A phosphorylated 34-kDa protein and a subpopulation of polyhedrin are thiol linked to the carbohydrate layer surrounding a baculovirus occlusion body

Abstract

Surrounding baculovirus occlusion bodies is an electron-dense layer reported to be composed of carbohydrate which we term calyx. Incubation of Autographa californica nuclear polyhedrosis virus occlusion bodies (AcMNPV OBs) with dilute alkaline saline (DAS) followed by centrifugation at 12,000 g resulted in the sedimentation of calyx material which contained pp34, residual polyhedrin (p32), and entrapped occluded virions (DAS P-12 fraction). Incubation of the DAS P-12 fraction with sodium dodecyl sulfate (SDS) resulted in solubilization of the entrapped virions and the majority of p32, while calyx material, pp34, and some p32 remained sedimentable at 12,000 g. Immunofluorescence microscopy of DAS-solubilized OBs using monoclonal antibody to pp34 and p32 revealed that both pp34 and p32 are closely associated with the calyx. When DAS P-12 fractions were resuspended in SDS and reducing agent, not only were the entrapped virions solubilized, but pp34 and the remaining p32 were also liberated, indicating that pp34 and a subpopulation of p32 are associated with the calyx via thiol linkages. Immunoblot analysis and peptide mapping demonstrated that pp34 is neither immunologically nor structurally related to p32. The kinetics of pp34 synthesis were also examined by immunoprecipitation of infected cell polypeptides using pp34-specific monoclonal antibody. pp34 was detected initially 15 hr postinfection (p.i.) and continued to be phosphorylated until 60–70 hr p.i. This study demonstrates that the AcMNPV calyx has a proteinaceous component and we propose that other occluded baculoviruses may also have a calyx-associated protein analogous to pp34.