A Fluorescence Polarization Assay for High-Throughput Screening of Inhibitors against HIV-1 Nef-Mediated CD4 Downregulation.

Therapeutic inhibition of the viral protein Nef is an intriguing direction of antiretroviral drug discovery-it may revitalize immune mechanisms to target, and potentially clear, HIV-1-infected cells. Of the many cellular functions of Nef, the most conserved is the downregulation of surface CD4, which takes place through Nef hijacking the clathrin adaptor protein complex 2 (AP2)-dependent endocytosis. Our recent crystal structure has unraveled the molecular details of the CD4-Nef-AP2 interaction. Guided by the new structural knowledge, we have developed a fluorescence polarization-based assay for inhibitor screening against Nef's activity on CD4. In our assay, AP2 is included along with Nef to facilitate the proper formation of the CD4-binding pocket, and a fluorescently labeled CD4 cytoplasmic tail binds competently to the Nef-AP2 complex generating the desired polarization signal. The optimized assay has a good signal-to-noise ratio, excellent tolerance of DMSO and detergent, and the ability to detect competitive binding at the targeted Nef pocket, making it suitable for high-throughput screening.

A Fluorescence Polarization Assay for High-Throughput Screening of Inhibitors against HIV-1 Nef-Mediated MHC-I Downregulation.

The multifunctional, HIV-1 accessory protein Nef enables infected cells to evade host immunity and thus plays a key role in viral pathogenesis. One prominent function of Nef is the downregulation of major histocompatibility complex class I (MHC-I), which disrupts antigen presentation and thereby allows the infected cells to evade immune surveillance by the cytotoxic T cells. Therapeutic inhibition of this Nef function is a promising direction of antiretroviral drug discovery as it may revitalize cytotoxic T cells to identify, and potentially clear, hidden HIV-1 infections. Guided by the crystal structure of the protein complex formed between Nef, MHC-I, and the hijacked clathrin adaptor protein complex 1 (AP1), we have developed a fluorescence polarization-based assay for inhibitor screening against Nef's activity on MHC-I. The optimized assay has a good signal-to-noise ratio, substantial tolerance of DMSO, and excellent ability to detect competitive inhibition, indicating that it is suitable for high-throughput screening.

Structural basis of CD4 downregulation by HIV-1 Nef.

The HIV-1 Nef protein suppresses multiple immune surveillance mechanisms to promote viral pathogenesis and is an attractive target for the development of novel therapeutics. A key function of Nef is to remove the CD4 receptor from the cell surface by hijacking clathrin- and adaptor protein complex 2 (AP2)-dependent endocytosis. However, exactly how Nef does this has been elusive. Here, we describe the underlying mechanism as revealed by a 3.0-Å crystal structure of a fusion protein comprising Nef and the cytoplasmic domain of CD4 bound to the tetrameric AP2 complex. An intricate combination of conformational changes occurs in both Nef and AP2 to enable CD4 binding and downregulation. A pocket on Nef previously identified as crucial for recruiting class I MHC is also responsible for recruiting CD4, revealing a potential approach to inhibit two of Nef's activities and sensitize the virus to immune clearance.