Required for aphid transmission and also has proteolytic activity. Only cleaves a Gly-Gly dipeptide at its own C-terminus. Interacts with virions and aphid stylets. Acts as a suppressor of RNA-mediated gene silencing, also known as post-transcriptional gene silencing (PTGS), a mechanism of plant viral defense that limits the accumulation of viral RNAs. May have RNA-binding activity. Allows efficient cell to cell propagation, by bypassing the host cell wall barrier. Transports viral genome to neighboring plant cells directly through plasmosdesmata, without any budding (By similarity). Hydrolyzes a Gly-|-Gly bond at its own C-terminus, commonly in the sequence -Tyr-Xaa-Val-Gly-|-Gly, in the processing of the potyviral polyprotein. Interacts (via PIPO domain) with host PCaP1 protein; this interaction may help to anchor the movement complex to the plasma membrane from which the complex could move to the plasmodesmata. The N-terminus of helper component proteinase is involved in interaction with stylets. The central part is involved in interaction with virions and the C-terminus is involved in cell-to cell movement of the virus (By similarity). Potyviral RNA is expressed as two polyproteins which undergo post-translational proteolytic processing. Genome polyprotein is processed by NIa-pro, P1 and HC-pro proteinases resulting in the production of at least ten individual proteins. P3N-PIPO is cleaved by P1 and HC-pro proteinases resulting in the production of three individual proteins. The P1 proteinase and the HC-pro cleave only their respective C-termini autocatalytically (By similarity). Produced by -1 ribosomal frameshifting in P3 ORF. Belongs to the potyviridae P3N-PIPO polyprotein family.