Myosin II Controls Junction Fluctuations to Guide Epithelial Tissue Ordering.

Under conditions of homeostasis, dynamic changes in the length of individual adherens junctions (AJs) provide epithelia with the fluidity required to maintain tissue integrity in the face of intrinsic and extrinsic forces. While the contribution of AJ remodeling to developmental morphogenesis has been intensively studied, less is known about AJ dynamics in other circumstances. Here, we study AJ dynamics in an epithelium that undergoes a gradual increase in packing order, without concomitant large-scale changes in tissue size or shape. We find that neighbor exchange events are driven by stochastic fluctuations in junction length, regulated in part by junctional actomyosin. In this context, the developmental increase of isotropic junctional actomyosin reduces the rate of neighbor exchange, contributing to tissue order. We propose a model in which the local variance in tension between junctions determines whether actomyosin-based forces will inhibit or drive the topological transitions that either refine or deform a tissue.

Thermal Tunneling of Homopolymers through Amphiphilic Membranes.

We propose a theory to predict the passive translocation of flexible polymers through amphiphilic membranes. By using a generic model for the potential felt by a monomer across the membrane we calculate the free energy profile for homopolymers as a function of their hydrophobicity. Our model explains the translocation window and the translocation rates as a function of chain hydrophobicity in quantitative agreement with simulation results. The potential model leads to a new adsorption transition where chains switch from a one-sided bound adsorbed state into a bridging state through the membrane core by increasing the hydrophobicity beyond a critical value. We demonstrate that the hydrophobicity leading to the fastest translocation coincides with the solution for the critical point of adsorption in the limit of long chains.