Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 106, Issue 52, Pages 22205-22210Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0910686106
Keywords
in vivo crystallization; molecular arms; occlusion body; self-assembly; virus evolution
Categories
Funding
- Royal Society of New Zealand
- University of Auckland Research Committee
- Maurice Wilkins Centre for Molecular Biodiscovery
- Kansai Bureau of Ministry of Economy, Trade and Industry
- CREST by Japan ScienceandTechnology Agency
- Japan Society for the Promotion of Science Invitation
- Tertiary Education Commission
- University of Auckland
- Swiss National Centre of Competence in Research Structural Biology
- National Health and Medical Research Council
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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 alpha-helical. These results suggest homology between the proteins and a common structural basis for viral polyhedra. Here, we present the 2.2-angstrom 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 I23 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.
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