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Visualization of a missing link in retrovirus capsid assembly.

Cardone G, Purdy JG, Cheng N, Craven RC, Steven AC - Nature (2009)

Bottom Line: 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.Both are icosahedrally symmetric: one is composed of 12 pentamers, and the other of 12 pentamers and 20 hexamers.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.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Structural Biology, National Institute for Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.

ABSTRACT
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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RSV CA protein assembles in vitro in 0.5M phosphate buffer into small isometric particles. a, Cryo-electron micrograph of capsids; most are ~ 17 nm in diameter, while a few are ~ 30 nm - white circles. Bar, 50 nm. b – d, Averaged images. b, 17-nm capsids, unclassified. c, 17-nm capsids, projecting the 3-fold view. d, 30-nm capsids, unclassified, showing two concentric shells. Bar, 10 nm.
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Figure 1: RSV CA protein assembles in vitro in 0.5M phosphate buffer into small isometric particles. a, Cryo-electron micrograph of capsids; most are ~ 17 nm in diameter, while a few are ~ 30 nm - white circles. Bar, 50 nm. b – d, Averaged images. b, 17-nm capsids, unclassified. c, 17-nm capsids, projecting the 3-fold view. d, 30-nm capsids, unclassified, showing two concentric shells. Bar, 10 nm.

Mentions: In vitro assembly of full-length RSV-CA produces a diversity of structures, including spheroids, tubes, and planar arrays14. In buffer containing 0.5 M phosphate at near-neutral pH, the protein can also form angular structures, resembling capsids inside native virions26. However, a major portion of the assemblies formed in 0.5 M phosphate is small isometric particles - Fig. 1a. Most are ~ 17 nm in diameter; a few are larger, at ~ 30 nm. The 17-nm particles suggest an interpretation of the small rings with the wall-thickness of capsids observed in tomographic slices of RSV virions (Fig. 3b of Butan et al.16), i.e. they are likely to represent T=1 capsids. It appears, therefore, that such capsids, although too small to confine genomes, are also assembled in situ. The similar polymorphisms exhibited in vitro and in situ imply that form determination is largely an intrinsic property of CA protein, although other factors may steer assembly in certain directions – for instance, by affecting nucleation4.


Visualization of a missing link in retrovirus capsid assembly.

Cardone G, Purdy JG, Cheng N, Craven RC, Steven AC - Nature (2009)

RSV CA protein assembles in vitro in 0.5M phosphate buffer into small isometric particles. a, Cryo-electron micrograph of capsids; most are ~ 17 nm in diameter, while a few are ~ 30 nm - white circles. Bar, 50 nm. b – d, Averaged images. b, 17-nm capsids, unclassified. c, 17-nm capsids, projecting the 3-fold view. d, 30-nm capsids, unclassified, showing two concentric shells. Bar, 10 nm.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2721793&req=5

Figure 1: RSV CA protein assembles in vitro in 0.5M phosphate buffer into small isometric particles. a, Cryo-electron micrograph of capsids; most are ~ 17 nm in diameter, while a few are ~ 30 nm - white circles. Bar, 50 nm. b – d, Averaged images. b, 17-nm capsids, unclassified. c, 17-nm capsids, projecting the 3-fold view. d, 30-nm capsids, unclassified, showing two concentric shells. Bar, 10 nm.
Mentions: In vitro assembly of full-length RSV-CA produces a diversity of structures, including spheroids, tubes, and planar arrays14. In buffer containing 0.5 M phosphate at near-neutral pH, the protein can also form angular structures, resembling capsids inside native virions26. However, a major portion of the assemblies formed in 0.5 M phosphate is small isometric particles - Fig. 1a. Most are ~ 17 nm in diameter; a few are larger, at ~ 30 nm. The 17-nm particles suggest an interpretation of the small rings with the wall-thickness of capsids observed in tomographic slices of RSV virions (Fig. 3b of Butan et al.16), i.e. they are likely to represent T=1 capsids. It appears, therefore, that such capsids, although too small to confine genomes, are also assembled in situ. The similar polymorphisms exhibited in vitro and in situ imply that form determination is largely an intrinsic property of CA protein, although other factors may steer assembly in certain directions – for instance, by affecting nucleation4.

Bottom Line: 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.Both are icosahedrally symmetric: one is composed of 12 pentamers, and the other of 12 pentamers and 20 hexamers.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.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Structural Biology, National Institute for Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus