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Evidence for proteolytic cleavage of the baculovirus occlusion-derived virion envelope protein P74

Jeffrey M. Slack

USDA/ARS, Insect Biocontrol Laboratory, BARC-West, Bldg 011A, Rm 214, Beltsville, MD 20852-2350, USA

Correspondence
Susan D. Lawrence
lawrencs{at}ba.ars.usda.gov

Baculovirus occlusion-derived virions (ODVs) are released from occlusion bodies by the alkaline environment of the insect midgut. The ODV envelope protein P74 is required for oral infectivity. A soluble form of the Autographa californica multiple nucleopolyhedrovirus P74 protein, P74^sol, was engineered as part of a chimeric protein with jellyfish green fluorescent protein (GFP). P74^sol–GFP was overproduced by the baculovirus expression system and purified away from the wild-type P74. Brush border membrane vesicles (BBMVs) were prepared from the midguts of third-instar Helicoverpa zea larvae. When P74^sol–GFP was incubated under alkaline conditions with BBMVs, a P74^sol–GFP product with a smaller molecular mass was produced. Immunoblots indicated that the smaller product was generated by N-terminal cleavage of P74. This cleavage was prevented by soybean trypsin inhibitor. Analysis of the peptide sequences of P74 homologues identified a conserved trypsin cleavage site that could generate the observed P74^sol–GFP BBMV-specific cleavage product.

Present address: Great Lakes Forestry Centre, Canadian Forestry Service, Natural Resources Canada, Sault Ste Marie, ON, Canada P6A 2E5.