Structural Comparison of Membrane-Bound Retroviral Gag Proteins

Abstract

he Gag polyprotein is essential for the assembly of retroviral particles. It is expressed in the cellular cytoplasm and targets the surface of the plasma membrane, where assembly occurs. All Gag proteins from various retroviral species contain structured domains - the membrane-binding matrix (MA), the capsid (CA) and the genome-binding nucleocapsid (NC) domains - that are separated by unstructured linker regions. Cryo-EM of immature virions shows assemblies on the plasma membrane in which Gag polyproteins are in an extended conformation, apparently stabilized by lateral interactions between their CA domains. However, the structural dynamics are complex. In a recent study with neutron reflection (NR), we showed that in HIV-1 Gag, the MA-CA linker is so flexible that the polyprotein binds a model membrane in a backfolded structure in the absence of nucleic acid, and that a transformation into an extended conformation is triggered upon nucleic acid binding (Datta et al., J. Mol. Biol. 406, 2011, 205). Here we investigate the membrane-binding of murine leukemia virus (MLV) and Rous sarcoma virus (RSV) Gag proteins, which also include unstructured linkers between their MA and CA domains. Both wild-type MLV and RSV Gag bind membranes in vitro in folded conformations, as shown by NR, indicating structural flexibility of their MA-CA linkers. In contrast, an MLV mutant, Δp12, in which the MA-CA linker has been eliminated, binds the membrane in an extended conformation. An investigation of the RSV mutant, Δ104-220, that is analogous to MLV-Δp12, is currently in progress. These studies suggest that the disordered linker regions in retroviral Gag proteins may function to prevent unproductive assembly processes in the host cytosol or on the membrane in the absence of viral genome.