Amino Acids at Positions 156 and 332 in the E Protein of the West Nile Virus Subtype Kunjin Virus Classical Strain OR393 Are Involved in Plaque Size, Growth, and Pathogenicity in Mice.

The West Nile virus (WNV) subtype Kunjin virus (WNV) is endemic to Australia. Here, we characterized the classical WNV strain, OR393. The original OR393 strain contained two types of viruses: small plaque-forming virus (SP) and large plaque-forming virus (LP). The amino acid residues at positions 156 and 332 in the E protein (E and E) of SP were Ser and Lys (E), respectively, whereas those in LP were Phe and Thr (E). SP grew slightly faster than LP in vitro. The E protein of SP was N-glycosylated, whereas that of LP was not. Analysis using two recombinant single-mutant LP viruses, rKUNV-LP-E and rKUNV-LP-E, indicated that E enlarged plaques formed by LP, but E potently reduced them, regardless of the amino acid at E. rKUNV-LP-E showed significantly higher neuroinvasive ability than LP, SP, and rKUNV-LP-E. Our results indicate that the low-pathogenic classical WNV can easily change its pathogenicity through only a few amino acid substitutions in the E protein. It was also found that Phe at E of the rKUNV-LP-E was easily changed to Ser during replication in vitro and in vivo, suggesting that E is advantageous for the propagation of WNV in mammalian cells.