THE INTERACTIONS OF THE FLAVIVIRUS ENVELOPE PROTEINS - IMPLICATIONS FOR VIRUS ENTRY AND RELEASE

Viral membrane proteins play an important role in the assembly and dis assembly of enveloped viruses. Oligomerization and proteolytic cleavag e events are involved in controlling the functions of these proteins d uring virus entry and release. Using tick-borne encephalitis virus as a model we have studied the role of the flavivirus envelope proteins E and prM/M in these processes. Experiments with acidotropic agents pro vide evidence that the virus is taken up by receptor-mediated endocyto sis and that the acidic pH in endosomes plays an important role for vi rus entry. The envelope glycoprotein E undergoes irreversible conforma tional changes at acidic pH, as indicated by the loss of several monoc lonal antibody-defined epitopes, which coincide with the viral fusion activity in vitro. Sedimentation analysis reveals that these conformat ional changes lead to aggregation of virus particles, apparently by th e exposure of hydrophobic sequence elements. None of these features ar e exhibited by immature virions containing E and prM rather than E and M. Detergent solubilization, sedimentation, and crosslinking experime nts provide evidence that prM forms a complex with protein E which pre vents the conformational changes necessary for fusion activity. The fu nctional role of prM before its endoproteolytic cleavage by a cellular protease thus seems to be the protection of protein E from acid-inact ivation during its passage through acidic trans Golgi vesicles in the course of virus release.