Multilayered Mechanism of CD4 Downregulation by HIV-1 Vpu Involving Distinct ER Retention and ERAD Targeting Steps

Javier G Magadán,Klaus Strebel,Yihong Ye,Rachid Sougrat,F Javier Pérez-Victoria,Juan S Bonifacino,Michael Emerman

doi:10.1371/journal.ppat.1000869

Abstract

A key function of the Vpu protein of HIV-1 is the targeting of newly-synthesized CD4 for proteasomal degradation. This function has been proposed to occur by a mechanism that is fundamentally distinct from the cellular ER-associated degradation (ERAD) pathway. However, using a combination of genetic, biochemical and morphological methodologies, we find that CD4 degradation induced by Vpu is dependent on a key component of the ERAD machinery, the VCP-UFD1L-NPL4 complex, as well as on SCFβ-TrCP-dependent ubiquitination of the CD4 cytosolic tail on lysine and serine/threonine residues. When degradation of CD4 is blocked by either inactivation of the VCP-UFD1L-NPL4 complex or prevention of CD4 ubiquitination, Vpu still retains the bulk of CD4 in the ER mainly through transmembrane domain interactions. Addition of a strong ER export signal from the VSV-G protein overrides this retention. Thus, Vpu exerts two distinct activities in the process of downregulating CD4: ER retention followed by targeting to late stages of ERAD. The multiple levels at which Vpu engages these cellular quality control mechanisms underscore the importance of ensuring profound suppression of CD4 to the life cycle of HIV-1.

Highlights

Human Immunodeficiency Virus-1 and -2 (HIV-1 and -2), as well as Simian Immunodeficiency Virus (SIV), selectively target helper T-lymphocytes and macrophages/monocytes by binding of their viral envelope protein, Env, to a combination of two celltype-specific surface receptors: a type 1 transmembrane protein, CD4, and a seven-transmembrane chemokine receptor, CXCR4 or CCR5 [1]
We show that Vpu utilizes at least part of the endoplasmic reticulumassociated degradation machinery to dispose of CD4
Disabling this machinery prevents CD4 degradation induced by Vpu but, surprisingly, does not allow transport of CD4 to the cell surface

Summary

Introduction

Human Immunodeficiency Virus-1 and -2 (HIV-1 and -2), as well as Simian Immunodeficiency Virus (SIV), selectively target helper T-lymphocytes and macrophages/monocytes by binding of their viral envelope protein, Env, to a combination of two celltype-specific surface receptors: a type 1 transmembrane protein, CD4, and a seven-transmembrane chemokine receptor, CXCR4 or CCR5 [1]. An early and lasting effect of infection is the downregulation of CD4 from the host cell surface [2,3]. CD4 downregulation compromises the ability of T-lymphocytes to become activated in response to immunogenic peptides bound to MHC class II molecules on the surface of antigen-presenting cells [7]. These effects all contribute to propagation of the infection, eventually leading to depletion of CD4-positive cells and development of acquired immunodeficiency syndrome (AIDS) in untreated individuals

Methods

Results

Discussion

Conclusion