Single-Molecule Atomic-Force Spectroscopy Captures a Novel Class of Molecular Nanosprings with Robust Stepwise Refolding Properties

Abstract

Biological systems are constantly under mechanical stress either during movement or when acted upon by external forces. The identification of proteins motifs that behave as biological springs will be important for understanding how cells respond to mechanical stimuli and can also propel the design of non-biological nanomaterials. We report here identification of a large class of alpha-helical spiral or solenoid-shaped proteins comprised of ANK-R, ARM, or HEAT repeats that rapidly and forcefully refold following stretching. Each of these repeats unfolds and refolds in equilibrium through discrete events involving individual repeats or their alpha-helical subunits. We also present evidence for the capture of unfolding/refolding transient events while stretching or relaxing by AFM and analyze differences in refolding lengths and forces for each repeat. This class of stacked helical-repeats behave as molecular nanosprings, are likely important for cellular mechanosensation, and can be used as platforms for structural elements of nanomechanical systems based on proteins. Supported by the NIH (PEM) and HHMI (VB).