S T R U C T U R A L B I O L O G Y

S C I E N T I F I C H I G H L I G H T S

3 8 H I G H L I G H T S 2 0 2 2 I

These processes include oligomeric reorganisations of the two envelope proteins E and prM as well as a proteolytic activation cleavage of prM into pr and M (Figure 28, top panels).

Using beamlines ID23-1 and ID29, the crystal structure was determined of a key intermediate form of the TBE virus envelope glycoproteins, which elucidated molecular details of the missing link in understanding of the processes leading to fusogenic and infectious flavivirus particles (Figure 28, lower panel). Specifically, the crystal

Fig. 28: Maturation process of flavivirus envelope proteins. Top: Immature virions

containing trimeric spikes of E-prM heterodimers are assembled at neutral

pH in the endoplasmic reticulum and transported through exocytosis. The acidic pH in late Golgi compartments induces an

oligomeric rearrangement of E and prM as well as cleavage of prM into pr and M,

forming smooth-surfaced intermediate immature particles. Upon release from cells

and encountering neutral pH, pr falls off to generate fusogenic mature infectious

virions (modified from F. Rey et al., EMBO Rep. 2018). Bottom: Side view of the

crystal structure of the TBE virus (pr-E)2 complex at acidic pH.

structure enabled the identification of a structural element in the viral fusion protein (the 150-loop) that acts as a pH-dependent snap lock and controls the structural and oligomeric rearrangements during the pathways of viral assembly and maturation (Figure 29).

This new data identifies a new and important site of vulnerability common to all flaviviruses that can become a target for substances inhibiting virus replication, thus providing a lead towards the development of specific antiviral drugs.

Fig. 29: Snap-lock mechanism of the 150-loop in E during virus maturation.

At acidic pH (in the trans-Golgi network) the 150-loop opens up to create a positively charged cavity that accommodates pr in immature particles. At neutral pH (upon

release of virus particles from infected cells), pr is ejected from the E dimer by the

150-loop, which closes as a hinged-lid to protect the fusion loop (FL) of E. During

virus entry, when encountering the acidic pH of endosomes, this snap lock opens again and the exposed fusion loop can

initiate membrane fusion.