Cryogenic light microscopy of vitrified samples with angstrom precision

Abstract

High-resolution studies in structural biology are often limited by the challenges of crystallization and low contrast in the cellular native environment. The exquisite labeling specificity of fluorescence microscopy gets around these issues and allows superresolution microscopy, but to date, these works have used chemically fixed samples. To establish light microscopy as a workhorse in structural biology, two main requirements must be fulfilled: near-native sample preservation and near-atomic optical resolution. Here, we introduce single-particle cryogenic light microscopy (spCryo-LM) as a technique that satisfies these key criteria. We adapt established protocols from cryogenic electron microscopy (Cryo-EM) for shock-freezing samples and use a high-vacuum cryogenic shuttle system to transfer them in and out of a liquid-helium cryostat that houses a superresolution fluorescence microscope. By exploiting the enhanced photophysics at low temperature, angstrom precision can be achieved in localizing several fluorophores attached to proteins separated by a few hundred nanometers. We present various characterization studies on vitreous ice, single-molecule photoblinking behavior, and the effects of laser intensity and benchmark our method by resolving the heptameric membrane protein alpha-hemolysin in a synthetic lipid membrane. Additionally, we report on the technique’s capability to resolve membrane proteins in their native cellular membrane environment. spCryo-LM enables structural studies of proteins in their native environment without chemical fixation or protein isolation, and can be integrated with other superresolution or spectroscopic techniques. We believe our approach establishes light microscopy as a powerful tool in structural biology and sets the stage for correlative microscopy with Cryo-EM and related techniques.