Effects of site-directed mutagenesis on the presumed catalytic triad and substrate-binding pocket of grapevine fanleaf nepovirus 24-kDa proteinase.

Grapevine fanleaf nepovirus (GFLV) has a bipartite plus-sense RNA genome. Its structural and functional proteins originate from polyprotein maturation by at least one virus-encoded proteinase. Here we describe the cloning of the 24-kDa proteinase cistron located between the virus-linked protein (VPg) and the RNA-dependent RNA polymerase cistron in GFLV RNA1 (nucleotides 3966 to 4622). Proteinase expressed from this clone is able to cleave GFLV polyprotein P2 in order to produce the coat protein and a 66-kDa protein which is further processed to the 38-kDa presumed movement protein. The GFLV 24-kDa proteinase sequence contains sequence similarities with other nepovirus and comovirus proteinases, particularly at the level of the conserved domains corresponding to the hypothetical catalytic triad and to the substrate-binding pocket (amino acids 192 to 200). Site-directed mutagenesis of residues His43, Glu87, and Leu197 abolished proteinase activity. Inactivation of the enzyme is also observed if the catalytic residue Cys179 was substituted by isoleucine, but replacement by a serine at the same position produced a mutant with an activity identical to that of native proteinase. All our data show that GFLV cysteine proteinase presents structure similarities to the proteinases of cowpea mosaic virus and potyviruses but is most closely related to trypsin.