In biomembranes, function is tied to nanoscopic motion and morphological dynamics of macromolecules. These phenomena can be interrogated at the ensemble level with well-established spectroscopic or scattering-based techniques, in the form of snapshots using conventional high-resolution ex situ imaging methods, or at relatively small time scales (&lt;µs) by means of molecular simulation. However, filming nanoscopic molecular morphology fluctuations experimentally has remained a methodological challenge. By combining graphene encapsulation, low dose imaging, molecular modeling, and theoretical concepts from electron optics and biophysics, we used liquid-phase transmission electron microscopy to image and derive insight from the nanoscopic morphological dynamics of individual bioassemblies in liquid water. We focus primarily on nanodiscs—a colloidal form of membrane proteins. By observing them in real-time, we found that nanodisc shape fluctuations are dominated by the molecular mechanics of encircling scaffold proteins, in the form of a measurable effective tension and bending rigidity. The same fluctuations are sensitive to noncovalent interactions between lipids and the membrane protein inside a nanodisc, because they delineate a structurally and dynamically modified “oasis”, which also protects nanodiscs from drastic fingering fluctuations. This work thereby illustrates an experimental approach to image, quantify, and understand biomolecular dynamics at the nanometer scale, and ongoing efforts to incorporate machine learning and other advanced data analysis methods may help it extend to diverse soft and biological materials systems that have so far eluded high-resolution in situ imaging.

Electron videography of nanoscale dynamics in membrane protein nanodiscs

ACS Spring 2022

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