Structural Model of the Tubular Assembly of the Rous Sarcoma Virus Capsid Protein

The orthoretroviral capsid protein (CA) assembles into polymorphic capsids, whose architecture, assembly, and stability are still being investigated. The N-terminal and C-terminal domains of CA (NTD and CTD, respectively) engage in both homotypic and heterotypic interactions to create the capsid. Hexameric turrets formed by the NTD decorate the majority of the capsid surface. We report nearly complete solid-state NMR (ssNMR) resonance assignments of Rous sarcoma virus (RSV) CA, assembled into hexamer tubes that mimic the authentic capsid. The ssNMR assignments show that, upon assembly, large conformational changes occur in loops connecting helices, as well as the short 3₁₀ helix initiating the CTD. The interdomain linker becomes statically disordered. Combining constraints from ssNMR and cryo-electron microscopy (cryo-EM), we establish an atomic resolution model of the RSV CA tubular assembly using molecular dynamics flexible fitting (MDFF) simulations. On the basis of comparison of this MDFF model with an earlier-derived crystallographic model for the planar assembly, the induction of curvature into the RSV CA hexamer lattice arises predominantly from reconfiguration of the NTD-CTD and CTD trimer interfaces. The CTD dimer and CTD trimer interfaces are also intrinsically variable. Hence, deformation of the CA hexamer lattice results from the variable displacement of the CTDs that surround each hexameric turret. Pervasive H-bonding is found at all interdomain interfaces, which may contribute to their malleability. Finally, we find helices at the interfaces of HIV and RSV CA assemblies have very different contact angles, which may reflect differences in the capsid assembly pathway for these viruses.