Visualization of a missing link in retrovirus capsid assembly


For a retrovirus such as HIV to be infectious, a properly formed capsid is needed; however, unusually among viruses, retrovirus capsids are highly variable in structure. According to the fullerene conjecture, they are composed of hexamers and pentamers of capsid protein (CA), with the shape of a capsid varying according to how the twelve pentamers are distributed and its size depending on the number of hexamers. Hexamers have been studied in planar and tubular arrays, but the predicted pentamers have not been observed. Here we report cryo-electron microscopic analyses of two in-vitro-assembled capsids of Rous sarcoma virus. Both are icosahedrally symmetric: one is composed of 12 pentamers, and the other of 12 pentamers and 20 hexamers. Fitting of atomic models of the two CA domains into the reconstructions shows three distinct inter-subunit interactions. These observations substantiate the fullerene conjecture, show how pentamers are accommodated at vertices, support the inference that nucleation is a crucial morphologic determinant, and imply that electrostatic interactions govern the differential assembly of pentamers and hexamers.

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Figure 1: RSV CA protein assembles in vitro in 0.5 M phosphate buffer into small isometric particles.
Figure 3: The 30-nm RSV CA double-layer capsid.
Figure 2: Three-dimensional reconstruction of the RSV CA T = 1 capsid.
Figure 4: Pseudo-atomic models of the RSV-CA subunit, pentamer and hexamer.
Figure 5: Inter-subunit interactions in the T = 1 capsid, represented in stereo views.


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We thank J. Flanagan for advice on protein purification and analytic methods and access to equipment, R. Meyers for assistance in electron microscopy at the Penn State College of Medicine and B. Heymann for advice on data analysis. This work was supported by the Intramural Research Program of NIAMS and the IATAP Program (A.C.S.), and funding from NIH grant CA100322, the Pennsylvania Department of Health and the Penn State Cancer Institute (R.C.C.).

Author Contributions A.C.S. and R.C.C. designed the project; J.G.P. prepared the capsids with guidance from R.C.C.; N.C. performed the cryo-electron microscopy; G.C. performed the image reconstruction and modelling; and A.C.S. and G.C. wrote the paper with input from the other authors.

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Correspondence to Rebecca C. Craven or Alasdair C. Steven.

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Cardone, G., Purdy, J., Cheng, N. et al. Visualization of a missing link in retrovirus capsid assembly. Nature 457, 694–698 (2009).

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