The HIV capsid is a fullerene cone: a variably curved, closed shell composed of approximately 250 hexamers and exactly 12 pentamers of the viral CA protein. Atomic resolution structures of the hexameric and pentameric HIV-1 capsomers have not been previously reported. This is presumably due to the low intrinsic stability of CA hexamers and pentamers, and the challenges of preparing discrete, soluble CA assemblies. Recently, we isolated soluble, assembly-competent CA hexamers by two independent methods: (1) stabilization of the hexameric ring by exogenous intermolecular disulfide bonds, and (2) fusion of CA to a stable hexameric template, the bacterial carboxysome protein, CcmK4. Using these novel reagents, we derived four crystallographically independent structures of the CA hexamer at atomic resolution (see figure). A ring of six CA N-terminal domains form an apparently rigid core, surrounded by an outer ring of C-terminal domains. Mobility of the outer ring appears to be an underlying mechanism for generating the variably curved lattice in authentic capsids. Hexamer-stabilizing interfaces are highly hydrated, and this property may be key to the formation of quasi-equivalent interactions within hexamers and pentamers. The structures also clarify the molecular basis for capsid assembly inhibition and should facilitate structure-based drug design strategies

Figure: Four crystallographically independent structures of the HIV-1 CA hexamer, with the N-terminal domains (NTD) colored in orange and the C-terminal domains (CTD) in blue, and shown as Calpha trace. The structures were superimposed at each of the rotationally equivalent positions. Calpha deviations were then calculated for each residue and then mapped onto a transluscent sausage representation. The width and coloring of the sausage are directly proportional to the magnitude of the deviation, with the minimum in white and the maximum in red. This illustrates that the NTD ring is invariant across the four hexamer structures, whereas the CTD “belt” undergoes rigid-body motions.