Structural Polymorphism of F-Actin is Coupled with its Mechanical Properties

Abstract

Actin plays a major role in many cellular processes including cell motility, cell division, endocytosis, and exocytosis. External and internal mechanical forces lead to cytoskeletal rearrangements in the cell. Recently, we proposed that actin filaments may be directly involved in mechanosensing. Single molecule experiments suggest that F-actin is stiff with respect to extension. We used cryo-electron microscopy and image analysis to examine variation in the axial rise (the distance between two adjacent actin protomers along the one-start left handed helix) within frozen-hydrated actin filaments. We show that F-actin can be found in both stretched and compressed structural states. The magnitude of such mechanical deformation is far beyond what has been suggested for F-actin, and the axial rise can vary from 25 to 30 Angstroms. We demonstrate that the structural state of the filament strongly correlates with the variability of the axial rise, and actin's stretching stiffness depends upon the structural state of the subdomain 2, just as we had previously shown for the bending stiffness. We show that actin binding proteins that cross-link adjacent actin protomers, such as cofilin and the actin binding domain 2 of fimbrin, dramatically reduce the variability of the axial rise within the decorated filaments. Our results demonstrate the coupling between the mechanical properties of F-actin and its structural state and provide a structural basis for actin's role in mechanosensing.