Review 3: "MX2 Restricts HIV-1 and Herpes Simplex Virus Type 1 by Forming Cytoplasmic Biomolecular Condensates that Mimic Nuclear Pore Complexes"

RR:C19 Evidence Scale rating by reviewer:

• Strong. The main study claims are very well-justified by the data and analytic methods used. There is little room for doubt that the study produced has very similar results and conclusions as compared with the hypothetical ideal study. The study’s main claims should be considered conclusive and actionable without reservation.

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Review: The human myxovirus resistance protein 2 (MX2) restricts the replication of HIV-1 and herpes viruses by interacting with the viral capsids in infected cells. However, the mechanisms of MX2 action and the roles of additional host factors in this process remain largely unknown. This study shows that MX2 forms cytoplasmic condensates together with nuclear pore proteins, which trap viral capsids and induce premature genome release, thereby impairing proper nuclear targeting of the respective viral genomes.

State-of-the-art technology was used to reveal that MX2 can interact with a number of proteins typically present in nuclear pores. Follow-up experiments aimed at demonstrating the importance of particular interaction partners for MX2-mediated antiviral activity did not yield very conclusive results, presumably due to insufficient RNAi-mediated knockdown of the candidate host factors. In contrast, however, the confocal microscopy data showing that MX2 and some of the newly identified partner proteins can form dynamic cytoplasmic condensates are very convincing and well controlled. In particular, the super-resolution confocal microscopy used to map the MX2 domains required for condensate formation and to visualize the fate of incoming viral particles yielded conclusive results. 

The available data provide a new and fascinating hypothesis of how MX2 might confer antiviral activity. The new model proposes that MX2 triggers the formation of cytoplasmic condensates that serve as nuclear pore decoys which trap incoming viral capsids. Such decoy-based mechanisms of MX2 action have previously not been considered. Thus, the current study will likely boost additional research and will greatly enhance the current knowledge in the field. 

Disappointingly, the discussion devoted to the broader significance of the new findings is too limited. Some speculation about whether a similar mode of action might likewise explain the antiviral activity of MX1 (a related cellular restriction factor that confers resistance to several pathogenic viruses, including influenza A viruses) would have been appropriate. In fact, the laboratory of Pravin Sehgal (DOI: 10.1007/s12038-021-00187-x) recently showed that MX1 can form metastable biomolecular cytoplasmic condensates, pretty much like MX2. Thus, the possibility exists that MX2 and MX1 proteins are both able to confer virus resistance by inducing decoy structures which disturb proper intracellular trafficking of viral components, as already suggested for MX1 in La Crosse virus restriction (DOI: 10.1073/pnas.052430399). If confirmed by future work, the insights provided in this manuscript will have a lasting impact on our understanding of innate antiviral immune responses.