Magic numbers in polymer phase separation -- the importance of being rigid

Cells possess non-membrane-bound bodies, many of which are now understood as phase-separated condensates. One class of such condensates is composed of two polymer species, where each consists of repeated binding sites that interact in a one-to-one fashion with the binding sites of the other polymer. Previous biologically-motivated modeling of such a two-component system surprisingly revealed that phase separation is suppressed for certain combinations of numbers of binding sites. This phenomenon, dubbed the "magic-number effect", occurs if the two polymers can form fully-bonded small oligomers by virtue of the number of binding sites in one polymer being an integer multiple of the number of binding sites of the other. Here we use lattice-model simulations and analytical calculations to show that this magic-number effect can be greatly enhanced if one of the polymer species has a rigid shape that allows for multiple distinct bonding conformations. Moreover, if one species is rigid, the effect is robust over a much greater range of relative concentrations of the two species. Our findings advance our understanding of the fundamental physics of two-component polymer-based phase-separation and suggest implications for biological and synthetic systems.Comment: 8 pages + 15 pages S

Stoichiometry controls the dynamics of liquid condensates of associative proteins

Multivalent associative proteins with strong complementary interactions play a crucial role in phase separation of intracellular liquid condensates. We study the internal dynamics of such "bond-network" condensates comprised of two complementary proteins via scaling analysis and molecular dynamics. We find that when stoichiometry is balanced, relaxation slows down dramatically due to a scarcity of alternative partners following a bond break. This microscopic slow-down strongly affects the bulk diffusivity, viscosity and mixing, which provides a means to experimentally test our predictions

The eukaryotic CO2-concentrating organelle is liquid-like and exhibits dynamic reorganization

Approximately 30%–40% of global CO 2 fixation occurs inside a non-membrane-bound organelle called the pyrenoid, which is found within the chloroplasts of most eukaryotic algae. The pyrenoid matrix is densely packed with the CO 2-fixing enzyme Rubisco and is thought to be a crystalline or amorphous solid. Here, we show that the pyrenoid matrix of the unicellular alga Chlamydomonas reinhardtii is not crystalline but behaves as a liquid that dissolves and condenses during cell division. Furthermore, we show that new pyrenoids are formed both by fission and de novo assembly. Our modeling predicts the existence of a “magic number” effect associated with special, highly stable heterocomplexes that influences phase separation in liquid-like organelles. This view of the pyrenoid matrix as a phase-separated compartment provides a paradigm for understanding its structure, biogenesis, and regulation. More broadly, our findings expand our understanding of the principles that govern the architecture and inheritance of liquid-like organelles

Learning the non-equilibrium dynamics of Brownian movies

Time-lapse microscopy imaging provides direct access to the dynamics of soft and living systems. At mesoscopic scales, such microscopy experiments reveal intrinsic thermal and non-equilibrium fluctuations. These fluctuations, together with measurement noise, pose a challenge for the dynamical analysis of these Brownian movies. Traditionally, methods to analyze such experimental data rely on tracking embedded or endogenous probes. However, it is in general unclear, especially in complex many-body systems, which degrees of freedom are the most informative about their non-equilibrium nature. Here, we introduce an alternative, tracking-free approach that overcomes these difficulties via an unsupervised analysis of the Brownian movie. We develop a dimensional reduction scheme selecting a basis of modes based on dissipation. Subsequently, we learn the non-equilibrium dynamics, thereby estimating the entropy production rate and time-resolved force maps. After benchmarking our method against a minimal model, we illustrate its broader applicability with an example inspired by active biopolymer gels

Sévérité de la dermatite atopique et taux de vitamine D : étude transversale

International audienceBackground Contradictory findings have recently been published on the association between atopic dermatitis (AD) severity and vitamin D deficiency. The aim of this study was to examine the correlation between 25-hydroxyvitamin D (25(OH)D) concentration and \AD\ severity. Patients and methods A cross-sectional study was conducted from June 2011 to March 2013 in dermatology departments in adults and children with a diagnosis of AD. The severity of \AD\ was assessed using the \SCORAD\ and PO-SCORAD indexes and serum 25(OH)D concentrations were determined for all patients. Results Sixty patients were included: 30 with severe \AD\ and 30 with mild-to-moderate AD. The 25(OH)D concentration was lower in patients with severe \AD\ than in patients with mild-to-moderate \AD\ (15.9 ± 8.3 ng/mL vs. 21.5 ± 8.2 ng/mL; P = 0.01). There was a negative correlation between 25(OH)D concentration and respectively, \SCORAD\ (r = −0.47; P < 0.001) and PO-SCORAD (r = −0.4; P = 0.004) values. The correlation between 25(OH)D concentrations and \SCORAD\ values remained valid after adjustment for age, phototype and season. Conclusion This study demonstrated an association between vitamin D deficiency and \AD\ severity but showed no causal link between these variables. Confounding variables such as sun exposure and socioeconomic status were not recorded. A large-sale, comparative interventional study could confirm a real link between these two variables