V2 Protein Enhances the Replication of Genomic DNA of Mulberry Crinkle Leaf Virus.

Mulberry crinkle leaf virus (MCLV), identified in mulberry plants ( L.), is a member of the genus in the family . The functions of the V2 protein encoded by MCLV remain unclear. Here, -mediated infectious clones of a wild-type MCLV vII (MCLV) and two V2 mutant MCLV vIIs, including MCLV (with a mutation of the start codon of the V2 ORF) and MCLV (5'-end partial deletion of the V2 ORF sequence), were constructed to investigate the roles of V2 both in planta and at the cellular level. Although all three constructs (pCA-1.1MCLV, pCA-MCLV, and pCA-MCLV) were able to infect both natural host mulberry plants and experimental tomato plants systematically, the replication of the MCLV and MCLV genomes in these hosts was significantly reduced compared to that of MCLV. Similarly, the accumulation of MCLV and MCLV in protoplasts of plants was significantly lower than that of MCLV either 24 h or 48 h post-transfection. A complementation experiment further confirmed that the decreased accumulation of MCLV in the protoplasts was due to the absence of V2 expression. These results revealed that MCLV-encoded V2 greatly enhances the level of MCLV DNA accumulation and is designated the replication enhancer protein of MCLV.