Mutation of three cysteine residues in Tomato yellow leaf curl virus-China C2 protein causes dysfunction in pathogenesis and posttranscriptional gene-silencing suppression

The nuclear localized C2 protein of the monopartite begomovirus Tomato yellow leaf curl virus-China (TYLCV-C) contributes to viral pathogenicity. Here, we have investigated TYLCV-C C2 protein domains that play a role in the phenotype. Alignment of the C2 protein with 67 homologues from monopartite and bipartite begomoviruses revealed that a putative zinc-ringer Motif C-36-X1-C-38-X7-C-46-X6-H-53-X4-H58C59 and four potential phosphorylation sites (T-52, S-61, Y-68, and S-74) are highly conserved. When expressed from a Potato virus X (PVX) vector, TYLCV-C C2 protein mutants C2-T52M, C2-H58S, C2-C59S, C2-S61R, and C2-S74D, like the wild-type C2 protein, induced local necrotic ringspots and systemic necrosis in Nicotiana benthamiana plants. Mutants C2-H53P and C2-Y68D produced irregular necrotic lesions on inoculated leaves that were distinct from the wild-type phenotype. In contrast, mutants C2-C36R, C2-C38N, and C2-C46I induced chlorosis and mosaic symptoms rather than necrosis. We demonstrate that TYLCV-C C2, like its counterpart in the bipartite begomovirus African cassava mosaic virus, mediates suppression of posttranscriptional gene silencing (PTGS). Moreover, the individual mutations C36R, C38N, and C46I abolished the ability of C2 protein to suppress PTGS. These results suggest that the three cysteine residues within the putative zinc-ringer motif are essential for C2 protein to induce necrosis and to act as a suppressor of PTGS.