Elucidating the Structural Bases of HIV-1-Induced CD4 Degradation

点击翻译按钮获取中文摘要

PROJECT SUMMARY: Currently available antiretroviral therapy is effective in treating HIV infection but cannot eliminate it. Once treat- ment is stopped, viral rebound typically occurs within weeks. Infected individuals must take antiretroviral drugs throughout their lives and, as a consequence, may experience drug resistance and side effects. Novel antiretro- virals that can better treat or even eliminate HIV infection are highly desired. One promising and yet underap- preciated approach is to develop therapeutics that disrupt HIV-induced downregulation of the CD4 receptor and thereby unleash CD4’s remarkable inhibitory power to combat the infection. CD4 is the entry receptor for HIV and plays a vital role in the initial stage of the infection, but its presence later in the virus’ replication cycle strongly inhibits viral replication and sensitizes infected cells to immune-clearance. The potency of CD4 as an inhibitor of the virus is reflected by the stringent, highly concerted mechanisms HIV uses to antagonize CD4. Two viral proteins, Nef and Vpu, are involved in downregulating CD4 from the cell surface and from the endoplasmic reticulum, respectively, leading to degradation of CD4 in either the lysosome (Nef-mediated pathway) or the proteasome (Vpu-mediated pathway). The level of effort here by HIV is arguably unmatched—no other host factors including the well-known restriction factors are antagonized in such a multifaceted manner. This suggests that restoring CD4 levels in infected cells may be significantly damaging to the virus and significantly beneficial to the host. Designing or developing therapeutics to restore CD4 levels, however, is greatly hindered by the lack of high-resolution structural information on the pertinent molecular assemblies, e.g., how Nef and Vpu each recruit CD4 into hijacked host trafficking and degradation machineries. In this project, we will solve such high- resolution structures to gain the knowledge necessary for this drug discovery approach. Our specific aims are: 1) Elucidate how the viral Nef protein hijacks the clathrin adaptor protein AP1 to enable the retention of CD4 in endosomes, thus facilitating the delivery of CD4 to lysosomes for degradation. 2) Reveal how Nef hijacks the host trafficking protein ALIX to channel CD4 into multivesicular bodies and lysosomes for degradation; investigate whether and, if so how, the ALIX-like protein PTPN23 participates in Nef-mediated degradation of CD4 and/or other host factors. 3) Elucidate how the viral Vpu protein targets newly synthesized CD4 in the ER to mediate its polyubiquiti- nation via the β-TrCP/cullin1 complex, thereby redirecting CD4 to the proteasome for degradation. Successful completion of this work should reveal opportunities for the design and/or development of novel ther- apeutics capable of disrupting HIV-induced CD4 degradation, thus restoring CD4 in infected cells to inhibit HIV replication or even eliminate the infection.