Biology subjects

Papoian, G.

Publications and source records attributed to Papoian, G..

5 recordsLinked to original sources

bioRxiv2022

The Molecular and Evolutionary Principles of Histone Folding in Eukarya and Archaea

Histones are the dominant proteins to compact and store DNA in both Eukarya and Archaea. For a long time, histones are observed to exist in the unit of dimers but diverge into different formats such as heterodimers in Eukarya or homodimers in Archaea. Here, by studying 11 types of histone proteins, both monomers and their dimeric complexes, using multiscale molecular dynamics (MD) simulations combined with NMR and circular dichroism experiments, we confirm the widely applied "folding upon binding" mechanism of histone structures. A histone dimer appears to form the longest 2 helices followed by other shorter helices and inter-molecular tertiary structures. We report an alternative conformation, namely, the inverted non-native dimer, which has a minimum free energy state. Protein sequence analysis indicates that the inverted conformation can be attributed to a hidden head-tail sequence symmetry underlying all histone proteins. This finding strongly support previously proposed histone evolution hypotheses. Finally, we separately used the MD-based AWSEM and AI-based AlphaFold-Multimer model to predict eukaryotic histone homodimer structures and performed extensive allatom MD simulations to examine their structural stabilities. Our results suggest that eukaryotic histones can also form stable homodimers, whereas their disordered tails-- the structurally asymmetrical region--may tip the balance towards the formation of heterotypic dimers.

biophysics↗

bioRxiv2021

Protein structure dynamic prediction: a Machine Learning/Molecular Dynamic approach to investigate the protein conformational sampling

Proteins exist in several different conformations. These structural changes are often associated with fluctuations at the residue level. Recent findings show that co-evolutionary analysis coupled with machine- learning techniques improves the precision by providing quantitative distance predictions between pairs of residues. The predicted statistical distance distribution from Multi Sequence Analysis (MSA) reveals the presence of different local maxima suggesting the flexibility of key residue pairs. Here we investigate the ability of the residue-residue distance prediction to provide insights into the protein conformational ensemble. We combine deep learning approaches with mechanistic modeling to a set of proteins that experimentally showed conformational changes. The predicted protein models were filtered based on energy scores, RMSD clustering, and the centroids selected as the lowest energy structure per cluster. The models were compared to the experimental-Molecular Dynamics (MD) relaxed structure by analyzing the backbone residue torsional distribution and the sidechain orientations. Our pipeline not only allows us to retrieve the global experimental folding but also the experimental structural dynamics. We show the potential correlation between the experimental structure dynamics and the predicted model ensemble demonstrating the susceptibility of the current state-of-the-art methods in protein folding and dynamics prediction and pointing out the areas of improvement.

bioinformatics↗

bioRxiv2021

A tug of war between filament treadmilling and myosin induced contractility generates actin cortex

In most eukaryotic cells, actin filaments assemble into a shell-like actin cortex under the plasma membrane, controlling cellular morphology, mechanics, and signaling. The actin cortex is highly polymorphic, adopting diverse forms such as the ring-like structures found in podosomes, axonal rings, and immune synapses. The biophysical principles that underlie the formation of actin rings and cortices remain unknown. Using a molecular simulation platform, called MEDYAN, we discovered that varying the filament treadmilling rate and myosin concentration induces a finite size phase transition in actomyosin network structures. We found that actomyosin networks condense into clusters at low treadmilling rates or high myosin concentration but form ring-like or cortex-like structures at high treadmilling rates and low myosin concentration. This mechanism is supported by our corroborating experiments on live T cells, which exhibit ring-like actin networks upon activation by stimulatory antibody. Upon disruption of filament treadmilling or enhancement of myosin activity, the pre-existing actin rings are disrupted into actin clusters or collapse towards the network center respectively. Our analyses suggest that the ring-like actin structure is a preferred state of low mechanical energy, which is, importantly, only reachable at sufficiently high treadmilling rates.

biophysics↗

bioRxiv2021

Membrane-MEDYAN: Simulating Deformable Vesicles Containing Complex Cytoskeletal Networks

The plasma membrane defines the shape of the cell and plays an indispensable role in bridging intra- and extra-cellular environments. Mechanochemical interactions between plasma membrane and cytoskeleton are vital for cell biomechanics and mechanosensing. A computational model that comprehensively captures the complex, cell-scale cytoskeleton-membrane dynamics is still lacking. In this work, we introduce a triangulated membrane model that accounts for membranes elastic properties, as well as for membrane-filament steric interactions. The corresponding force-field was incorporated into the active biological matter simulation platform, MEDYAN ("Mechanochemical Dynamics of Active Networks"). Simulations using the new model shed light on how actin filament bundling affects generation of tubular membrane protrusions. In particular, we used membrane-MEDYAN simulations to investigate protrusion initiation and dynamics while varying geometries of filament bundles, membrane rigidities and local G-Actin concentrations. We found that bundles protrusion propensities sensitively depend on the synergy between bundle thickness and inclination angle at which the bundle approaches the membrane. The new model paves the way for simulations of biological systems involving intricate membrane-cytoskeleton interactions, such as occurring at the leading edge and the cortex, eventually helping to uncover the fundamental principles underlying the active matter organization in the vicinity of the membrane.

biophysics↗

bioRxiv2018

The Oligomerization Landscape of Histones

In eukaryotes, DNA is packaged through nucleosomes. Each nucleosome is typically centered around an octameric histone protein core: one central tetramer plus two separate dimers. Studying the assembly mechanisms of histones is essential for understanding the dynamics of entire nucleosomes and higher-order DNA packaging. Here we investigate the canonical histone assembly and that of the centromere-specific histone variant CENP-A using molecular dynamics simulations. We quantitatively characterize their thermodynamical and dynamical features, showing that the canonical H3 tetramer exhibits large instability around the central interface manifested via a swiveling motion of two halves, supporting the recently observed DNA handedness flipping of the tetrasome. In contrast, the variant CENP-A encodes a distinctive stability to its tetramer with a rigid but twisted interface compared to the crystal structure, implying the diverse structural possibilities of the histone variant. Interestingly, the observed tetramer dynamics alter significantly and appear to reach a new dynamics balance when H2A/H2B dimers are present. In all, these data reveal key mechanistic insights and structural details for the assembly of canonical and variant CENP-A histone tetramers and octamers, providing theoretical quantifications and physical interpretations for longstanding and recent experimental observations.

biophysics↗