Processive Nanostepping of Formin mDia1 Loosely Coupled with Actin Polymerization

Direct observation of the conformational states of formin mDia1 at actin filament barbed ends and along the filament

The fine regulation of actin polymerization is essential to control cell motility and architecture and to perform essential cellular functions. Formins are key regulators of actin filament assembly, known to processively elongate filament barbed ends and increase their polymerization rate. Different models have been extrapolated to describe the molecular mechanism governing the processive motion of formin FH2 domains at polymerizing barbed ends. Using negative stain electron microscopy, we directly identified for the first time two conformations of the mDia1 formin FH2 domains in interaction with the barbed ends of actin filaments. These conformations agree with the speculated open and closed conformations of the "stair-stepping" model. We observed the FH2 dimers to be in the open conformation for 79% of the data, interacting with the two terminal actin subunits of the barbed end while they interact with three actin subunits in the closed conformation. In addition, we identified and characterized the structure of single FH2 dimers encircling the core of actin filaments, and reveal their ability to spontaneously depart from barbed ends.

Application of piconewton forces to individual filopodia reveals mechanosensory role of L-type Ca2+ channels

Filopodia are ubiquitous membrane projections that play crucial role in guiding cell migration on rigid substrates and through extracellular matrix by utilizing yet unknown mechanosensing molecular pathways. As recent studies show that Ca²⁺ channels localized to filopodia play an important role in regulation of their formation and since some Ca²⁺ channels are known to be mechanosensitive, force-dependent activity of filopodial Ca²⁺ channels might be linked to filopodia's mechanosensing function. We tested this hypothesis by monitoring changes in the intra-filopodial Ca²⁺ level in response to application of stretching force to individual filopodia of several cell types using optical tweezers. Results show that stretching forces of tens of pN strongly promote Ca²⁺ influx into filopodia, causing persistent Ca²⁺ oscillations that last for minutes even after the force is released. Several known mechanosensitive Ca²⁺ channels, such as Piezo 1, Piezo 2 and TRPV4, were found to be dispensable for the observed force-dependent Ca²⁺ influx, while L-type Ca²⁺ channels appear to be a key player in the discovered phenomenon. As previous studies have shown that intra-filopodial transient Ca²⁺ signals play an important role in guidance of cell migration, our results suggest that the force-dependent activation of L-type Ca²⁺ channels may contribute to this process. Overall, our study reveals an intricate interplay between mechanical forces and Ca²⁺ signaling in filopodia, providing novel mechanistic insights for the force-dependent filopodia functions in guidance of cell migration.

Tug-of-war between actomyosin-driven antagonistic forces determines the positioning symmetry in cell-sized confinement

Symmetric or asymmetric positioning of intracellular structures including the nucleus and mitotic spindle steers various biological processes such as cell migration, division, and embryogenesis. In typical animal cells, both a sparse actomyosin meshwork in the cytoplasm and a dense actomyosin cortex underneath the cell membrane participate in the intracellular positioning. However, it remains unclear how these coexisting actomyosin structures regulate the positioning symmetry. To reveal the potential mechanism, we construct an in vitro model composed of cytoplasmic extracts and nucleus-like clusters confined in droplets. Here we find that periodic centripetal actomyosin waves contract from the droplet boundary push clusters to the center in large droplets, while network percolation of bulk actomyosin pulls clusters to the edge in small droplets. An active gel model quantitatively reproduces molecular perturbation experiments, which reveals that the tug-of-war between two distinct actomyosin networks with different maturation time-scales determines the positioning symmetry.

Detection of tBid Oligomerization and Membrane Permeabilization by Graphene-Based Single-Molecule Surface-Induced Fluorescence Attenuation

The permeabilization of organelle membranes by BCL-2 family proteins is a pivotal step during the regulation of apoptosis; the underlying mechanisms remain unclear. Based on the fluorescence attenuation by graphene oxide, we developed a single-molecule imaging method termed surface-induced fluorescence attenuation (smSIFA), which enabled us to track both vertical and lateral kinetics of singly labeled BCL-2 family protein tBid during membrane permeabilization. We found that tBid monomers lie shallowly on the lipid bilayer, where they self-assemble to form oligomers. During the initiation phase of self-assembly, the two central hydrophobic helices (α₆ and α₇) of tBid insert halfway into the phospholipid core, while the other helices remain on the surface. In oligomerized tBid clusters, α₆ and α₇ prefer to float up, and the other helices may sink to the bottom of the membrane and cause the formation of transient two-dimensional, micelle-like pore structures, which are responsible for the permeabilization of membranes and the induction of apoptosis. Our results shed light on the understanding of tBid-induced apoptosis, and this nanotechnology-based smSIFA approach could be used to dissect the kinetic interaction between membrane protein and lipid bilayer at the single-molecule level with subnanometer precision.