Letter

Nature 446, 97-101 (1 March 2007) | doi:10.1038/nature05628; Received 29 November 2006; Accepted 31 January 2007

The molecular organization of cypovirus polyhedra

Fasséli Coulibaly1, Elaine Chiu1, Keiko Ikeda2, Sascha Gutmann3, Peter W. Haebel4, Clemens Schulze-Briese3, Hajime Mori5 & Peter Metcalf1

  1. School of Biological Sciences, University of Auckland, 1010, New Zealand
  2. Protein Crystal Corporation, Osaka 541-0053, Japan
  3. Swiss Light Source at Paul Scherrer Institute, Villigen 5232, Switzerland
  4. Altana Pharma AG, Konstanz 78467, Germany
  5. Kyoto Institute of Technology, Kyoto 606-8585, Japan

Correspondence to: Peter Metcalf1 Correspondence and requests for materials should be addressed to P.M. (Email: peter.metcalf@auckland.ac.nz).

Cypoviruses and baculoviruses are notoriously difficult to eradicate because the virus particles are embedded in micrometre-sized protein crystals called polyhedra1, 2. 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 insecticides3, 4. Although polyhedra have been extensively characterized since the early 1900s5, their atomic organization remains elusive6. 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 determination7. 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 applications8 such as microarrays9 and biopesticides4.

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