HIV Viral Infectivity Factor BC-box in complex with ElonginB and ElonginC

David Torres '16 and Joey Duronio '16


I. Introduction

The Human Immunodefiency Virus (HIV) as well as other similar retroviruses contain a viral infectivity factor (Vif) that inhibits certain anti-viral activity in human cells. Vif is used in viral replication by hijacking the Cullin5 ECS ubiquitin ligase, which consists of Cullin5, ElonginBC, andRbx2, and targeting APOBEC3G for degradation. APOBEC3G is a cellular enzyme and part of the APOBEC superfamily of proteins. This family of proteins plays a role in anti-viral immunity. Vif inhibits APOBEC3G from entering the new budding virus and hypermutating the genome to render the virus essentially dead.

Vif is able to hijack the ECS ubiquitin ligase by binding to ElonginBC via the BC-box domain. It then uses the hosts own ubiquitination and degradation system to destroy the APOBEC3G.

II. General Structure of Vif and Elongin-Cullin-SOCS-box (ECS)

     The Vif BC-box binds ElonginC and ElonginB similarly to how the human cellular SOCS box binds the ECS ubiquitin ligase.A normal cellular ECS ubiquitin ligase is made up ofElonginB(EloB) , ElonginC(EloC), Cullin5 (Cul5) , Rbx2, and a SOCS-box protein. SOCS-box is formally known as the suppressor of cytokine signaling box proteins. SOCS box proteins are characterized by a conserved interaction domain, which offers a link between cellular substrates and the E3 ubiquitin ligase. This conserved domain of SOCS box includes the BC box, which binds EloB and EloC (EloBC), and the Cullin box, which is hypothesized to participate in the binding of either Cul5 or Cul2. Crystal structures show that the SOCS box has a conserved region composed of a BC box helix, a small 90 turn, a second short helix ending in a region rich with prolines, a loop, and a third helix. The first helix is considered as the BC box and the rest is referred to as the Cullin box.

           The structure of Vif itself can be divided into two domains, with a zinc ion binding between them. The larger domain (b domain)binds one side of a protein called CBF-b. The smaller domain of Vif (a domain) interacts with EloC and Cul5. The carboxy-terminus of CBF-b is sandwiched between the two domains of Vif. By interacting with CBF-b, Cul5 and EloC, Vif organizes the formation of the pentameric complex. This unique zinc-finger motif of Vif located between the two domains makes no contacts with the other proteins but stabilizes the conformation of the smaller domain, which may be important in Vif-Cul5 binding.


III. Vif Binding APOBEC3G and ElonginBC

            Vif uses conserved regions to influence the degradation of APOBEC3G. Vif acts as the substrate of APOBEC3G by mimicking the SOCS box domain of a normal cellular ECS ubiquitin ligase. Research proposes that the Vif N-terminal domain interacts with APOBEC3G, while its C-terminal domain recruits the ECS ligase through two conserved regions. HIV Vif contains a BC-box region that interacts with EloC and is crucial for recruiting the cellular E3 ubiquitin ligase. A number of Vif residues V142 , L145 , L148,L149,A152 , AND L153 create the hydrophobic face that interacts with EloC. Hydrogen bonding between the backbone carbonyl of Vif G143 and EloC Y76 also contributes to the Vif-EloC interaction. A second ECS ligase-binding domain is a conserved region containing histidine and cysteine residues (this region is called the zinc-binding motif). This zinc-binding motif interacts with Cul5. Mutations of these His or Cys residues destroys the Vif-Cul5 interaction.

            The interaction between a third Vif C-terminal domain, the Vif Cullin box, and the ECS ubiquitin ligase has not been thoroughly studied. However, the Vif-Cul5 interaction has been identified by many experiments. These experiments were not able to identify a direct interaction between the Vif Cullin box and Cul5.

IV. Future Studies

    In the past few years, much has been learned about how Vif recruits the ECS ubiquitin ligase to destroy APOBEC3G. However, many of the details still remain unclear due to the lack of structural data on Vif itself (i.e. Vif not in complex with the EloBC box). Vif has yet to be fully crystallized on its own which is why the zinc motif residues and the terminal domains cannot be visualized here. Another issue in studying how Vif interacts with ECS ubiquitin ligase is the lack of structural data on the Cullin box. A possible future study ito find out more details of the structure of these two molecules would be to isolate Vif and the Cullin box and perform X-ray crystallography on them to obtain the crystal three-dimensional structure of Vif and the Cullin box. This crystal structure could provide insight into more of the chemical interactions that occur between Vif, ECS ubiquitin ligase, and the Cullin box. These studies could also assist in the creation of a new anti-retroviral drug therapy targeting Vif and allowing the human immune system to fight HIV.


V. References

Stanley Bradford J, Ehrlich Elana S, Short Leslie, Yu Yunkai, Xiao Zuoxiang, Yu Fang-Xiao, Xiong Yong. Structural Insight into the Human Immunodeficiency Virus Vif SOCS Box and Its Role in Human E3 Ubiquitin Ligase. Assembly Department of Molecular Biophysics and Biochemistry, Yale University, P.O. Box 208114, New Haven, Connecticut 06510,1 and Department of Molecular Microbiology and Immunology, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland 21205. 2010.

Bergeron JRC, Huthoff H, Veselkov DA, Beavil RL, Simpson PJ, et al. (2010) The SOCS-Box of HIV-1 Vif Interacts with ElonginBC by Induced-Folding to Recruit Its Cul5-Containing Ubiquitin Ligase Complex. PLoS Pathog 6(6): e1000925. doi:10.1371/journal.ppat.1000925. 2010.

Marcisin Sean R, Engen John R. Molecular Insight into the Conformational Dynamics of the Elongin BC Complex and Its Interaction with HIV-1 Vif. Department of Chemistry and Chemical Biology and the Barnett Institute of Chemical and Biological Analysis, Northeastern University, Boston, MA 02115, USA. 2010.

Rose Kristine M, Mariana Marin, Kozak Susan L, Kabat David. The viral infectivity factor (Vif) of HIV-1 unveiled. Trends in Molecular Medicine. Vol 10, Issue 6. 2004.

Guo Yingying, Dong Liyong, Qiu Xiaolin, Wang Yishu, Zhang Bailing, Liu Hongnan, Yu You, Zang Yi, Yang Maojun, and Huang Zhiwei. Structural basis for hijacking CBF-β and CUL5 E3 ligase complex by HIV-1 Vif. Nature 505, 229–233. January 08 2014.



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