Capsid-dependent innate sensing and restriction or retroviral replication complexes

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The main objective of this project is to elucidate the differential role of intact retroviral capsids regarding formation and nuclear entry of functional replication complexes with concomitant evasion from innate sensing of the viral nucleic acids. Our studies focus on HIV-1 (and partially HIV-2) and MLV since these two viruses exhibit fundamental differences in the relevant functions. In the first funding period, we have: (i) established and validated assays to detect and characterize functional replication complexes for both viruses, (ii) observed and characterized unexpected differences in the architecture of HIV and MLV mature capsids by structural analysis, (iii) identified a function of the capsid-binding host factor CPSF6 in nuclear entry and nucleoplasmic localization of HIV-1 replication complexes, and (vi) obtained evidence for a different mechanism of the capsid-targeting drug PF74 in blocking HIV replication. Together with the recently reported identification of IP6 as natural capsid-stabilizing molecule and of NONO as a potential bridging factor targeting cGAS sensing to the nuclear retroviral complex via capsid binding, these results provide the basis for the proposed work programme in the second funding period. The main hypothesis remains that the retroviral capsid shields the viral nucleic acids from innate sensing, while itself being the target of antiviral recognition, and these aspects differ depending on cell type and retrovirus. Specifically, we have the following aims for the second funding period: (i) comparative characterization of structure and stability of wild-type and mutant HIV-1 and MLV capsids, (ii) analysis of the association of retroviral replication complexes with known and presumed host factors and nucleic acid sensing molecules, and (iii) isolation and characterization of bona fide replication complexes and identification of associated host factors.