Arch Virol (2001) 146: 1297–1306 Inhibition of potato virus Y NIa activity: preparation of monoclonal antibody directed against PVY NI protein that inhibits cleavage of PVY polyprotein S. Rouis 1 , F. Traincard 4 , R. Gargouri 1 , S. Dartevelle 3 , O. Jeannequin 3 , J. C. Mazié 3 , and H. Ayadi 2,∗ 1 Centre de Biotechnologie de Sfax, Tunisia 2 Laboratoire de Genetique Moleculaire Humaine, Faculté de Médecine, Sfax, Tunisia 3 Laboratoire d’Ingenierie des Anticorps, Institut Pasteur, Paris, France 4 Unité de Régulation Enzymatique des Activités Cellulaires, Institut Pasteur, Paris, France Accepted February 15, 2001 Summary. A partially purified nuclear inclusion (NI) fraction was obtained from tobacco plants infected by potato virus Y (PVY). Four monoclonal antibodies (MAbs) were produced and characterized using this semipurified fraction as anti- gen. Data showed that only one was directed against NIa whereas two were directed against cytoplasmic inclusion (CI) protein and the last one against coat protein (CP). These results were due to the fact that the semipurified NI frac- tion was usually contaminated with CI and CP proteins. When used on in situ immunofluorescence method the anti-NIa MAb showed accumulation of the NIa protein in both nucleus and cytoplasm. In vivo, this MAb was able to detect dif- ferent forms of the NIa protein including precursors and cleavage products. It was also able to inhibit the cleavage of the polyprotein detected in the semipurified NI. Introduction Potato virus Y (PVY) is the type species of the genus Potyvirus in the family Potyviridae with a positive sense single-strand RNA genome of about 10 kb. This RNA is covalently linked to a virally-encoded protein (VPg) at the 5 ′ end and is polyadenylated at the 3 ′ end [3, 4]. The viral RNA encodes a polyprotein of about 350 kDa that is autocatalytically cleaved producing eight structural and nonstructural viral proteins [8, 12]. One of these proteins, the cylindrical inclusion protein (CI), accumulates within the cytoplasm of infected cells inducing the