Structural changes and functional control of the tick-borne encephalitis virus glycoprotein E by the heterodimeric...

We have used tick-borne encephalitis virus to study the involvement of acidic compartments during the entry and release phases of flavivirus infection and to elucidate the role of protein prM in immature virions. Elevation of the pH in acidic intracellular compartments by either bafilyomycin A1, a specific inhibitor of the vacuolar type H(+)-ATPase or by NH4Cl had a strong inhibitory effect during virus penetration and also prevented the cleavage of prM when added in the late phase of the viral life cycle. In the latter case the release of virus particles was not impaired. These immature (prM-containing) virions exhibited a 20- to 50-fold lower specific infectivity and HA activity than mature virions and in contrast to these did not undergo low pH-triggered aggregation. The presence of prM also affected the binding of monoclonal antibodies to protein E, especially at sites which have been shown to undergo acid pH-induced conformational changes in mature virions. Crosslinking, solubilization, and sedimentation analyses revealed the existence of prM-E heterooligomeric complexes, suggesting that the function of prM is to protect protein E from undergoing the irreversible conformational changes in acidic compartments of the secretory pathway that are necessary for triggering fusion activity in the endosome during virus entry.