Post-transcriptional gene silencing (PTGS) represents a major antiviral defense mechanism in plants. It is triggered by the presence of double stranded RNAs that are cleaved into short interfering RNAs (siRNAs) of 21 to 26 nucleotides. The incorporation of siRNA into the RNA induced silencing complex (RISC) guides sequence specific degradation of viral RNAs presenting homologies to the siRNAs. Most plant viruses evolved strategies to counteract this defense mechanism. Poleroviruses, a group of plant viruses belonging to the Luteoviridae family, encode a strong PTGS suppressor, P0, in their positive sense ssRNA genome. This 29 kDa protein has been shown to possess an F-box domain through which it interacts with plant SKP-like proteins, components of E3 ubiquitin ligases, suggesting that it could interfere with the ubiquitin proteasome pathway (1). Recently, it has been shown that P0 targets ARGONAUTE1 (AGO1) for degradation (2, 3). Since AGO1 is a key member of RISC bearing the RNA slicer activity, its degradation may lead to the inhibition of silencing mechanism. To further unravel P0’s mode of action, we carried out subcellular localization tests in Nicotiana benthamiana leaves which transiently expressed P0. We used P0 proteins of two different poleroviruses, Turnip Yellows Virus (TuYV, formerly Beet Western Yellows Virus-FL1) and Cucurbit Aphid-borne Yellows Virus (CABYV), and observed a nuclear and cytoplasmic localization for both of them, with the difference that the CABYV P0 also targeted the nucleolus. This observation suggests that there might exist another yet unknown function of the nucleolar protein. By analysing the properties of several P0 mutants, we could draw the conclusion that the nucleolar localisation was not correlated to the silencing suppression mechanism. Viral proteins often have several functions. In order to investigate P0's potential roles in the viral infection cycle we performed a yeast two-hybrid screen of an Arabidopsis thaliana phloem cDNA library and we identified small heat shock proteins (sHSP) as cellular partners for P0. sHSPs are involved in cellular response to various forms of stress. The role of this interaction in viral infection remains to be deciphered. The results that we’ll present support a model in which P0 could act as nucleo-cytoplasmic shuttling protein and open new investigation areas implying other functions for our silencing suppressor protein in the viral lifecycle.