Structure and mechanism of a novel cytomegaloviral DCAF mediating interferon antagonism

Human cytomegalovirus (CMV) is a highly relevant and ubiquitously distributed human pathogen. Its rodent counterparts such as mouse and rat CMV serve as common infection models. Here, we conducted the first global proteome profiling of rat CMV-infected cells and uncovered a pronounced loss of the transcription factor STAT2, which is crucial for interferon signalling. Deletion mutagenesis documented that STAT2 is targeted by the viral protein E27. Cellular and in vitro analyses showed that E27 exploits host-derived Cullin4-RING ubiquitin ligases (CRL4) to induce poly-ubiquitylation and proteasomal degradation of STAT2. A cryo-electron microscopic structure determination revealed how E27 mimics molecular surface properties of cellular CRL4 substrate receptors called DDB1- and Cullin4-associated factors (DCAFs) to displace them from the catalytic core of CRL4. Moreover, structural analyses elucidated the mechanism of STAT2 recruitment and indicate that E27-binding additionally disturbs STAT2-dependent interferon signalling by occupying its IRF9 binding interface. For the first time, these data provide structural insights into cytomegalovirus-encoded interferon antagonism and establish an atomic model for STAT2 counteraction by CRL4 misappropriation with important implications for viral immune evasion.