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. Retrovirus capsids are polymorphic, consisting of variable hexamer and pentamer aggregates with structures thought to resemble fullerenes. A cryo-electon microscopy/molecular modelling study of capsids assembled from the full-length Rous sarcoma virus capsid protein now confirms the existence of fullerene-like capsid architecture. Two different forms were observed, one composed of 12 pentamers and the other of 12 pentamers and 20 hexamers. Retrovirus capsids are polymorphic, consisting of variable hexamer and pentamer aggregates that are thought to reflect fullerenes. This paper reports the cryo-electron microscopy analysis of RSV capsid protein visualizing pentamers, and confirms that retrovirus capsid has a fullerene-based architecture.