Wheat Streak Mosaic Virus Infects Systemically Despite Extensive Coat Protein Deletions: 2 Identification of Virion Assembly and Cell-to-Cell Movement Determinants

23 24 Viral coat proteins function in virion assembly and virus biology in a tightly coordinated manner 25 with a role for virtually every amino acid. In this study, we demonstrated that the coat protein 26 (CP) of Wheat streak mosaic virus (WSMV) (genus Tritimovirus; family Potyviridae) is 27 unusually tolerant of extensive deletions with continued virion assembly and/or systemic 28 infection. A series of deletion and point mutations were created in the CP cistron of wild-type 29 and/or GFP-tagged WSMV and examined the effects of these mutations on cell-to-cell and 30 systemic transport and virion assembly of WSMV. Mutants with overlapping deletions 3132 terminal 14 or 17 amino acids systemically infected wheat with different efficiencies. However, 33 mutation of conserved amino acids in the core domain, which may be involved in a salt bridge, 34 abolished virion assembly and cell-to-cell movement. The N-terminal amino acids 6 to 27 and 85 35 to 100 are required for efficient virion assembly and cell-to-cell movement, while the C-terminal 36 65 amino acids are dispensable for virion assembly but required for cell-to-cell movement, 37 suggesting that the C-terminus of CP functions as a dedicated cell-to-cell movement determinant. 38 In contrast, amino acids 36 to 84 are expendable with no obvious effects on systemic infection 39 and virion assembly. In total, 152 amino acids (6 to 27, 36 to 100 and 65 amino acids at the C-40 terminus end) of 349 amino acids of CP are dispensable for systemic infection and/or virion 41 assembly, which is rare for multifunctional viral CPs.