Imaging Biopolymers In Vivo Directly with Cryo-ET: Trim5 Nets and IRE-1 Helices

In the last ten years electron cryotomography (cryo-ET) has made it possible to visualize large macromolecular assemblies inside intact cells in a near-native, "frozen-hydrated" state in 3-D to a few nanometers resolution. increasingly, atomic models of individual proteins and smaller complexes obtained by X-ray crystallography, NMR spectroscopy, or other methods can be fit into cryotomograms to reveal how the various pieces work together inside cells. A few good pictures is therefore sometimes all that is really needed to distinguish between competing models. To illustrate these points, I will present examples of current results from our recent work on the HIV-restriction protein Trim5α and the unfolded protein response sensor IRE1α, both of which form polymers inside cells. Trim5α polymerizes into hexagonal lattices near sites of macroautophagy. IRE1α polymerizes into left-handed double helices inside thin ER tubules. Time permitting, I will also describe ongoing technology development efforts that promise to dramatically expand the applicability of cryo-ET.