Together with movement protein P17, plays an essential role in virus long distance movement. Homodimer (PubMed:30373157). Heterodimer with movement protein P17 (PubMed:30373157). P3a directs P17 to the mitochondrial outer membrane while P17 regulates the localization of the P3a-P17 heterodimer to plastids. The C-terminus regulates the targeting to host mitochondria. Belongs to the polerovirus movement protein P3a family. This sequence initiates at a non-canonical ATA isoleucine codon.