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Autographa californica multiple nucleopolyhedrovirus and Choristoneura fumiferana multiple nucleopolyhedrovirus v-cath genes are expressed as pre-proenzymes

¹ Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON N1G 2W1, Canada
² Laboratory for Molecular Virology, Great Lakes Forestry Centre, Sault Ste Marie, ON P6A 2E5, Canada

Correspondence
Peter J. Krell
pkrell{at}uoguelph.ca

Intracellular processing and trafficking of the baculovirus v-cath expressed cathepsin (V-CATH), which lacks canonical targeting signals, are poorly understood. The cathepsins of Autographa californica multiple nucleopolyhedrovirus (AcMNPV), Choristoneura fumiferana multiple nucleopolyhedrovirus (CfMNPV) and most other alphabaculovirus group I nucleopolyhedroviruses have well-conserved N-termini containing overlapping chymotrypsin-cleavage (Y¹¹) and myristoylation (G¹²) motifs, which are suggestive of proteolytic signal-peptide cleavage to generate proV-CATH and subsequent acylation. To determine proteolytic N-terminal processing of V-CATH, haemagglutinin epitope-coding tags were fused to the 5' and/or 3' ends of AcMNPV and CfMNPV v-cath. Immunoblot analysis suggested that a small N-terminal peptide is cleaved for both viruses, indicating that v-cath is expressed as a pre-proenzyme. The two viral homologues undergo similar proteolytic processing, but have different glycosylation or other post-translational modifications. An AcMNPV V-CATH–DsRED fusion protein co-localized to the endoplasmic reticulum with an HDEL motif-containing green fluorescent protein. Based on these findings, pre-proV-CATH processing and trafficking mechanisms are postulated.

Supplementary methods, references and a table of primer sequences are available with the online version of this paper.