High-resolution cryoEM structure determination of soluble proteins after soft-landing electrospray ion beam deposition

Abstract

A protein's structure in the gas phase underpins the interpretation of native mass spectrometry. Yet how fold and conformation respond to dehydration has never been resolved at the residue level, due to the unavailability of a method to image gas-phase proteins at near-atomic resolution. Here, we determine near-atomic-resolution cryoEM structures (2.5-4.8$\,\r{A}$) of four soluble protein complexes ($\beta$-Galactosidase, GDH, RuBisCo, and GroEL) prepared by soft-landing electrospray ion beam deposition (ESIBD) and show the retention of secondary and tertiary structure. Comparison with the corresponding solution structures reveals dehydration-induced structural change is governed by local solvent exposure: interior residues retain high-resolution density while solvent-exposed regions are likely to rearrange. Coherent rearrangements preserve secondary and tertiary structure, incoherent changes manifest as local loss of resolution. Dedicated instrumentation provided the required control over deposition energy, sample environment, and growth of thin vitreous ice films embedding the landed proteins. ESIBD+cryoEM thereby links the chemical selectivity of native mass spectrometry directly to near-atomic structural resolution.