Biology subjects

Berro, J.

Publications and source records attributed to Berro, J..

4 recordsLinked to original sources

bioRxiv2018

Conditions for optimal mechanical power generation by a bundle of growing actin filaments

Bundles of actin filaments are central to a large variety of cellular structures, such as filopodia, stress fibers, cytokinetic rings or focal adhesions. The mechanical properties of these bundles are critical for proper force transmission and force bearing. Previous mathematical modeling efforts have focused on bundles rigidity and shape. However, it remains unknown how bundle length and thickness are controlled by external physical factors, and how the attachment of the bundle to a load affects its ability to transmit forces. In this paper, we present a biophysical model for dynamic bundles of actin filaments that takes into account individual filaments, their interaction with each other and with an external load. In combination with in vitro motility assays of beads coated with formins, our model allowed us to characterize conditions for bead movement and bundle buckling. From the deformation profiles, we determined key biophysical properties of tethered actin bundles, such as their rigidity and filament density. Our model also demonstrated that filaments undulate under lateral constraints applied by external forces or by neighboring filaments of the bundle. Last, our model allowed us to identify optimum conditions in filament density and barbed end tethering to the load for a maximal yield of mechanical power by a dynamic actin bundle.

biophysics↗

bioRxiv2018

Adaptation of actin dynamics and membrane tension control for yeast endocytosis

During clathrin-mediated endocytosis in eukaryotes, actin assembly is required to overcome large membrane tension and turgor pressure. However, the molecular mechanisms by which the actin machinery adapts to varying membrane tension remain unknown. In addition, how cells reduce their membrane tension when they are challenged by hypotonic shocks remains unclear. We used quantitative microscopy to demonstrate that cells rapidly reduce their membrane tension using three parallel mechanisms. In addition to using their cell wall for mechanical protection, yeast cells disassemble eisosomes to buffer moderate changes in membrane tension on a minute time scale. Meanwhile, a temporary reduction of the rate of endocytosis for 2 to 6 minutes, and an increase in the rate of exocytosis for at least 5 minutes allow cells to add large pools of membrane to the plasma membrane. We built on these results to submit the cells to abrupt increases in membrane tension and determine that the endocytic actin machinery of fission yeast cells rapidly adapts to perform clathrin-mediated endocytosis. Our study sheds light on the tight connection between membrane tension regulation, endocytosis and exocytosis.

cell biology↗

bioRxiv2017

Structural organization and energy storage in crosslinked actin-assemblies

During clathrin-mediated endocytosis in yeast cells, short actin filaments (< 200nm) and crosslinking protein fimbrin assemble to drive the internalization of the plasma membrane. However, the organization of the actin meshwork during endocytosis remains largely unknown. In addition, only a small fraction of the force necessary to elongate and pinch off vesicles can be accounted for by actin polymerization alone. In this paper, we used mathematical modeling to study the self-organization of rigid actin filaments in the presence of elastic crosslinkers in conditions relevant to endocytosis. We found that actin filaments condense into either a disordered meshwork or an ordered bundle depending on filament length and the mechanical and kinetical properties of the crosslinkers. Our simulations also demonstrated that these nanometer-scale actin structures can store a large amount of elastic energy within the crosslinkers (up to 10kBT per crosslinker). This conversion of binding energy into elastic energy is the consequence of geometric constraints created by the helical pitch of the actin filaments, which results in frustrated configurations of crosslinkers attached to filaments. We propose that this stored elastic energy can be used at a later time in the endocytic process. As a proof of principle, we presented a simple mechanism for sustained torque production by ordered detachment of crosslinkers from a pair of parallel filaments.

biophysics↗

bioRxiv2016

New single-molecule imaging of the eisosome BAR domain protein Pil1p reveals filament-like dynamics

Molecular assemblies can have highly heterogeneous dynamics within the cell, but the limitations of conventional fluorescence microscopy can mask nanometer-scale features. We have developed a novel, broadly applicable, fluorescent labeling and imaging protocol, called Single-molecule Recovery After Photobleaching (SRAP), which allowed us to reveal the heterogeneous dynamics of the eisosome, a multi-protein structure on the cytoplasmic face of the plasma membrane in fungi. By fluorescently labeling only a small fraction of cellular Pil1p, the core eisosome BAR domain protein in fission yeast, we visualized whole eisosomes and, after photobleaching, recorded the binding of individual Pil1p molecules with ~20 nm precision. Further analysis of these dynamic structures and comparison to computer simulations allowed us to show that Pil1p exchange is spatially heterogeneous, supporting a new model of the eisosome as a dynamic filament.\n\nAbbreviations used

cell biology↗