Type I interferons (IFN-alpha/beta) are potent antiviral cytokines and modulators of the adaptive immune system. They are induced by viral infection or by double-stranded RNA (dsRNA), a by-product of viral replication, and lead to the production of a broad range of antiviral proteins and immunoactive cytokines. Viruses, in turn, have evolved multiple strategies to counter the IFN system which would otherwise stop virus growth early in infection. Here we discuss the current view on the balancing act between virus-induced IFN responses and the viral counterplayers.

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GTPase is a key mediator of cell-autonomous innate immunityHis-tagged full-length human MxA (Fig. 1a) was recombinantly expressed in bacteria and purified to homogeneity (Methods, Supp. Fig. 1). In crystallization trials, small needle-shaped protein crystals were obtained which represented proteolytic cleavage products of the MD and GED (Supp. Fig. 2). We solved the phase problem by a single anomalous dispersion protocol and could build and refine a model containing two molecules in the asymmetric unit (Methods, Supp. Table 1 and 2). Each monomer spans nearly the complete MD and the amino(N-)-terminal part of the GED (amino acids 366-633) which together fold into an elongated anti-parallel fourhelical bundle where the MD contributes three helices and the GED one (Fig. 1b, Supp. Fig. 3). This segment corresponds to the stalk region of dynamin 7 , and we refer to it as stalk of MxA. The first visible amino acid, Glu366, is 15 amino acids downstream of the last visible residue of the corresponding G-domain structure in rat dynamin (Supp. Fig. 3) 8 . It marks the start of helix α1 in the MxA stalk which is divided in α1 N and α1 C by a 10 amino acid long loop, L1, introducing a 30° kink. A putative loop L2 (amino acids 438-447) opposite of the deduced position of the G-domain is not visible in our structure. L2 was previously demonstrated to be the target of a functionally neutralising monoclonal antibody 9,10 . Helix α2 runs anti-parallel to α1 back to the G-domain. It ends in a short loop L3 and is followed by helix α3 that extends in parallel to α1. The 40 amino acid long loop L4 (residues 532-572) is at the equivalent sequence position as the PH domain of dynamin (Fig. 1a, Supp. Fig. 3) and is absent in our model. L4 is predicted to be unstructured and was previously shown to be proteinase K sensitive 11 .At the C-terminus, the GED supplies 44 residues to helix α4 which proceeds in parallel to helix α2 back to the G-domain. It is followed by a short helix α5 which directs the polypeptide chain towards the N-terminus of the MD. The carboxy(C-)-terminal 30 highly conserved residues of the GED known to be involved in antiviral specificity 12 are missing in our model. In dynamin, the corresponding residues were shown to directly interact with the G-domain 13 . The stalk of MxA is divergent from the corresponding structures of other dynamin superfamily members, such as GBP1 14 , EHD2 15 and BDLP 16 although some features are shared (Supp. Fig. 4). 4 In the crystal lattice, each MxA stalk is assembled in a criss-cross pattern resulting in a linear oligomer, where each stalk contributes three distinct interfaces (Fig. 1c). Such an arrangement of the stalks is plausible for the Mx oligomer since all G-domains would be located at one side of the oligomer whereas the putative substrate-binding site in L2 and L4 would be located at the opposite side (Fig. 1b, c).The hydrophobic interface-1 covering 1300 Å 2 is conserved among Mx proteins and dynamins and has a two-fold symmetry between the associating monomers (Fig....

Mx proteins are interferon-induced GTPases that belong to the dynamin superfamily of large GTPases. Similarities include a high molecular weight, a propensity to self-assemble, a relatively low affinity for GTP, and a high intrinsic rate of GTP hydrolysis. A unique property of Mx GTPases is their antiviral activity against a wide range of RNA viruses, including bunyaand orthomyxoviruses. The human MxA GTPase accumulates in the cytoplasm of interferon-treated cells, partly associating with the endoplasmic reticulum. In the case of bunyaviruses, MxA interferes with transport of the viral nucleocapsid protein (N) to the Golgi compartment, the site of virus assembly. In the case of Thogoto virus (an orthomyxovirus), MxA prevents the incoming viral nucleocapsids from being transported into the nucleus, the site of viral transcription and replication. In both cases, the GTP-binding and carboxyterminal effector functions of MxA are required for target recognition. In general, Mx GTPases appear to detect viral infection by sensing nucleocapsid-like structures. As a consequence, these viral components are trapped and sorted to locations where they become unavailable for the generation of new virus particles.

Rift Valley fever virus (RVFV), a phlebovirus of the family Bunyaviridae, is a major public health threat in Egypt and sub-Saharan Africa. The viral and host cellular factors that contribute to RVFV virulence and pathogenicity are still poorly understood. All pathogenic RVFV strains direct the synthesis of a nonstructural phosphoprotein (NSs) that is encoded by the smallest (S) segment of the tripartite genome and has an undefined accessory function. In this report, we show that MP12 and clone 13, two attenuated RVFV strains with mutations in the NSs gene, were highly virulent in IFNAR ؊/؊ mice lacking the alpha/beta interferon (IFN-␣/␤) receptor but remained attenuated in IFN-␥ receptor-deficient mice. Both attenuated strains proved to be excellent inducers of early IFN-␣/␤ production. In contrast, the virulent strain ZH548 failed to induce detectable amounts of IFN-␣/␤ and replicated extensively in both IFN-competent and IFN-deficient mice. Clone 13 has a defective NSs gene with a large in-frame deletion. This defect in the NSs gene results in expression of a truncated protein which is rapidly degraded. To investigate whether the presence of the wild-type NSs gene correlated with inhibition of IFN-␣/␤ production, we infected susceptible IFNAR ؊/؊ mice with S gene reassortant viruses. When the S segment of ZH548 was replaced by that of clone 13, the resulting reassortants became strong IFN inducers. When the defective S segment of clone 13 was exchanged with the wild-type S segment of ZH548, the reassortant virus lost the capacity to stimulate IFN-␣/␤ production. These results demonstrate that the ability of RVFV to inhibit IFN-␣/␤ production correlates with viral virulence and suggest that the accessory protein NSs is an IFN antagonist.Alpha/beta interferons (IFNs-␣/␤) are key components of the innate immune mechanisms that protect the host against invading viruses (23,31,42). The extraordinary power of the IFN system has prompted many viruses to adopt strategies that inhibit IFN production or action (for a review, see reference 13). We therefore considered the possibility that virulent strains of Rift Valley fever virus (RVFV) differ from attenuated strains in their capacity to actively antagonize the IFN response of the host. RVFV is a mosquito-borne virus which belongs to the Bunyaviridae family (Phlebovirus genus). Periodically, the virus causes epidemics and epizootics in subSaharan countries of Africa and in Egypt. In humans, infection leads to a wide spectrum of clinical symptoms that range from a benign fever to severe encephalitis, retinitis, and fatal hepatitis with hemorrhagic fever (27). Among animals, sheep and goats are severely affected.Like all members of the family, RVFV possesses a singlestranded segmented RNA genome composed of a large (L), a medium (M), and a small (S) segment (for reviews, see references 9, 11, and 40). The L and M segments are of negative polarity. The L segment codes for the RNA-dependent RNA polymerase. The M segment codes for a polyprotein which is the precursor to the...