Structural Visualization of Cytoskeletal Force Transduction

Carl Ayala

A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for The degree of Doctor of Philosophy

Abstract

Cells adhere to their surroundings, mechanically interfacing their intracellular actin cytoskeletons with their local extracellular environments. This enables contractile forces generated by myosin motors to mediate transduction of mechanical cues into biochemical signaling pathways by unclear mechanisms. In this thesis, I show that myosin forces elicit conformational transitions in actin filaments (F-actin) that modulate interactions between F-actin and the force-activated cell adhesion protein α-catenin. In vitro reconstitution and cryo-electron microscopy reveal myosin force-evoked superhelical F-actin spirals. Three-dimensional reconstruction and variability analysis uncover extensive asymmetric remodeling of F-actin’s helical lattice. This is recognized by α-catenin, which cooperatively binds along individual strands, preferentially engaging interfaces featuring extended inter-subunit distances while simultaneously suppressing rotational deviations to regularize the lattice. Collectively, I find that myosin forces can deform F-actin, generating a conformational landscape that is detected and reciprocally modulated by α-catenin, providing a direct structural glimpse at force transduction through the cytoskeleton. This mechanism was further explored in a cellular context using correlative cryo-fluorescence microscopy and cryo-electron tomography. Using these advanced tools, I identify F-actin featuring nanoscale domains of oscillating curvature reminiscent of F-actin superhelices observed in vitro.This F-actin morphology was enriched in cytoskeleton-adhesion interfaces marked by the force-sensitive F-actin repair protein zyxin.These findings support a mechanism in which forces on the actin cytoskeleton in cells cause morphological changes to actin filaments within networks, serving as a site of recognition and accumulation by mechanically regulated proteins. Finally, this thesis explores mechanical regulation of a class of tandem calponin homology (CH) domain actin binding proteins (ABPs) found at the nuclear membrane of the cell called Nesprins. Using an in vitro myosin reconstitution system and TIRF microscopy, I found that Nesprin isoforms 1 and 2 differentially engage tensed F-actin despite their high sequence similarity. Structural studies of both Nesprins bound to F-actin reveal similar binding of the filament throughCH1, but differential disorder of CH2. Preliminary structure-guided protein engineering suggests a mechanism in which CH2 mediates Nesprin’s recognition of force-induced state of F-actin, potentially by modulating CH1’s actin-binding activity.