GTP-bound human MxA protein interacts with the nucleocapsids of Thogoto virus (Orthomyxoviridae)

Human MxA protein is an interferon-induced member of the dynamin superfamil y of large GTPases. MxA inhibits the multiplication of several RNA viruses, including Thogoto virus, an influenza virus-like orthomyxovirus transmitte d by ticks. Previous studies have indicated that GTP binding is required fo r antiviral activity, but the mechanism of action is still unknown. Here, w e have used an in vitro cosedimentation assay to demonstrate, for the first time, a GTP-dependent interaction between MxA GTPase and a viral target st ructure. The assay is based on highly active MxA GTPase as effector molecul es, Thogoto virus nucleocapsids as viral targets, and guanosine 5'-O-(3-thi otriphosphate) (GTP gamma S) as a stabilizing factor. We show that MxA tigh tly interacts with viral nucleocapsids by binding to the nucleoprotein comp onent. This interaction requires the presence of GTP gamma S and is mediate d by domains in the carboxyl-terminal moiety of MxA. We propose that GTP-bo und MxA adopts an antivirally active conformation that allows interaction w ith viral nucleocapsids, thereby impairing their normal function.