A FUNCTIONAL GTP-BINDING MOTIF IS NECESSARY FOR ANTIVIRAL ACTIVITY OFMX PROTEINS

Mx proteins are interferon-induced GTPases that inhibit the multiplica tion of certain negative-stranded RNA viruses. However, it has been un clear whether GTPase activity is necessary for antiviral function. Her e, we have introduced mutations into the tripartite GTP-binding consen sus elements of the human MxA and mouse Mx1 proteins. The invariant ly sine residue of the first consensus motif, which interacts with the be ta- and gamma-phosphates of bound GTP in other GTPases, was deleted or replaced by methionine or alanine. These Mx mutants and appropriate c ontrols were then tested for antiviral activity, GTP-binding capacity, and GTPase activity. We found a direct correlation between the GTP-bi nding capacities and GTP hydrolysis activities of the purified Mx muta nts in vitro and their antiviral activities in transfected 3T3 cells, demonstrating that a functional GTP-binding motif is necessary for vir us inhibition. Our results, thus, firmly establish antiviral activity as a novel function of a GTPase, emphasizing the enormous functional d iversity of GTPase superfamily members.