Mx GTPases: efficient host defense against viral intruders.

Mx proteins are interferon-induced members of the dynamin superfamily of large GTPases. They inhibit a wide range of viruses by blocking early steps in the viral replication cycle. Recent evidence suggests that the human MxA (MX1) protein provides a barrier against zoonotic introduction of influenza A viruses into the human population, whereas the related human MxB (MX2) protein is an inhibitor of HIV-1 and other primate lentiviruses. Structural and functional data suggest that Mx proteins target the nucleocapsids of Mx-sensitive viruses and thereby inhibit their transcriptional and replicative function. Evolutionary studies revealed that Mx GTPases are subject to recurrent arms races with viral targets that shape their specificity determinants while the overall architecture is conserved. Here we briefly review the most salient features of Mx GTPases and their antiviral action as molecular machines.