Self-interaction of ORF II protein through the leucine zipper is essential for Soybean chlorotic mottle virus infectivity

The ORF II protein (PII) of Soybean chlorotic mottle virus (SbCMV) is essential for the virus life cycle. We investigated the interactions of SbCMV PII with itself and with other essential virus proteins using a Gal4-based yeast two-hybrid system. PII interacted only with itself and not with any other virus proteins. The PII-PII interaction was confirmed by a Sos-based yeast two-hybrid system and a far-western analysis. Deletion mutagenesis mapped the self-interacting domain to the C-terminal 48 amino acids (amino acids 154-201), which contain two putative leucine zipper motifs. Introduction of amino acid substitutions to leucine/isoleucine in zipper sequences prevented the PII-PII interaction and abolished the infectivity of SbCMV. These results revealed that the self-interaction of PII through a leucine zipper is necessary for virus infection.