Navigating the oligomeric landscape of the periplasmic stress response protease-chaperone DegP with charge detection mass spectrometry

DegP is a periplasmic protease-chaperone essential for protein quality control and virulence factor trafficking in Gram-negative bacteria. In its apo form, DegP adopts a dynamic ensemble of oligomers derived from trimer building blocks through two competing self-assembly pathways. Upon engaging client proteins, apo DegP oligomers redistribute into discrete cage structures inside which the clients are encapsulated. The cage ensembles formed depend on the size of the bound clients, and notably can include 12mers, 24mers, and 60mers. Previous studies mapped the DegP oligmeric landscape using dynamic light scattering, analytical ultracentrifugation, nuclear magnetic resonance spectroscopy, and electron cryomicroscopy, modalities which in general report ensemble averages and often cannot directly delineate closely related coexisting species. Here, we apply charge detection mass spectrometry (CDMS) to directly measure the masses of individual DegP ions, resolving the complete oligomeric distribution in the absence and presence of four clients of increasing size. We reveal previously undetected odd-numbered oligomers and quantify the relative abundance of each assembly. Through heat-cool cycling CDMS experiments, we track cage distribution changes and reveal client protection and refolding, providing a direct view of the chaperone capabilities of DegP. These results further establish CDMS as a powerful single-molecule tool for dissecting heterogeneous protein assembly landscapes.