Discovery of a Novel Antiviral Effect of the Restriction Factor SPOC1 against Human Cytomegalovirus.

The chromatin-remodeler SPOC1 (PHF13) is a transcriptional co-regulator and has been identified as a restriction factor against various viruses, including human cytomegalovirus (HCMV). For HCMV, SPOC1 was shown to block the onset of immediate-early (IE) gene expression under low multiplicities of infection (MOI). Here, we demonstrate that SPOC1-mediated restriction of IE expression is neutralized by increasing viral titers. Interestingly, our study reveals that SPOC1 exerts an additional antiviral function beyond the IE phase of HCMV replication. Expression of SPOC1 under conditions of high MOI resulted in severely impaired viral DNA replication and viral particle release, which may be attributed to inefficient viral transcription. With the use of click chemistry, the localization of viral DNA was investigated at late time points after infection. Intriguingly, we detected a co-localization of SPOC1, RNA polymerase II S5P and polycomb repressor complex 2 (PRC2) components in close proximity to viral DNA in areas that are hypothesized to harbor viral transcription sites. We further identified the N-terminal domain of SPOC1 to be responsible for interaction with EZH2, a subunit of the PRC2 complex. With this study, we report a novel and potent antiviral function of SPOC1 against HCMV that is efficient even with unrestricted IE gene expression.

IE1 of Human Cytomegalovirus Inhibits Necroptotic Cell Death via Direct and Indirect Modulation of the Necrosome Complex.

Programmed necrosis is an integral part of intrinsic immunity, serving to combat invading pathogens and restricting viral dissemination. The orchestration of necroptosis relies on a precise interplay within the necrosome complex, which consists of RIPK1, RIPK3 and MLKL. Human cytomegalovirus (HCMV) has been found to counteract the execution of necroptosis during infection. In this study, we identify the immediate-early 1 (IE1) protein as a key antagonist of necroptosis during HCMV infection. Infection data obtained in a necroptosis-sensitive cell culture system revealed a robust regulation of post-translational modifications (PTMs) of the necrosome complex as well as the importance of IE1 expression for an effective counteraction of necroptosis. Interaction analyses unveiled an association of IE1 and RIPK3, which occurs in an RHIM-domain independent manner. We propose that this interaction manipulates the PTMs of RIPK3 by promoting its ubiquitination. Furthermore, IE1 was found to exert an indirect activity by modulating the levels of MLKL via antagonizing its interferon-mediated upregulation. Overall, we claim that IE1 performs a broad modulation of innate immune signaling to impede the execution of necroptotic cell death, thereby generating a favorable environment for efficient viral replication.