Cross-linking Measurements of the <i>Potato leafroll virus</i> Reveal Protein Interaction Topologies Required for Virion Stability, Aphid Transmission, and Virus–Plant Interactions

Protein interactions are critical determinants of insect transmission for viruses in the family <i>Luteoviridae</i>. Two luteovirid structural proteins, the capsid protein (CP) and the readthrough protein (RTP), contain multiple functional domains that regulate virus transmission. There is no structural information available for these economically important viruses. We used Protein Interaction Reporter (PIR) technology, a strategy that uses chemical cross-linking and high resolution mass spectrometry, to discover topological features of the <i>Potato leafroll virus</i> (PLRV) CP and RTP that are required for the diverse biological functions of PLRV virions. Four cross-linked sites were repeatedly detected, one linking CP monomers, two within the RTP, and one linking the RTP and CP. Virus mutants with triple amino acid deletions immediately adjacent to or encompassing the cross-linked sites were defective in virion stability, RTP incorporation into the capsid, and aphid transmission. Plants infected with a new, infectious PLRV mutant lacking 26 amino acids encompassing a cross-linked site in the RTP exhibited a delay in the appearance of systemic infection symptoms. PIR technology provided the first structural insights into luteoviruses which are crucially lacking and are involved in vector–virus and plant–virus interactions. These are the first cross-linking measurements on any infectious, insect-transmitted virus