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Trypsin cleavage of the baculovirus occlusion-derived virus attachment protein P74 is prerequisite in per os infection

¹ Great Lakes Forestry Centre, Sault Ste Marie, ON P6A 2E5, Canada
² US Department of Agriculture, Beltsville, MD 20705-2350, USA
³ Department of Molecular and Cellular Biology, University of Guelph, ON N1G 2W1, Canada

Correspondence
Jeffrey M. Slack
jslack{at}nrcan.gc.ca

Baculovirus occlusion-derived virions (ODVs) contain a number of infectivity factors essential for the initiation of infection in larval midgut cells. Deletion of any of these factors neutralizes infectivity by the per os route. We have observed that P74 of the group I alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is N-terminally cleaved when a soluble form of the protein was incubated with insect midgut tissues under alkaline conditions and that cleavage was prevented by soybean trypsin inhibitor (SBTI). Presently, biological assays were carried out that suggest SBTI inhibits and trypsin enhances baculovirus per os infectivity. We developed a method to rescue per os infectivity of a P74 null virus involving co-transfection of viral DNA with a plasmid that transiently expresses p74. We used this plasmid rescue method to functionally characterize P74. A series of site-directed mutants were generated at the N terminus to evaluate if trypsin cleavage sites were necessary for function. Mutagenesis of R195, R196 and R199 compromised per os infectivity and rendered P74 resistant to midgut trypsin.

Published online ahead of print on 23 July 2008 as DOI 10.1099/vir.0.2008.002543-0.

Supplementary tables are available with the online version of this paper.

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