Improved technology for reconstructing cryo-electron microscopy (cryo-EM) images has now made it possible to determine secondary structural features of membrane proteins in enveloped viruses. The structure of mature dengue virus particles was determined to a resolution of 9.5 Å by cryo-EM and image reconstruction techniques, establishing the secondary structural disposition of the 180 envelope (E) and 180 membrane (M) proteins in the lipid envelope. The α-helical 'stem' regions of the E molecules, as well as part of the N-terminal section of the M proteins, are buried in the outer leaflet of the viral membrane. The 'anchor' regions of E and the M proteins each form antiparallel E-E and M-M transmembrane α-helices, leaving their C termini on the exterior of the viral membrane, consistent with the predicted topology of the unprocessed polyprotein. This is one of only a few determinations of the disposition of transmembrane proteins in situ and shows that the nucleocapsid core and envelope proteins do not have a direct interaction in the mature virus.
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We thank E. Strauss for helpful discussions and R. Ashmore, C.R. Xiao, Y. Ji and D. Marinescu for the use of their various computer programs that were essential for calculating the image reconstruction. We are grateful for an equipment grant from the Keck Foundation. This work was supported by a US National Institutes of Health (NIH) Program Project grant to M.G.R., R.J.K. and T.S.B., by NIH grants to T.S.B. and J.H.S. and by a US National Science Foundation grant to T.S.B.