light on the microscopic origin of this difference by investigating mixtures of polyU oligonucleotides with either polyR/polyK peptides. In agreement with experiments, our simulations indicate that arginine has a higher affinity for polyU than lysine both in highly diluted conditions and in concentrated solutions with a biomolecular density comparable to cellular condensate. The analysis of intermolecular contacts suggests that this differential behavior is due to the propensity of arginine side chains to simultaneously form a higher number of specific interactions with oligonucleotides, including hydrogen bonds and stacking interactions. Our results provide a molecular description of how the multivalency of the guanidinium group enables the coordination of multiple RNA groups by a single arginine residue, thus ultimately stabilizing protein/RNA condensates.

biophysics↗

bioRxiv2020

Unraveling molecular interactions in a phase-separating protein by atomistic simulations

Membraneless organelles are dynamical cellular condensates formed by the liquid-liquid phase separation of proteins and RNA molecules. Multiple evidence suggests that disordered proteins are structural scaffolds that drive the condensation by forming a dynamic network of inter- and intra-molecular contacts. Despite the blooming research activity in this field, the structural characterization of these condensates is very limited and we still do not understand how the phase behaviour is encoded in the amino-acid sequences of the scaffolding proteins. Here we exploited explicit-solvent atomistic simulations to disentangle the molecular interactions governing the phase behaviour of the N-terminal disordered region of DEAD-box helicase 4 (NDDX4), which is a well-established model for phase separation in vitro and in vivo. Single-molecule simulations clarified the interplay between the intramolecular interactions that shape NDDX4 conformational ensemble and the known determinants of its phase behaviour, such as the attraction between oppositely-charged regions and the presence of arginine and phenylalanine. We then investigated intermolecular interactions associated with phase separation via a divide-and-conquer strategy based on the simulations of various NDDX4 fragments at high concentration. Our approach allowed us to probe conditions mimicking real condensates and revealed, in agreement with mutagenesis results, how these interactions arise from the complex interplay of diverse molecular mechanisms. Particularly, we characterized the transient formation of clusters of arginine and aromatic residues, which may stabilize the assembly of several MLOs. Overall, our results reveal the potential of atomistic simulations in the investigation of biomolecular phase separation paving the way for future studies.

biophysics↗