A single amino acid change in the coat protein of Maize streak virus abolishes systemic infection, but not interaction with viral DNA or movement protein

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Abstract
Summary: Functional coat protein (CP) is important for host plant infection by monopartite geminiviruses. We identified a proline‐cysteine‐lysine (PCK) motif at amino acids 180–182 of the maize streak virus (MSV) CP that is conserved in most of the cereal–infecting Mastreviruses. Substitution of the lysine (K) with a valine (V) in the CP of MSV to produce mutant MSVCP182V abolished systemic infection in maize plants, although the mutant replicated around the inoculation site and, unlike other MSV CP mutants, enabled single‐stranded (ss) DNA accumulation in suspension cells. The stability of the mutant protein, CP182V, in infected cells was confirmed by immunoblotting, but virions could not be detected. Like the wild‐type (wt) CP, CP182V localized to the nucleus when expressed in insect and tobacco cells, and the Escherichia coli‐expressed protein bound both ss and double‐stranded DNA and interacted with movement protein in vitro. Taken together, these data suggest that mutation of amino acid 182 affects virion formation of MSV, either by affecting encapsidation per se or by affecting particle stability, and that virions are necessary for the long‐distance movement of MSV in maize plants.