Precursor from which the VPg molecule is probably released at the onset of the RNA synthesis. Essential for virus replication (By similarity). The C-terminus part of protein P1 and P1-C25 displays RNA-binding properties. Specific enzymatic cleavages in vivo yield mature proteins. The protease probably cleaves itself and releases the VPg protein (By similarity). Produced by conventional translation. Belongs to the peptidase S39B family.