The cysteine-histidine-rich region of the movement protein of Cucumber mosaic virus contributes to plasmodesmal targeting, zinc binding and pathogenesis

Viral movement proteins (MPs) are central to the establishment of viral pathogenesis, and yet relatively little is understood about the structural and functional aspects of MPs or about the host factors on which they depend. Through chemical mutagenesis of transgenic Arabidopsis expressing Cucumber mosaic virus (CMV) MP fused with the green fluorescent protein, we have studied the function of a central region of the MP, defined by a number of conserved cysteine and histidine residues (Cys-His-rich region), which potentially functions as a zinc-binding domain. Transient expression of mutant MPs identified through an in planta screen for altered MP function or constructed with altered putative zinc ligands through site-directed mutagenesis showed that mutations in the Cys-His-rich region affected localization to and trafficking through plasmodesmata. In vitro zinc-binding analysis revealed that wild type (wt) CMV MP had the ability to bind zinc and that movement-defective mutants bound zinc with less affinity than wt MP. Furthermore, a correlation between the association of the MP with plasmodesmata and virus pathogenesis was shown. We discuss roles of the Cys-His region in biochemical and biological functions of the MP during virus movement.