Conformational analysis of the GTP-binding protein MxA using limited proteolysis.

Guanosine triphosphate (GTP)-binding proteins are known to function as molecular switches that cycle between GTP-bound and guanosine diphosphate (GDP)-bound states. Switching is achieved by the fact that G-proteins in the GTP-bound conformation can interact with a certain set of effector molecules while they interact with a different set of partners in their GDP-bound conformation. The antiviral properties of the interferon-induced MxA protein are critically dependent on the ability of MxA to bind GTP. Using limited proteolysis we analyzed the conformations of the MxA protein under nucleotide-free, GDP-bound, and GTP-bound conditions. We find that whereas the conformations of nucleotide-free MxA and GDP-bound MxA are essentially similar, GTP-binding causes a dramatic change in the conformation of MxA.