A domain of the readthrough protein of barley yellow dwarf virus (NY-RPV isolate) is essential for aphid transmission

Luteoviruses are obligately transmitted by aphids and contain two capsid proteins, the coat protein (CP) coded for by open reading frame (ORF) 3, and the readthrough protein (RTP), produced by readthrough of the amber termination codon of ORF 3 into the contiguous ORF 5. Previous studies have suggested that it is the RTP that determines transmissibility and vector specificity. To investigate which capsid protein or protein part contains determinants for the transmission of the NY-RPV isolate of barley yellow dwarf virus (BYDV) by its vectorRhopalosiphum padi, we produced three fusion proteins by expressing NY-RPV cDNA inE. coli. These respectively represented the CP alone (P3), a region of the RTP immediately following the amber termination codon (P5a), and the remainder of the RTP (P5b). Polyclonal antisera raised against the P3, P5a and P5b proteins each gave distinctive reactions against purified NY-RPV on Western blots. Also, in ELISA tests, antisera raised against all three fusion proteins detected purified intact virions. When mixed with purified virions and fed toR. padi through Parafilm membranes, immunoglobulins (Igs) from antisera raised against P3 and P5b had no effect on transmission, whereas Ig from antiserum against P5a interfered with transmission. P5a antiserum Ig had no effect on the transmission of the P-PAV isolate of BYDV byR. padi. The results demonstrate that while neither the CP itself nor the terminal region of the RTP are key determinants for transmission, a specific domain in the central part of the RTP is an important determinant in the transmission of NY-RPV byR. padi, though apparently not of P-PAV.