Precursor from which the VPg molecule is probably released at the onset of the RNA synthesis. Essential for virus replication. Participates, together with the proteins P0 and P7, in the inhibition of the induction of aphid-induced host phytohormones (PubMed:31758809). This could play a role in the attraction to the infected plants by aphids (PubMed:31758809). The isoform P1 is produced by conventional translation, whereas the isoform RNA-directed RNA polymerase is produced by -1 ribosomal frameshifting between codons 487 and 488. The C-terminus part of protein P1 and VPg/P1-C25 displays RNA-binding properties. Specific enzymatic cleavages in vivo yield mature proteins. The protease probably cleaves itself and releases the VPg protein. The VPg protein is probably further cleaved in its C-terminus. Belongs to the peptidase S39B family.