The helper component-proteinase (HC-Pro) of potyviruses is a multifunctional protein involved in aphid transmission, polyprotein processing, cell-to-cell and long-distance movement, genome amplification and symptom expression. The HC-Pros of several potyviruses interact with themselves but the key domains responsible for self-interaction are apparently not conserved. In our experiments, yeast two-hybrid assays and bimolecular fluorescence complementation showed that Turnip mosaic virus (TuMV) HC-Pro interacted with itself in yeast cells, plant cells and insect cells. It was also shown that the central and C-terminal regions of the HC-Pro participated in these self-interactions. Fluorescence microscopy showed that TuMV HC-Pro was present in the cytoplasm and formed aggregates along the ER.