Polyhedrin

Polyhedrin is a highly abundant structural protein encoded by baculoviruses, most notably by nucleopolyhedroviruses (NPV). It is the major component of occlusion bodies, crystalline bodies that enclose virions and are collectively called polyhedra. These occlusion bodies protect virions in the external environment and facilitate oral transmission to insect hosts.

Composition and expression: Polyhedrin is a small, thermostable protein, typically around 25-30 kDa, assembled into densely packed crystals. It is expressed at very late stages of infection at exceptionally high levels under the control of the strong polyhedrin promoter.

Biological role: Occlusion bodies shield virions from UV radiation, desiccation, and proteolysis, enabling survival outside the host. In the insect gut, alkaline conditions dissolve the crystal, releasing occluded virions that infect midgut cells.

Genetics and use: The gene is essential for occlusion body formation; without polyhedrin, occlusion bodies are not produced, reducing environmental stability and transmission, though viral replication in cell culture can proceed. The polyhedrin promoter is widely used in baculovirus expression vector systems to drive high-level expression of recombinant proteins in insect cells, making polyhedrin a key tool in biotechnology.