Featured Article - November 2011
Short description: NMR studies reveal the dimer interface of the effector domain (ED) of NS1A from influenza A virus and provide insight into how dimerization of the ED provides cooperative dsRNA binding and other functions of NS1A.
With the rise of new strains of influenza virus and the emergence of antiviral-resistant strains, identifying new targets for antiviral therapeutics is becoming increasingly important. One such target is the nonstructural protein 1 of influenza A (NS1A). NS1A binds nonspecifically to double-stranded RNA (dsRNA) through its N-terminal RNA-binding domain (RBD) and to a host of cellular proteins through its C-terminal effector domain (ED). These interactions allow the virus to evade the host antiviral system by affecting interferon-induced and other innate immune response pathways.
NS1A has been the focus of several recent structural studies. The ED has been shown to adopt a novel α-helix β-crescent fold that forms a homodimer. However, the biological dimer interface has remained controversial, as structures of the ED of NS1A from different influenza strains have revealed three different dimer interfaces.
Aramini, Ma, Montelione and colleagues (PSI NESG), in a PSI Community Outreach project with R. Krug at the University of Texas, Austin, have recently revealed the biologically relevant dimer of the ED from NS1A of H3N2 influenza A/Udorn/307/1972 (Ud) virus. They determined that the affinity for dimerization is relatively weak (micromolar range) and proceeded to map the dimer interface by NMR. Rotational correlation measurements, chemical shift mapping and NOE analysis all confirm a helix-helix dimer interface. These data are consistent with some, but not all, of the available crystal structures of the NS1A ED, resolving a controversy in the literature in which several alternative dimer interfaces have been observed in various crystal forms.
The authors went on to confirm the importance of a particular residue, Trp187, for dimerization. Mutation of Trp187 in the context of full-length Ud NS1A disrupts higher-order oligomerization and impairs its ability to bind to dsRNA. The mutant NS1A protein also shows reduced cooperativity in its binding of long stretches of dsRNA. On the basis of these results, Aramini and colleagues propose a model by which Ud NS1A forms a tube around the dsRNA, and the ED is involved in either cooperative interactions between NS1A dimers along the dsRNA, or in binding to other proteins, depending on the local NS1A, dsRNA and binding-partner concentrations.
This study highlights the ability of NMR to characterize weak dimers in solution, which are sometimes incorrectly characterized by crystallography as a result of lattice-packing effects. Furthermore, knowing the affinity for dimerization of the ED of Ud NS1A will be important for drug screening, as such experiments can be done under conditions at which the ED of Ud NS1A is monomeric.
J. M. Aramini, L.-C. Ma et al. The dimer interface of the effector domain of non-structural protein 1 from influenza A virus: an interface with multiple functions.
J. Biol. Chem. 286, 26050-26060 (2011). doi:10.1074/jbc.M111.248765