Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) contains a 966 bp ORF that encodes a papain type cysteine proteinase with cathepsin L-like characteristics. Using Western blot analysis of infected cell extracts we showed that v-cath proteinase has 35.5 kDa and 32 kDa precursor forms which are processed to a 27.5kDa mature form in a manner characteristic of papain and cathepsin L. V-cath proteinase activity was greatest under acidic conditions (pH 5.0) and was reduced in the presence of the cysteine proteinase inhibitors, leupeptin and E64. Urea, a known enhancer of cathepsin L activity, also enhanced v-cath proteinase activity. AcMNPV v-cath proteinase was detected post-mortem in tissues of insects infected with wild-type (wt) virus. Insects infected with a v-cath deletion mutant did not become flaccid after death as is normally observed with wt AcMNPV infections. These findings indicate a link between v-cath activity and degradation of host tissues during virus pathogenesis.

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The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) protein p74 is associated with the occlusion-derived virus (ODV) envelope. p74 is essential for oral infectivity of ODV and has been proposed to play a role in midgut attachment and/or fusion. In this study, p74 protein was expressed in-frame with green fluorescent protein (GFP) to create a p74-GFP chimera. The Cterminal GFP portion of the chimera facilitated visualization of the trafficking of p74 in baculovirusinfected Spodoptera frugiperda (Sf-9) cells. p74-GFP chimeric proteins localized in the intranuclear ring zone of the nucleus and were found to co-precipitate with the microvesicle fraction of cell lysates. A series of truncations of p74 was expressed as p74-GFP chimeras in recombinant baculoviruses. When C-terminal region S580-F645 was deleted from p74, p74-GFP chimera localization became non-specific and chimeras became soluble. p74 region S580-F645 directed GFP to the intranuclear ring zone in a similar pattern to full-length p74. The hydrophobic C terminus of p74 plays a role in protein localization and possibly in transmembrane anchoring and insertion.

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. INTRODUCTIONBaculoviruses are a group of arthropod-specific viruses (Zanotto et al., 1993) that have been applied as insecticides and as vectors for the expression of exogenous genes. They have a biphasic replication strategy producing two distinct viral phenotypes: the budded virion (BV) and the occlusion-derived virion (ODV). Both virion phenotypes have bilayer lipid envelopes surrounding bacillus-shaped nucleocapsids and contain double-stranded, circular DNA genomes. However, the integral protein composition of their envelopes and their roles in infection are distinct.BVs spread viral infection throughout host tissues by attaching to and entering host cells via receptor-mediated endocytosis (Volkman & Goldsmith, 1985). Endosomal acidification triggers envelope fusion with the endosomal membrane to release the viral nucleocapsid into the host cell (Leikina et al., 1992). In the case of group I alphabaculoviruses (Jehle et al., 2006), budding, attachment and envelope fusion are mediated by the viral protein GP64 (Blissard & Wenz, 1992) and for other baculovirus types these processes are mediated by an F protein (Pearson et al., 2000; Westenberg et al., 2002).ODVs are required in the horizontal transmission of baculoviruses between insect hosts (Kozlov et al., 1986). The ODVs of all baculoviruses are occluded into proteinaceous occlusion bodies (OBs) prior to release from the host (Rohrmann, 1986). The distinctive protein structure of OBs has a dual function. It serves to protect virions from deleterious environmental factors, and also acts as a delivery mechanism to transport the ODVs to the alkaline midgut where the cells are susceptible to infection.ODV envelopes are derived from the inner nuclear membrane (Braunagel & Summers, 1994) and contain envelope proteins that can survive the protease-rich environment of the insect midgut. ODVs attach to midgut columnar epithelial cells and fuse th...