OLIGOMERIC REARRANGEMENT OF TICK-BORNE ENCEPHALITIS-VIRUS ENVELOPE PROTEINS INDUCED BY AN ACIDIC PH

The flavivirus envelope protein E undergoes irreversible conformationa l changes at a mildly acidic pH which are believed to be necessary for membrane fusion in endosomes. In this study we used a combination of chemical cross-linking and sedimentation analysis to show that the env elope proteins of the flavivirus tick-borne encephalitis virus also ch ange their oligomeric structure when exposed to a mildly acidic enviro nment. Under neutral or slightly alkaline conditions, protein E on the surface of native virions exists as a homodimer which can be isolated by solubilization with the nonionic detergent Triton X-100. Solubiliz ation with the same detergent after pretreatment at an acidic pH, howe ver, yielded homotrimers rather than homodimers, suggesting that expos ure to an acidic pH had induced a simultaneous weakening of dimeric co ntacts and a strengthening of trimeric ones. The pH threshold for the dimer-to-trimer transition was found to be 6.5. Because the pH depende nce of this transition parallels that of previously observed changes i n the conformation and hydrophobicity of protein E and that of virus-i nduced membrane fusion, it appears likely that the mechanism of fusion with endosomal membranes involves a specific rearrangement of the pro teins in the viral envelope. Immature virions in which protein E is as sociated with the uncleaved precursor (prM) of the membrane protein M did not undergo a low-ps-induced rearrangement. This is consistent wit h a protective role of protein prM for protein E during intracellular transport of immature virions through acidic compartments of the trans -Golgi network.