SARS-CoV-2 Mpro responds to oxidation by forming disulfide and NOS/SONOS bonds.

The main protease (Mpro) of SARS-CoV-2 is critical for viral function and a key drug target. Mpro is only active when reduced; turnover ceases upon oxidation but is restored by re-reduction. This suggests the system has evolved to survive periods in an oxidative environment, but the mechanism of this protection has not been confirmed. Here, we report a crystal structure of oxidized Mpro showing a disulfide bond between the active site cysteine, C145, and a distal cysteine, C117. Previous work proposed this disulfide provides the mechanism of protection from irreversible oxidation. Mpro forms an obligate homodimer, and the C117-C145 structure shows disruption of interactions bridging the dimer interface, implying a correlation between oxidation and dimerization. We confirm dimer stability is weakened in solution upon oxidation. Finally, we observe the protein's crystallization behavior is linked to its redox state. Oxidized Mpro spontaneously forms a distinct, more loosely packed lattice. Seeding with crystals of this lattice yields a structure with an oxidation pattern incorporating one cysteine-lysine-cysteine (SONOS) and two lysine-cysteine (NOS) bridges. These structures further our understanding of the oxidative regulation of Mpro and the crystallization conditions necessary to study this structurally.

MHz frame rate hard X-ray phase-contrast imaging using synchrotron radiation.

Third generation synchrotron light sources offer high photon flux, partial spatial coherence, and ~10-10 s pulse widths. These enable hard X-ray phase-contrast imaging (XPCI) with single-bunch temporal resolutions. In this work, we exploited the MHz repetition rates of synchrotron X-ray pulses combined with indirect X-ray detection to demonstrate the potential of XPCI with millions of frames per second multiple-frame recording. This allows for the visualization of aperiodic or stochastic transient processes which are impossible to be realized using single-shot or stroboscopic XPCI. We present observations of various phenomena, such as crack tip propagation in glass, shock wave propagation in water and explosion during electric arc ignition, which evolve in the order of km/s (µm/ns).

Cockroaches probably cleaned up after dinosaurs.

Dinosaurs undoubtedly produced huge quantities of excrements. But who cleaned up after them? Dung beetles and flies with rapid development were rare during most of the Mesozoic. Candidates for these duties are extinct cockroaches (Blattulidae), whose temporal range is associated with herbivorous dinosaurs. An opportunity to test this hypothesis arises from coprolites to some extent extruded from an immature cockroach preserved in the amber of Lebanon, studied using synchrotron X-ray microtomography. 1.06% of their volume is filled by particles of wood with smooth edges, in which size distribution directly supports their external pre-digestion. Because fungal pre-processing can be excluded based on the presence of large particles (combined with small total amount of wood) and absence of damages on wood, the likely source of wood are herbivore feces. Smaller particles were broken down biochemically in the cockroach hind gut, which indicates that the recent lignin-decomposing termite and cockroach endosymbionts might have been transferred to the cockroach gut upon feeding on dinosaur feces.