A novel cleavage site within the potato leafroll virus P1 polyprotein

To study the proteolytic processing of the potato leafroll virus replicase proteins, the multidomain P1 protein with a c-myc epitope tag attached at the N terminus was expressed in insect cells by using the baculovirus system. Western blotting showed that P1 was cleaved at a site upstream of the serine protease domain, in addition to the cleavage site downstream of the protease domain. Mutational analysis showed that the serine protease domain within P1 was responsible for this cleavage. To characterize this novel cleavage site further, a portion of the P1 protein comprising the protease domain and the two cleavage sites was expressed in Escherichia coli. A similar cleavage event was observed in bacteria and was abolished when the P1 protease was inactivated by mutation. Peptide-sequencing studies indicated that this cleavage occurred at a Glu/Arg junction, separating the N-terminal 204 residues from the serine protease domain of P1.