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J. Virol., 12 1997, 9313-9322, Vol 71, No. 12
AL Wassenaar, WJ Spaan, AE Gorbalenya and EJ Snijder
The C-terminal half of the replicase ORF1a polyprotein of the arterivirus
equine arteritis virus is processed by a chymotrypsinlike serine protease
(SP) (E. J. Snijder et al., J. Biol. Chem. 271:4864- 4871, 1996) located in
nonstructural protein 4 (nsp4). Three probable SP cleavage sites had
previously been identified in the ORF1a protein. Their proteolysis
explained the main processing products generated from the C-terminal part
of the ORF1a protein in infected cells (E. J. Snijder et al., J. Virol.
68:5755-5764, 1994). By using sequence comparison, ORF1a expression
systems, and site-directed mutagenesis, we have now identified two
additional SP cleavage sites: Glu-1430 / Gly and Glu-1452 / Ser. This means
that the ORF1a protein can be cleaved into eight processing end products:
nsp1 to nsp8. By microsequence analysis of the nsp5 and nsp7 N termini, we
have now formally confirmed the specificity of the SP for Glu / (Gly/Ser)
substrates. Importantly, our studies revealed that the C-terminal half of
the ORF1a protein (nsp3-8) can be processed by the SP following two
alternative pathways, which appear to be mutually exclusive. In the
majority of the nsp3-8 precursors the SP cleaves the nsp4/5 site, yielding
nsp3-4 and nsp5-8. Subsequently, the latter product is cleaved at the
nsp7/8 site only, whereas the newly identified nsp5/6 and nsp6/7 sites
appear to be inaccessible to the protease. In the alternative proteolytic
cascade, which is used at a low but significant level in infected cells, it
is the nsp4/5 site which remains uncleaved, while the nsp5/6 and nsp6/7
sites are processed to yield a set of previously unnoticed processing
products. Coexpression studies revealed that nsp3-8 has to interact with
cleaved nsp2 to allow processing of the nsp4/5 junction, the first step of
the major processing pathway. When the nsp2 cofactor is absent, the nsp4/5
site cannot be processed and nsp3-8 is processed following the alternative,
minor pathway.
Copyright © 1997, American Society for Microbiology
Alternative proteolytic processing of the arterivirus replicase ORF1a polyprotein: evidence that NSP2 acts as a cofactor for the NSP4 serine protease
Department of Virology, Leiden University Medical Center, The Netherlands.
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