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Journal of Virology, May 2002, p. 4321-4330, Vol. 76, No. 9
0022-538X/02/$04.00+0     DOI: 10.1128/JVI.76.9.4321-4330.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Molecular Organization of Mason-Pfizer Monkey Virus Capsids Assembled from Gag Polyprotein in Escherichia coli

National Institute for Biological Standards and Control, South Mimms, Hertfordshire EN6 3QG, United Kingdom,¹ UMR 6026 CNRS, Équipe CRM, Université de Rennes 1, Rennes, France,² Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences,³ Institute of Chemical Technology and Center for Integrated Genomics, Prague, Czech Republic,⁴ UAB Center for AIDS Research, University of Alabama at Birmingham, Birmingham, Alabama 35294-2170⁵

Received 13 November 2001/ Accepted 17 January 2002

We describe the results of a study by electron microscopy and image processing of Gag protein shells—immature capsids—of Mason-Pfizer monkey virus assembled in Escherichia coli from two truncated forms of the Gag precursor: p4Gag, in which the C-terminal p4Gag was deleted, and Pro(-)CA.NC, in which the N-terminal peptides and proline 1 of the CA domain were deleted. Negative staining of capsids revealed small patches of holes forming a trigonal or hexagonal pattern most clearly visible on occasional tubular forms. The center-to-center spacing of holes in the network was 7.1 nm in p4Gag capsids and 7.4 nm in Pro(-)CA.NC capsids. Image processing of p4Gag tubes revealed a hexagonal network of holes formed by six subunits with a single subunit shared between rings. This organization suggests that the six subunits are contributed by three trimers of the truncated Gag precursor. Similar molecular organization was observed in negatively stained Pro(-)CA.NC capsids. Shadowed replicas of freeze-etched capsids produced by either construct confirmed the presence of a hexagonal network of holes with a similar center-to-center spacing. We conclude that the basic building block of the cage-like network is a trimer of the p4Gag or Pro(-)CA.NC domains. In addition, our results point to a key role of structurally constrained CA domain in the trimeric interaction of the Gag polyprotein.

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