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Imaging live virus to monitor the viral entry process is essential to understand virus-host interactions during pathogen infection. However, methods for efficient labeling of live viruses, in particular labeling non-enveloped viruses and tracing virus entry processes, remain limited. Recently, labeling by using organometallic palladium reagents has provided a highly efficient and selective way to bioconjugate cysteines of virus proteins. Here, site-specific bioorthogonal labeling mediated by an organometallic palladium reagent on the surface of live enterovirus-71 (EV71) was used to visualize its entry into live cells. In contrast to currently used immunofluorescence and membrane-anchored dyes, this site-specific and quantitative labeling of live EV71 allows temporal imaging of its entry into host cell membranes on the timescale of seconds with little negative impact on its virulence. This method revealed details of EV71 virus entry and has broad applicability for monitoring virus entry that is difficult to assess by using conventional protein-labeling approaches.

Cuticle-degrading proteases are involved in the breakdown of cuticle/eggshells of nematodes or insects, a hard physical barrier against fungal infections. Understanding the 3-dimensional structures of these proteins can provide crucial information for improving the effectiveness of these fungi in biocontrol applications, e.g., by targeted protein engineering. However, the structures of these proteases remain unknown. Here, we report the structures of two cuticle-degrading proteases from two species of nematophagous fungi. The two structures were solved with X-ray crystallography to resolutions of 1.65 A (Ver112) and 2.1 A (PL646), respectively. Crystal structures of PL646 and Ver112 were found to be very similar to each other, and similar to that of proteinase K from another fungus Tritirachium album. Differences between the structures were found among residues of the substrate binding sites (S1 and S4). Experimental studies showed that the enzymes differed in hydrolytic activity to synthetic peptide substrates. Our analyses of the hydrophobic/hydrophilic and electrostatic features of these two proteins suggest that their surfaces likely play important roles during fungal infection against nematodes. The two crystal structures provide a solid basis for investigating the relationship between structure and function of cuticle-degrading proteases.