New advances in imaging polymers at near-atomic resolution

Most protein in a cell can often be found in the form of helical filaments, but there have been many limitations on the resolution that can be routinely achieved by electron microscopy of helical polymers. There is nothing that maintains long-range order in a filament, so all disorder accumulates. To overcome this problem, we have developed single-particle approaches to helical reconstruction that have become widely used. When these approaches are applied to cryo-EM images obtained with direct electron detectors, we can now obtain near-atomic resolution from helical filaments that are quite variable in twist. Four examples will be presented showing the biological insights possible with this resolution of better than 4 Å: F-actin, PYD filaments from the inflammasome, I. hospitalis adhesion filaments, and a polymer built from a synthetic polypeptide that allows us to study in vitro evolution.