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High-resolution atomic structures have been reported recently for two types of viral polyhedra, intracellular protein crystals produced by ubiquitous insect viruses. Polyhedra contain embedded virus particles and function as the main infectious form for baculoviruses and cypoviruses, two distinct classes of viruses that infect mainly Lepitoptera species (butterflies and moths). Polyhedra are extremely stable and protect the virus particles once released in the environment. The extensive crystal contacts observed in the structures explain the remarkable stability of viral polyhedra and provide hints about how these crystals dissolve in the alkaline midgut, releasing embedded virus particles to infect feeding larvae. The stage is now set to answer intriguing questions about the in vivo crystallization of polyhedra, how virus particles are incorporated into polyhedra, and what determines the size and shape of the crystals. Large quantities of polyhedra can be obtained from infected larvae and polyhedra can also be produced using insect cell expression systems. Modified polyhedra encapsulating other entities in place of virus particles have potential applications as a means to stabilize proteins such as enzymes or growth factors, and the extremely stable polyhedrin lattice may provide a framework for future engineered micro-crystal devices., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Baculoviruses are ubiquitous insect viruses well known for their use as bioinsecticides, gene therapy vectors, and protein expression systems. Overexpression of recombinant proteins in insect cell culture utilizes the strong promoter of the polyhedrin gene. In infected larvae, the polyhedrin protein forms robust intracellular crystals called polyhedra, which protect encased virions for prolonged periods in the environment. Polyhedra are produced by two unrelated families of insect viruses, baculoviruses and cypoviruses. The atomic structure of cypovirus polyhedra revealed an intricate packing of trimers, which are interconnected by a projecting N-terminal helical arm of the polyhedrin molecule. Baculovirus and cypovirus polyhedra share nearly identical lattices, and the N-terminal region of the otherwise unrelated baculovirus polyhedrin protein sequence is also predicted to be α-helical. These results suggest homology between the proteins and a common structural basis for viral polyhedra. Here, we present the 2.2-Å structure of baculovirus polyhedra determined by x-ray crystallography from microcrystals produced in vivo. We show that the underlying molecular organization is, in fact, very different. Although both polyhedra have nearly identical unit cell dimensions and share 123 symmetry, the polyhedrin molecules are structurally unrelated and pack differently in the crystals. In particular, disulfide bonds and domain-swapped N-terminal domains stabilize the building blocks of baculovirus polyhedra and interlocking C-terminal arms join unit cells together. We show that the N-terminal projecting helical arms have different structural roles in baculovirus and cypovirus polyhedra and conclude that there is no structural evidence for a common evolutionary origin for both classes of polyhedra. [ABSTRACT FROM AUTHOR]

Cypoviruses and baculoviruses are notoriously difficult to eradicate because the virus particles are embedded in micrometre-sized protein crystals called polyhedra. The remarkable stability of polyhedra means that, like bacterial spores, these insect viruses remain infectious for years in soil. The environmental persistence of polyhedra is the cause of significant losses in silkworm cocoon harvests but has also been exploited against pests in biological alternatives to chemical insecticides. Although polyhedra have been extensively characterized since the early 1900s, their atomic organization remains elusive. Here we describe the 2 Å crystal structure of both recombinant and infectious silkworm cypovirus polyhedra determined using crystals 5–12 micrometres in diameter purified from insect cells. These are the smallest crystals yet used for de novo X-ray protein structure determination. We found that polyhedra are made of trimers of the viral polyhedrin protein and contain nucleotides. Although the shape of these building blocks is reminiscent of some capsid trimers, polyhedrin has a new fold and has evolved to assemble in vivo into three-dimensional cubic crystals rather than icosahedral shells. The polyhedrin trimers are extensively cross-linked in polyhedra by non-covalent interactions and pack with an exquisite molecular complementarity similar to that of antigen–antibody complexes. The resulting ultrastable and sealed crystals shield the virus particles from environmental damage. The structure suggests that polyhedra can serve as the basis for the development of robust and versatile nanoparticles for biotechnological applications such as microarrays and biopesticides. [ABSTRACT FROM AUTHOR]