The NS1 protein of tick-borne encephalitis virus forms multimeric species upon secretion from the host cell

Flaviviruses elicit a humoral immune response to two virus-encoded, membrane-associated glycoproteins. One is the major virion surface envelope protein (E), which is recognized by antibody, whereas the other is a secreted, heavily glycosylated non-structural protein (NS1). Inoculation with either protein can give rise to a protective immune response, as can the passive transfer of E and NS1 monospecific monoclonal antibodies. Experiments reported here demonstrate that the secreted form of NS1, whether from cells infected with tick-borne encephalitis virus (TBEV) or from cells infected with a defective recombinant adenovirus containing the NS1 gene, occurs chiefly as a pentamer or hexamer and occasionally as a decamer or dodecamer. Intracellular forms of this protein however occur only as dimers. The higher M(r) forms secreted from the cell are exquisitely sensitive to detergent, suggesting they are held together by hydrophobic bonds. Both intracellular and extracellular forms of the dimer can be dissociated by heat, but at different temperatures. Unlike similar proteins from mosquito-borne viruses. NS1 from TBEV-infected cells cannot be dissociated at ambient temperatures by extremes of pH. Studies on the antigenic structure of this protein show it to have several highly conserved epitopes, confirming similar earlier conclusions from amino acid sequence analyses.