Actin bundles are key factors in the mechanical support and dynamic reorganization of the cytoskeleton. High concentrations of multivalent counterions promote bundle formation through electrostatic attraction between actin filaments that are negatively charged polyelectrolytes. In this study, we evaluate how physiologically relevant divalent cations affect the mechanical, dynamic, and structural properties of actin bundles. Using a combination of total internal reflection fluorescence microscopy, transmission electron microscopy, and dynamic light scattering, we demonstrate that divalent cations modulate bundle stiffness, length distribution, and lateral growth. Molecular dynamics simulations of an all-atom model of the actin bundle reveal specific actin residues coordinate cation-binding sites that promote the bundle formation. Our work suggests that specific cation interactions may play a fundamental role in the assembly, structure, and mechanical properties of actin bundles.

The cellular environment is crowded with high concentrations of macromolecules that significantly reduce accessible volume for biomolecular interactions. Reductions in cellular volume can generate depletion forces that affect protein assembly and stability. The mechanical and structural properties of actin filaments play critical roles in various cellular functions, including structural support, cell movement, division, and intracellular transport. Although the effects of molecular crowding on actin polymerization have been shown, how crowded environments affect filament mechanics and structure is unknown. In this study, we investigate the effects of solution crowding on the modulations of actin filament bending stiffness and conformations both in vitro and in silico. Direct visualization of thermally fluctuating filaments in the presence of crowding agents is achieved by fluorescence microscopy imaging. Biophysical analysis indicates that molecular crowding enhances filament's effective bending stiffness and reduces average filament lengths. Utilizing the all-atom molecular dynamics simulations, we demonstrate that molecular crowding alters filament conformations and intersubunit contacts that are directly coupled to the mechanical properties of filaments. Taken together, our study suggests that the interplay between excluded volume effects and nonspecific interactions raised from molecular crowding may modulate actin filament mechanics and structure.

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The mechanical and structural properties of actin cytoskeleton drive various cellular processes, including structural support of the plasma membrane and cellular motility. Actin monomers assemble into double-stranded helical filaments as well as higher-ordered structures such as bundles and networks. Cells incorporate macromolecular crowding, cation interactions, and actin-crosslinking proteins to regulate the organization of actin bundles. Although the roles of each of these factors in actin bundling have been well-known individually, how combined factors contribute to actin bundle assembly, organization, and mechanics is not fully understood. Here, we describe recent studies that have investigated the mechanisms of how intracellular environmental factors influence actin bundling. This review highlights the effects of macromolecular crowding, cation interactions, and actin-crosslinking proteins on actin bundle organization, structure, and mechanics. Understanding these mechanisms is important in determining in vivo actin biophysics and providing insights into cell physiology.

Fascin and α‐actinin form higher‐ordered actin bundles that mediate numerous cellular processes including cell morphogenesis and movement. While it is understood crosslinked bundle formation occurs in crowded cytoplasm, how crowding affects the bundling activities of the two crosslinking proteins is not known. Here, we demonstrate how solution crowding modulates the organization and mechanical properties of fascin‐ and α‐actinin‐induced bundles, utilizing total internal reflection fluorescence and atomic force microscopy imaging. Molecular dynamics simulations support the inference that crowding reduces binding interaction between actin filaments and fascin or the calponin homology 1 domain of α‐actinin evidenced by interaction energy and hydrogen bonding analysis. Based on our findings, we suggest a mechanism of crosslinked actin bundle assembly and mechanics in crowded intracellular environments.

La televisión ecuatoriana tiene 57 años, tiempo en el que la ficción ha sido parte de la programación y se ha consolidado como uno de los géneros audiovisuales más ofertados de los canales de señal abierta. Actualmente, el humor es la marca de las ficciones nacionales que se han ubicado entre las más vistas y, al mismo tiempo, se han constituido en estudios de caso sobre interacciones con las audiencias por medios de comunicación distintos a la televisión y conversión de un formato a otro formato, sin desechar personajes y locaciones. Este artículo registra los resultados de una investigación longitudinal realizada entre 2010 y 2015 por OBITEL Ecuador sobre recepción transmediática y "transformatizada" de cinco ficciones nacionales de humor desde una perspectiva presente para comprender los cambios de ese proceso comunicativo. Como estrategia metodológica, se ha usado la etnografía virtual. Se concluye que las ficciones nacionales se han "transformatizado" y transmediatizado por razones internas a las emisoras mientras que otras se han transmediado por la aplicación de la ley, es Facebook en el las audiencias siguen mostrando un acompañamiento mayor de las ficciones de humor que tuvieron altos índices de audiencia en otros años. En un escenario mundial donde la tecnología está modificando las formas en las que se consumen los contenidos mediáticos, los resultados de este trabajo aportan a las investigaciones sobre recepción de la televisión ecuatoriana, proceso que precisa de más estudios por las dinámicas que está alcanzando.Palabras claves: televisión abierta, audiencias, transmediación, formatos, ficción. Transmediation and transformatization in Ecuador's television fiction: humor narratives' analysis of reception Ecuadorian television is 56 years old, time in which fiction has been part of the program and has established itself as one of the most offered genres in audiovisual channels

Mechanical studies focused on flagella and pili of bacteria, typically estimated by evaluating bending persistence length measurements or Young's modulus, fail to consider the behavior of the whole bacterial body, which can undergo significant changes during antimicrobial resistance. In this Letter, we investigate the biophysical properties of an important plant bacterial pathogen, Xanthomonas perforans, the causal agent for bacterial spot disease of tomato and pepper. The emergence of copper-tolerant strains of X. perforans is predicted to have a devastating impact on industrial scale productions, making it essential to advance approaches to distinguish copper-tolerant from the copper-sensitive X. perforans strains. After introducing a means to measure the bending stiffness of bacteria using atomic force microscopy (AFM), we propose a comparison of the longitudinal and transverse mechanical properties by acquiring high resolution images and force curves of the bacteria. A quantification of the longitudinal and transverse Young's modulus of bacteria reveals significant differences between copper-sensitive and copper-tolerant strains of X. perforans. The present approach is expected to significantly advance the fundamental understanding of bacterial phenomena, such as by monitoring the therapeutic efficacy of newly developed materials on individual X. perforans in situ to relate them to the behavior of their colonies or biofilms.

We have developed multi-modal Microelectrode Arrays (MEAs) with electrodes and microfluidics, with successful manipulation of actin filaments and bundles onto the devices for electro-mechanosensing studies. The application of our MEAs to the characterization of actin filaments/bundles will allow fundamental understanding of actin cytoskeleton's mechanical and electrodynamic properties in neurodegenerative disease signatures on a chip.