Interferon-induced MxA protein. GTP binding and GTP hydrolysis properties

MxA is a GTPase encoded by an interferon-activated human gene which inhibits the multiplication of several RNA viruses. Recombinant histidine-tagged MxA protein (His-MxA) was expressed in Escherichia coli and purified to near homogeneity. Gel filtration showed that it formed high molecular weight oligomers. Purified His-MxA exhibited specific GTP hydrolysis rates of up to 350 nmol of GTP/min/mg of protein, corresponding to a turnover number of 27 min-1. The Km for this reaction was 260 microM. Guanine nucleotides did not copurify with His-MxA. Binding experiments in solution with fluorescent-labeled nucleotides confirmed that His-MxA binds guanine nucleotides rather weakly and further showed that the fluorescent GDP analog N-methylanthraniloyl (mant)-GDP had a much lower affinity for His-MxA (Kd 20 microM, koff 8.5 s-1) than the nonhydrolyzable GTP analog mant-5'-guanylyl-beta,gamma-imidotriphosphate (mant-GMP-PNP) (Kd 0.75 microM, koff 0.012 s-1). Competitive binding studies with nonlabeled nucleotides revealed a similar binding preference of His-MxA for GTP over GDP: the Kd for GTP was 20 microM, whereas the Kd for GDP was 100 microM. Thus, a high percentage of MxA molecules may be complexed with GTP in vivo.