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34 p. ; In the Potyvirus genus, the P1 protein is the first N-terminal product processed from the viral polyprotein, followed by HCPro. In silencing suppression patch assays we found that Potato virus Y (PVY) HCPro expressed from a P1-HCPro sequence increased the accumulation of a reporter gene, whereas protein expressed from an HCPro sequence did not, even with P1 supplied in trans. This enhancing effect of P1 had been noticed in other potyviruses, but remained unexplained. We analyzed the accumulation of PVY HCPro in infiltrated tissue and found it higher when expressed from P1-HCPro than from HCPro sequences. Co-expression of heterologous suppressors increased the steady-state level of mRNA expressed from the HCPro sequence, but not that of protein. This suggested that in the absence of P1 upstream, either HCPro acquired a conformation that affected negatively its activity or stability, or that its translation was reduced. To test these options, we purified HCPro expressed in the presence or absence of upstream P1, and found no differences in purification patterns and final soluble states. By contrast, alteration of the Kozak context in the HCPro mRNA sequence to favor translation increased partially both suppressor accumulation and activity. Furthermore, it was as active as protein expressed from P1-HCPro sequences. Thus, a direct role for P1 on HCPro suppressor activity or stability by influencing its conformation during translation can be excluded. However, P1 could still have an indirect effect favoring HCPro accumulation. Our data highlight the relevance of cis-acting translational elements in the heterologous expression of HCPro. ; This work was supported by grant AGL2008-03482 from the Spanish Ministry of Innovation and Science and by a joint grant between the Spanish Ministry of Economy and Competitivity (AC1/2009-0855) to T. Canto and the Department of Science and Technology of the Government of India (DST/INT/SPAIN/P-9/2009) to S. Praveen. N. Sahana was funded by a grant from the Indian Agricultural Research Institute, New Delhi ; Peer reviewed