Structural insights into Cullin4-RING ubiquitin ligase remodelling by Vpr from simian immunodeficiency viruses

Author summary Due to the limited size of virus genomes, virus replication critically relies on host cell components. In addition to the host cell's energy metabolism and its DNA replication and protein synthesis apparatus, the protein degradation machinery is an attractive target for viral re-appropriation. Certain viral factors divert the specificity of host ubiquitin ligases to antiviral host factors, in order to mark them for destruction by the proteasome, to lift intracellular barriers to virus replication. Here, we present molecular details of how the simian immunodeficiency virus accessory protein Vpr interacts with a substrate receptor of host Cullin4-RING ubiquitin ligases, and how this interaction redirects the specificity of Cullin4-RING to the antiviral host factor SAMHD1. The studies uncover the mechanism of Vpr-induced SAMHD1 recruitment and subsequent ubiquitylation. Moreover, by comparison to related accessory proteins from other immunodeficiency virus species, we illustrate the surprising variability in the molecular strategies of SAMHD1 counteraction, which these viruses adopted during evolutionary adaptation to their hosts. Lastly, our work also provides deeper insight into the inner workings of the host's Cullin4-RING ubiquitylation machinery. Viruses have evolved means to manipulate the host's ubiquitin-proteasome system, in order to down-regulate antiviral host factors. The Vpx/Vpr family of lentiviral accessory proteins usurp the substrate receptor DCAF1 of host Cullin4-RING ligases (CRL4), a family of modular ubiquitin ligases involved in DNA replication, DNA repair and cell cycle regulation. CRL4(DCAF1) specificity modulation by Vpx and Vpr from certain simian immunodeficiency viruses (SIV) leads to recruitment, poly-ubiquitylation and subsequent proteasomal degradation of the host restriction factor SAMHD1, resulting in enhanced virus replication in differentiated cells. To unravel the mechanism of SIV Vpr-induced SAMHD1 ubiquitylation, we conducted integrative biochemical and structural analyses of the Vpr protein from SIVs infecting Cercopithecus cephus (SIVmus). X-ray crystallography reveals commonalities between SIVmus Vpr and other members of the Vpx/Vpr family with regard to DCAF1 interaction, while cryo-electron microscopy and cross-linking mass spectrometry highlight a divergent molecular mechanism of SAMHD1 recruitment. In addition, these studies demonstrate how SIVmus Vpr exploits the dynamic architecture of the multi-subunit CRL4(DCAF1) assembly to optimise SAMHD1 ubiquitylation. Together, the present work provides detailed molecular insight into variability and species-specificity of the evolutionary arms race between host SAMHD1 restriction and lentiviral counteraction through Vpx/Vpr proteins.

Automated summary

Viruses rely on host cell components for replication, with certain viral factors altering the specificity of host ubiquitin ligases to down-regulate antiviral factors. Studies have shown how viruses exploit the dynamic assembly of the CRL4(DCAF1) complex to optimize SAMHD1 ubiquitylation.