Our aims for Vif focus on the roles of CBFp, host complex modifications, and the biology of Vif interactions with newly identified partners. Our studies will establish new paradigms not only for Vif-host interactions, but also more generally for restriction factors that engage the ubiquitin-proteasome system. Our proteomics studies identified additional Vif partners, including AMRA1 and SQSTM implicated in autophagy, as well as with the transcriptional corepressor complex NCOR1/HDAC3/GPS2/TBL1R [2]. Following up these discoveries will potentially define unanticipated alternative roles for Vit such as regulating chromosomal gene expression, in HIV pathogenesis. Vif is a highly significant protein. It is essential for the spread of HIV and represents a conserved viral strategy for counteracting host restriction factors. Vif triggers degradation of members of the AP0BEC3 (A3) family of cytosine deaminases that otherwise halt viral HIV replication by causing lethal hypermutation of the viral genome [6, 7]. Vif hijacks a cellular Cullin-RING ubiquitin E3 ligase (CRL) that acts in the last step of a three-enzyme, E1-E2-E3 cascade to promote ubiquitination and subsequent degradation of A3 substrates [8- 1.0]. The Vif E3 ligase consists ofthe CUL5/RBX2 scaffold and substrate adaptors, Elongins B and C (ELOBC). ELOBC binds ~50 different Suppressor of Cytokine Signaling (SOCS) proteins, which are substrate specificity factors for CRL5 [11, 12]. SOCS proteins contain a three-helix motif (the SOCS box) that engages ELOC and CUL5, respectively [13, 14]. The C-terminus of Vif acts as a molecular mimic of cellular SOCS proteins [15,16], and additional regions of Vif that are important for function have been mapped [17-19]. Importantly, we identified CBFp as a critical host factor needed to stabilize Vit to bind the CRL5 machinery and trigger A3G degradation [1, 2]. CRL5 also requires modification by NEDD8 for Vif to counteract A3G [10]. NEDD8ylation of CRLs requires an E1-E2-E3 cascade much like ubiqultin [20]. The NEDD8 modification is coupled to NEDD E3 (DCN~ Defective in Cullin Neddylation) proteins, which bridge CRL with NEDD8 E2 via acetylation and may be antagonized by CRL phosphorylation [21-25], A potent, mechanism-based NEDD8 El inhibitor (MLN4924) is in Phase 1 clinical trials for treatment of hematologic cancers [26, 27], underscoring the role of NEDD8 in controlling cellular protein homeostasis. These observations suggest a novel antiretroviral strategy in which perturbing host NEDD8 pathways or PTMs could suppress CRL function and restore the innate immunity provided by restriction factors.
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