Molecular Mechanism Of Coat Protein Regulating The Cell-to-cell Movement Of Tobacco Vein Banding Mosaic Virus

The genus Potyvirus is the largest one of plant RNA viruses.Many potyviruses cause severe economic losses in crop production.Cell-to-cell movement is a critical step for plant viruses to establish systemic infection.Coat protein(CP)are known to be involved in potyviral cell-to-cell movement.However,roles of the CP in viral cell-to-cell movement remain largely unknown.In this study,based on tobacco vein banding mosaic virus(TVBMV),we investigated the role of CP in virus cell-to-cell movement and found that CP regulated TVBMV cell-to-cell movement in three dimensions,i.e.,CP structure,CP-RNA binding activity,and RNA structure.The main results are shown as below.1.The conserved aromatic residue W¹²² in the CP core region is involved in potyviral cell-to-cell movement by maintaining CP stability.Substitutions of alanine,glutamic acid,or lysine for the conserved residue tryptophan at position 122(W¹²²)in TVBMV CP abolished virus cell-to-cell movement in Nicotiana benthamiana plants.In agroinfiltrated N.benthamiana leaf patches,both the CP and RNA accumulation levels of three W¹²² mutant viruses were significantly reduced compared with those of wild-type TVBMV,and CP accumulated to a low level similar to that of a replication-deficient mutant.The results of polyprotein transient expression experiments indicated that CP instability was responsible for the significantly low CP accumulation levels of the three W¹²² mutant viruses.The substitution of W¹²² did not affect CP plasmodesmata localization or virus particle formation;however,the substitution significantly reduced the number of virus particles.The wild-type TVBMV CP could complement the reduced replication and abolished cell-to-cell movement of the mutant viruses.When the codon for W¹²² was mutated to that for a different aromatic residue,phenylalanine or tyrosine,the resultant mutant viruses moved systemically and accumulated up to 80%of the wild-type TVBMV level.Similar results were obtained for the corresponding amino acids of W¹²² in the watermelon mosaic virus and potato virus Y CPs.2.Mutiplex conserved aromatic residues in the CP core region is involved in potyviral cell-to-cell movement by forming?stackings to maintain CP stability.Analysis of CP three-dimensional model of TVBMV showed that 16 aromatic residues might form?-stackings.Transient expression experiments showed that deletion of any one of these aromatic residues reduced CP accumulation.Infectivity assays showed that deletion of any one of these 16aromatic residues in the TVBMV infectious clone abolished viral cell-to-cell movement and reduced viral replication.Substitution of aromatic residues Y¹⁰⁵ and Y¹⁴⁷ with non-aromatic residues alanine or glycine,respectively,reduced CP accumulation and abolished the ability of TVBMV to move intercellularly,while substitution of these two residues with aromatic residues phenylalanine or tryptophan,respectively,had no or little effect on CP accumulation,TVBMV systemic movement and replication.Similar results were obtained from the watermelon mosaic virus(genus Potyvirus).3.RNA-binding pocket of CP core region is involved in potyviral cell-to-cell movement by binding viral RNA.In this study,we predicted the three-dimensional model of TVBMV-encoded CP and found nine residues presumably located in the CP RNA-binding pocket.Substitutions of the two basic residues at positions of 192 and 225(R¹⁹² and K²²⁵)either with alanine,cysteine,or glutamic acid abolished TVBMV cell-to-cell and systemic movement in Nicotiana benthamiana plants.These substitutions also reduced the replication of the mutant viruses.Results from the electrophoretic mobility shift assay showed that the RNA binding activity of mutant CPs derived from R¹⁹² or K²²⁵ substitutions was significantly lower than that of wild-type CP.Analysis of purified virus particles showed that mutant viruses with R¹⁹² or K²²⁵substitutions formed RNA-free virus-like particles.Mutations of R¹⁹² and K²²⁵ did not change the CP plasmodesmata localization.The wild-type TVBMV CP could rescue the deficient cell-to-cell movement of mutant viruses.Moreover,deletion of any of the other seven residues also abolished TVBMV cell-to-cell movement and reduced the CP-RNA binding activity.The corresponding nine residues in watermelon mosaic virus CP were also found to play essential roles in viral cell-to-cell movement.4.The stability of a predicted stem-loop in CP-coding sequence is involved in TVBMV cell-to-cell movement by regulating virus replication.We introduced synonymous substitutions to the codon C₅₇₄G₅₇₅C₅₇₆ coding conserved residue arginine at position 192(R¹⁹²)of TVBMV CP.Substitution of the codon C₅₇₄G₅₇₅C₅₇₆ to A₅₇₄G₅₇₅A₅₇₆ or A₅₇₄G₅₇₅G₅₇₆,but not C₅₇₄G₅₇₅A₅₇₆,C₅₇₄G₅₇₅T₅₇₆,or C₅₇₄G₅₇₅G₅₇₆,reduced the replication,cell-to-cell movement,and accumulation of TVBMV in Nicotiana benthamiana plants,suggesting that C₅₇₄ was critical for replication of TVBMV.Nucleotides at 531 to 576 of the TVBMV CP-coding sequence were predicted to form a stem-loop structure.Synonymous substitutions of R¹⁷⁸-codon C₅₃₂G₅₃₃C₅₃₄to A532G533A534 and A532G533G534,but not C532G533A534,C532G533T534,or C532G533G534,reduced the replication levels,cell-to-cell,and systemic movement of TVBMV,suggesting that C₅₃₂was critical for TVBMV replication.Four consecutive C-G base pairs(C₅₇₆-G₅₃₁,C₅₃₂-G₅₇₅,C₅₇₄-G₅₃₃,and C₅₃₄-G₅₇₃)were located at the bottom of the stem.Synonymous substitutions disrupting base pairs C₅₇₆-G₅₃₁ and C₅₃₄-G₅₇₃ did not affect viral accumulation.After three series passage inoculation,the accumulation of spontaneous mutant viruses derived from the replication and movement reduced mutants was restored and codons have two consecutive C₅₇₆-G₅₃₁ and C₅₃₄-G₅₇₃base pairs.RNA secondary structure analysis showed that breaking either one of two consecutive base pairs resulted in changes in the stem-loop structure.In summary,TVBMV cell-to-cell movement was jointly regulated by the stability of CP three-dimensional structure,CP-RNA binding activity,and stability of a stem-loop structure.