The multi-domain potato leafroll virus replicase protein P1 was expressed in insect cells from the polyhedrin promoter of Autographa californica nucleopolyhedrovirus. Using antisera raised against P1, it was shown that P1 was cleaved near the VPg in insect cells in a manner similar to that in plant cells, to produce a approximately 27 kDa C-terminal fragment. Furthermore, it was shown that the proposed serine proteinase-like domain within P1 is responsible for this processing and that this can occur in a trans (intermolecular) reaction. Four conserved residues within the serine proteinase domain that are essential for catalysis have been identified, consistent with the proposal that this domain comprises a serine proteinase.