Flexible Fitting of Biomolecular Structures to Atomic Force Microscopy Images via Biased Molecular Simulations

Atomic force microscopy (AFM) is a prominent imaging technology that observes large-scale structural dynamics of biomolecules near the physiological condition, but the AFM data are limited to the surface shape of specimens. Rigid-body fitting methods were developed to obtain molecular structures that fit to an AFM image, without accounting for conformational changes. Here we developed a method to fit flexibly a three-dimensional biomolecular structure into an AFM image. First, we describe a method to produce a pseudo-AFM image from a given three-dimensional structure in a differentiable form. Then, using a correlation function between the experimental AFM image and the computational pseudo-AFM image, we developed a flexible fitting molecular dynamics (MD) simulation method, by which we obtain protein structures that well fit to the given AFM image. We first test it with a twin-experiment; for a synthetic AFM image produced from a protein structure different from its native conformation, the flexible fitting MD simulations sampled those that fit well the AFM image. Then, parameter dependence in the protocol is discussed. Finally, we applied the method to a real experimental AFM image for a flagellar protein FlhA, demonstrating its applicability. We also test the rigid-body fitting of a fixed structure to the AFM image. Our method will be a general tool for structure modeling based on AFM images and is publicly available through CafeMol software.