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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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Pseudo-atomic models of the RSV-CA subunit, pentamer, and hexamer. a shows the relative positions and orientations of the NTD (orange) and CTD (blue), as disposed in the T=1 capsid. The dashed line connects the C-terminus of the NTD to the N-terminus of the CTD. b and c show axial views of the pentamer and hexamer, respectively.
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Figure 4: Pseudo-atomic models of the RSV-CA subunit, pentamer, and hexamer. a shows the relative positions and orientations of the NTD (orange) and CTD (blue), as disposed in the T=1 capsid. The dashed line connects the C-terminus of the NTD to the N-terminus of the CTD. b and c show axial views of the pentamer and hexamer, respectively.

Mentions: Structures for both domains of RSV-CA have been determined13,14. The resolution of our T=1 reconstruction was high enough to give unambiguous solutions when the domains were fitted separately into it (Figs. 4a,b). As there are two CTD structures13,14, we fitted both and chose the one that correlated best with the density map. In the fit, all of the α-helices map on to well-defined elongated densities (Fig. 2c). The map also contains densities attributable to the single-turn 310 helix (residues 152 to 155) and the β-hairpin at the N-terminus of mature CA14,27. Next, we generated a pseudo-atomic model of the complete capsid (Supp. Fig. 1). The NTDs account for its protrusions, while the floor layer is composed of CTDs. In a pentamer, the CTD of one subunit underlies the NTD of its neighbor. The C-terminus of the NTD and the N-terminus of its CTD are 1.6 nm apart, connected by the (unseen) four residues not present in either domain structure (Fig. 2c, Fig 4a).


Visualization of a missing link in retrovirus capsid assembly.

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

Pseudo-atomic models of the RSV-CA subunit, pentamer, and hexamer. a shows the relative positions and orientations of the NTD (orange) and CTD (blue), as disposed in the T=1 capsid. The dashed line connects the C-terminus of the NTD to the N-terminus of the CTD. b and c show axial views of the pentamer and hexamer, respectively.
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Related In: Results  -  Collection

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

Figure 4: Pseudo-atomic models of the RSV-CA subunit, pentamer, and hexamer. a shows the relative positions and orientations of the NTD (orange) and CTD (blue), as disposed in the T=1 capsid. The dashed line connects the C-terminus of the NTD to the N-terminus of the CTD. b and c show axial views of the pentamer and hexamer, respectively.
Mentions: Structures for both domains of RSV-CA have been determined13,14. The resolution of our T=1 reconstruction was high enough to give unambiguous solutions when the domains were fitted separately into it (Figs. 4a,b). As there are two CTD structures13,14, we fitted both and chose the one that correlated best with the density map. In the fit, all of the α-helices map on to well-defined elongated densities (Fig. 2c). The map also contains densities attributable to the single-turn 310 helix (residues 152 to 155) and the β-hairpin at the N-terminus of mature CA14,27. Next, we generated a pseudo-atomic model of the complete capsid (Supp. Fig. 1). The NTDs account for its protrusions, while the floor layer is composed of CTDs. In a pentamer, the CTD of one subunit underlies the NTD of its neighbor. The C-terminus of the NTD and the N-terminus of its CTD are 1.6 nm apart, connected by the (unseen) four residues not present in either domain structure (Fig. 2c, Fig 4a).

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