S C

IE N

T IF

IC H

IG H

LI G

H T

S S

T R

U C

T U

R A

L B

IO LO

G Y

3 9 I H I G H L I G H T S 2 0 2 2

PRINCIPAL PUBLICATION AND AUTHORS

Evolution and activation mechanism of the flavivirus class II membrane-fusion machinery, M.-C. Vaney (a), M. Dellarole (a), S. Duquerroy (a), I. Medits (b), G. Tsouchnikas (b), A. Rouvinski (a), P. England (a), K. Stiasny (b), F.X. Heinz (b), F.A. Rey (a), Nat. Commun. 13(1), 3718 (2022); https:/doi.org/10.1038/s41467-022-31111-y (a) Institut Pasteur, Paris (France) (b) Center for Virology, Medical University of Vienna (Austria)

REFERENCES

[1] G. Gonzalez et al., Front. Microbiol. 13, 863725 (2022). [2] F.A. Rey et al., EMBO Rep. 19, 206 (2018). [3] F.A. Rey et al., Curr. Opin. Virol. 24, 132 (2017).

Structural insights into the infection cycle of Epstein-Barr virus

The infection cycle of Epstein-Barr virus (EBV) alternates between a latent phase, in which few viral proteins are produced, and a lytic phase, during which viral replication and transmission occurs. X-ray crystallography studies of a key EBV regulatory protein elucidated how the switch from latency to lytic replication is achieved.

EBV is a human herpesvirus that infects over 90% of the world population, causes infectious mononucleosis and is associated with numerous cancers. EBV primarily infects B lymphocytes, the cells responsible for producing antibodies. Following initial infection, the viral DNA is maintained in the nucleus of B cells as plasmid-like mini- chromosomes, which become progressively methylated at CpG dinucleotide motifs by the infected cell. This allows EBV to repress expression of viral antigens and to establish a strictly latent infection that leads to cell survival and escape from immune surveillance of the infected human host. Extracellular signals can subsequently reactivate the latent virus and induce lytic gene expression, leading to replication, release and transmission of the virus.

The EBV infection cycle is regulated by the viral protein ZEBRA, a homodimeric member of the AP-1 family of bZIP transcription factors. ZEBRA binds to and activates both viral and cellular promoters. Early in infection, when the EBV genome is unmethylated, ZEBRA activates cellular genes that promote B-cell proliferation and the establishment of viral latency. In contrast, the expression of ZEBRA during strict latency, when the EBV genome is methylated, leads to the activation of viral genes that trigger lytic replication. Underpinning these roles is ZEBRA s ability to recognise two distinct classes of DNA target sites (ZEBRA responsive elements or ZREs): CpG-free motifs resembling the consensus AP-1 site recognised by cellular bZIP proteins and CpG-containing motifs found in viral lytic gene promoters that are selectively bound by ZEBRA when methylated. Thus, by preferentially binding methylated target sites, ZEBRA overturns the host-mediated epigenetic silencing of the latent EBV genome, allowing the virus to reactivate from its latent state and to replicate.

Although the structure of ZEBRA bound to an AP-1 site is known [1], how ZEBRA preferentially recognises methylated promoters and triggers lytic activation was poorly understood. X-ray crystallography data collected at beamline ID23-2 allowed the crystal structure of ZEBRA

Fig. 30: Structure of the DNA-binding domain of ZEBRA in complex with methylated DNA. a) Global view of the complex. b) View of the methylated DNA half-site (top) and comparison with the corresponding AP-1 half-site (bottom). Red asterisks indicate hydrogen bonds unique to each structure.