Hendra and Nipah viruses are the prototypical species from the recently classified Henipavirus genus. These viruses are zoonotic infections that are harboured by Pteropus Flying Fox species. Henipaviruses cause severe respiratory disease, neural symptoms and encephalitis in a variety of animals and humans, with a mortality rate greater than 70%.
Of note is the Henipavirus Matrix protein, that drives assembly and budding of the virion, as it also performs non-structural functions as a Type I Interferon antagonist. Interestingly, Matrix undergoes nuclear trafficking. This allows for monoubiquitination for downstream cell sorting and membrane association that have been determined as critical for the budding process.
To date the literature supports a bipartite Nuclear Localisation Signal (NLS) as being functional in Matrix protein nuclear import, but the recent structural determination of Hendra virus Matrix protein revealed this NLS lies within an alpha helix; incompatible with Importin alpha (IMPA) major and minor binding sites. Matrix also contains a putative monopartite NLS that has been overlooked in functional studies, even though it is on an exposed flexible loop, compatible with Importin alpha binding sites.
The aim of this study was to gain an understanding of how Henipavirus Matrix protein traffics to the nucleus and this was achieved through protein purification, complexing of Matrix NLSs with Importins, and solving the binding interface with x-ray crystallography. It was determined that the monopartite NLS binds to IMPA2 and IMPA3, and a model of the full-length Matrix protein in complex with IMPA was generated, showing the anticipated binding conformation.
The structure was solved of the synthetic peptide Bimax2 in complex with IMPA3, indicating it is a potential inhibitor of Henipavirus Matrix nuclear import.
The structural data from the present study provides a greater understanding of the monopartite NLS mechanism involved in the nuclear import of Matrix. Abrogating nuclear import of Matrix could be a realistic approach to halting henipavirus infection. Given the widespread distribution of the reservoir host and mortality rate, exploration of inhibitors targeting the nuclear trafficking of Matrix should be considered with great importance.