ANTIVIRAL DETERMINANTS OF RAT MX GTPASES MAP TO THE CARBOXY-TERMINAL HALF

Rat Mx2 and rat Mx3 are two alpha/beta interferon-inducible cytoplasmi c GTPases that differ in three residues in the amino-terminal third, w hich also contains the tripartite GTP-binding domain, and that differ in five residues in the carboxy-terminal quarter, which also contains a dimerization domain. While Mx2 is active against vesicular stomatiti s virus (VSV), Mx3 lacks antiviral activity. We mapped the functional difference between Mx2 and Mx3 protein to two critical residues in the carboxy-terminal parts of the molecules. An exchange of either residu e 588 or 630 of Mx2 with the corresponding residues of Mx3 abolished a nti-VSV activity, and the introduction of the two Mx2 residues on an M x3 background partially restored anti-VSV activity, These results are consistent with the facts that Mx2 and Mx3 have similar intrinsic GTPa se activities and that the GTPase domain of Mx3 can fully substitute f or the GTPase domain of Mx2. Nevertheless, the amino-terminal third co ntaining the GTP-binding domain is necessary for antiviral activity, s ince an amino-terminally truncated Mx2 protein is devoid of anti-VSV a ctivity. Furthermore, Fab fragments of a monoclonal antibody known to neutralize antiviral activity block GTPase activity by binding an epit ope in the carboxyterminal half of Mx2 or Mx3 protein. The results are consistent with a two-domain model in which both the conserved amino- terminal half and the less-well-conserved carboxy-terminal half of Mx proteins carry functionally important domains.