Baculoviruses require an intact ODV entry-complex to resist proteolytic degradation of per os infectivity factors by co-occluded proteases from the larval host

Abstract

Baculoviruses orally infect caterpillars in the form of occlusion-derived viruses (ODVs). The ODV-envelope contains a number of proteins which are essential for oral infectivity, called per os infectivity factors (PIFs). Most of these PIFs are involved in the formation of an ODV-entry complex that consists of a stable core, formed by PIF1, PIF2, PIF3 and PIF4, and the more loosely associated PIFs P74 (PIF0) and P95 (PIF8). PIF1, PIF2 and PIF3 are essential for formation of the stable core, whereas deletion of the pif4 gene results in the formation of a smaller complex. P74 is not needed for formation of the stable core. We show here in larva-derived ODVs of the Autographa californica multicapsid nucleopolyhedrovirus that PIF-proteins are degraded by host-derived proteases after deletion of a single pif-gene. Constituents of the stable core-complex appeared to be more resistant to proteases as part of the complex than as monomer, as in ODVs of a p74 deletion mutant only the stable core was found but no PIF monomers. When the stable core lacks PIF4, it lost its proteolytic resistance as the resulting smaller core complex was degraded in a pif4 deletion mutant. We also identified PIF6 as a loosely associated component of the entry complex that appeared nevertheless important for the proteolytic resistance of the stable core, which was degraded after deletion of pif6. We conclude from these results that an intact entry-complex in the ODV-envelope is prerequisite for proteolytic resistance of PIF-proteins under the alkaline conditions of the larval midgut.

Baculovirus Per Os Infectivity Factor Complex: Components and Assembly

“…Consistent with previous reports (3,22,23), the AcMNPV PIF complex contained eight PIFs (PIF0 to -4 and PIF6 to -8) and lacked PIF5 (Fig. 1A).…”

“…Western blot analysis confirmed that the AcMNPV PIF complex was approximately 500 kDa ( Fig. 1A), as reported previously (3,22,23). By use of anti-AcMNPV PIF antibodies, a similar, ϳ500-kDa complex was detected in the closely related virus BmNPV (Fig.…”

“…While PIF1, -2, and -3 are essential for core complex formation, deletion of pif4 did not completely impair the stable core complex but resulted in a smaller stable complex of ϳ150 kDa (22). This ϳ150-kDa complex was later found to be sensitive to proteolytic degradation, and the apparent inconsistency is due to the fact that the partial denaturation method used did not completely inhibit proteolytic degradation by the OB's endogenous proteinase (23).…”

“…A proteomic study showed that PIF6, Ac5, and Ac108 were associated with PIF1 (22). Later, PIF6 was confirmed to be a component of the PIF complex (23), while Ac5 was not (24). Recently, PIF7 was also identified as a component of the complex (3).…”

“…All PIFs except PIF5 are associated with the PIF complex, which highlights the pivotal role of the complex for oral baculovirus infection (25). The intact complex was required to protect PIFs from proteolytic degradation by OB endogenous proteinases during ODV release from OBs (23). Understanding the structure and function of this unique complex calls for a detailed study, particularly in light of the ubiquitous nature of pif genes in a wide range of invertebrate large dsDNA viruses.…”