Tyrosine 78 of premembrane protein is essential for assembly of West Nile virus

Flavivirus premembrane (prM) protein plays an important role in conformational folding of the envelope (E) protein and protects it against premature fusion in acidic vesicles of the Golgi network. Currently, molecular determinants on the prM protein ectodomain which mediate critical steps during the flavivirus assembly process are poorly characterized. In this study, bioinformatics analysis and alanine scanning mutagenesis showed that the amino acid triplet valine 76, tyrosine 78 and glycine 79 is absolutely conserved among flavivirus prM ectodomains. Triple mutations engineered at these residues in prM ectodomain of West Nile virus (WNV) completely abrogated virus infectivity. Site-directed mutagenesis of prM protein revealed that tyrosine 78 of the amino acid triplet was required for virus infectivity and secretion. The mutation did not affect folding, post-translational modifications and trafficking of the prM and E proteins. Ultrastructural studies using transmission electron microscopy confirmed that virus particle formation was blocked by tyrosine 78 mutation. Specificity of assembly defect conferred by tyrosine 78 mutation was demonstrated by positive and negative trans complementation studies. Collectively, these results defined tyrosine 78 as a novel critical determinant present on prM protein ectodomain that is required for flavivirus assembly. Molecular dissection of prM protein function provides the crucial knowledge much needed in the elucidation of flavivirus particle formation.