Potyviral helper component-proteinase (HC-Pro) is a multifunctional protein involved in aphid transmission, long-distance movement, polyprotein processing, genome amplification and symptom expression. It has been proposed that the active form of HC-Pro is a dimer and that coat protein (CP)-HC-Pro interaction is required for aphid transmission. To test these proposed interactions between CP and HC-Pro of potato A potyvirus (PVA), the yeast two-hybrid system was used. HC-Pro was shown to interact with itself in vivo in yeast cells, as did CP. Taken together with previous observations, we conclude that the functional HC-Pro is a homodimer. Deletion analysis showed that a 24 aa domain in the N-terminal half and the C-terminal proteinase part of HC-Pro were required for the interaction between HC-Pro molecules. No interactions were found between HC-Pro and CP using the genes of aphid-transmissible as well as aphid non-transmissible strains of PVA.