High flavivirus structural plasticity demonstrated by a non-spherical morphological variant.

Previous flavivirus (dengue and Zika viruses) studies showed largely spherical particles either with smooth or bumpy surfaces. Here, we demonstrate flavivirus particles have high structural plasticity by the induction of a non-spherical morphology at elevated temperatures: the club-shaped particle (clubSP), which contains a cylindrical tail and a disc-like head. Complex formation of DENV and ZIKV with Fab C10 stabilize the viruses allowing cryoEM structural determination to ~10 Å resolution. The caterpillar-shaped (catSP) Fab C10:ZIKV complex shows Fabs locking the E protein raft structure containing three E dimers. However, compared to the original spherical structure, the rafts have rotated relative to each other. The helical tail structure of Fab C10:DENV3 clubSP showed although the Fab locked an E protein dimer, the dimers have shifted laterally. Morphological diversity, including clubSP and the previously identified bumpy and smooth-surfaced spherical particles, may help flavivirus survival and immune evasion.

Near-atomic resolution cryo-electron microscopic structure of dengue serotype 4 virus.

Dengue virus (DENV), a mosquito-borne virus, is responsible for millions of cases of infections worldwide. There are four DENV serotypes (DENV1 to -4). After a primary DENV infection, the antibodies elicited confer lifetime protection against that DENV serotype. However, in a secondary infection with another serotype, the preexisting antibodies may cause antibody-dependent enhancement (ADE) of infection of macrophage cells, leading to the development of the more severe form of disease, dengue hemorrhagic fever. Thus, a safe vaccine should stimulate protection against all dengue serotypes simultaneously. To facilitate the development of a vaccine, good knowledge of different DENV serotype structures is crucial. Structures of DENV1 and DENV2 had been solved previously. Here we present a near-atomic resolution cryo-electron microscopy (cryo-EM) structure of mature DENV4. Comparison of the DENV4 structure with similar-resolution cryo-EM structures of DENV1 and DENV2 showed differences in surface charge distribution, which may explain their differences in binding to cellular receptors, such as heparin. Also, observed variations in amino acid residues involved in interactions between envelope and membrane proteins on the virus surface correlate with their ability to undergo structural changes at higher temperatures.

The stem region of premembrane protein plays an important role in the virus surface protein rearrangement during dengue maturation.

BACKGROUND: Dengue virus surface proteins, envelope (E) and pre-membrane (prM), undergo rearrangement during the maturation process at acidic condition. RESULTS: prM-stem region binds tighter to both E protein and lipid membrane when environment becomes acidic. CONCLUSION: At acidic condition, E proteins are attracted to the membrane-associated prM-stem. SIGNIFICANCE: prM-stem region induces virus structural changes during maturation. Newly assembled dengue viruses (DENV) undergo maturation to become infectious particles. The maturation process involves major rearrangement of virus surface premembrane (prM) and envelope (E) proteins. The prM-E complexes on immature viruses are first assembled as trimeric spikes in the neutral pH environment of the endoplasmic reticulum. When the virus is transported to the low pH environment of the exosomes, these spikes rearrange into dimeric structures, which lie parallel to the virus lipid envelope. The proteins involved in driving this process are unknown. Previous cryoelectron microscopy studies of the mature DENV showed that the prM-stem region (residues 111-131) is membrane-associated and may interact with the E proteins. Here we investigated the prM-stem region in modulating the virus maturation process. The binding of the prM-stem region to the E protein was shown to increase significantly at low pH compared with neutral pH in ELISAs and surface plasmon resonance studies. In addition, the affinity of the prM-stem region for the liposome, as measured by fluorescence correlation spectroscopy, was also increased when pH is lowered. These results suggest that the prM-stem region forms a tight association with the virus membrane and attracts the associated E protein in the low pH environment of exosomes. This will lead to the surface protein rearrangement observed during maturation.

The localization of the Golgin GCC185 is independent of Rab6A/A' and Arl1.

Mammalian golgins of the trans-Golgi network (TGN) are small G protein effectors that are required for membrane transport and contain a Golgi targeting C-terminal GRIP domain. The localization of two TGN golgins, p230/golgin-245 and golgin-97, is mediated by the small GTPase Arl1, whereas recruitment of the TGN golgin GCC185 is controversial. Recently, GCC185 was proposed to localize to the Golgi by the co-operation of two small GTPases, Rab6A/A' and Arl1 (Burguete et al., 2008), a model based predominantly on in vitro interactions. Here we demonstrate that Golgi recruitment of endogenous GCC185 does not involve Rab6A/A' and Arl1. We find minimal colocalization between Rab6A/A' and endogenous GCC185 on Golgi membranes and failed to detect an interaction between Rab6A/A' and C-terminal domains of GCC185 by yeast two-hybrid analyses. Moreover, depletion of both Rab6A/A' and Arl1 also had no effect on the localization of endogenous GCC185 or the isolated GRIP domain of GCC185.