AP-2 Is the Crucial Clathrin Adaptor Protein for CD4 Downmodulation by HIV-1 Nef in Infected Primary CD4+ T Cells

AP-2 Adaptor Complex-Dependent Enhancement of HIV-1 Replication by Nef in the Absence of the Nef/AP-2 Targets SERINC5 and CD4

Human immunodeficiency virus type 1 (HIV-1) Nef hijacks the clathrin adaptor complex 2 (AP-2) to downregulate the viral receptor CD4 and the antiviral multipass transmembrane proteins SERINC3 and SERINC5, which inhibit the infectivity of progeny virions when incorporated. In Jurkat Tag T lymphoid cells lacking SERINC3 and SERINC5, Nef is no longer required for full progeny virus infectivity and for efficient viral replication. However, in MOLT-3 T lymphoid cells, HIV-1 replication remains highly dependent on Nef even in the absence of SERINC3 and SERINC5. Using a knockout (KO) approach, we now show that the Nef-mediated enhancement of HIV-1 replication in MOLT-3 cells does not depend on the Nef-interacting kinases LCK and PAK2. Furthermore, Nef substantially enhanced HIV-1 replication even in triple-KO MOLT-3 cells that simultaneously lacked the three Nef/AP-2 targets, SERINC3, SERINC5, and CD4, and were reconstituted with a Nef-resistant CD4 to permit HIV-1 entry. Nevertheless, the ability of Nef mutants to promote HIV-1 replication in the triple-KO cells correlated strictly with the ability to bind AP-2. In addition, knockdown and reconstitution experiments confirmed the involvement of AP-2. These observations raise the possibility that MOLT-3 cells express a novel antiviral factor that is downregulated by Nef in an AP-2-dependent manner. IMPORTANCE The HIV-1 Nef protein hijacks a component of the cellular endocytic machinery called AP-2 to downregulate the viral receptor CD4 and the antiviral cellular membrane proteins SERINC3 and SERINC5. In the absence of Nef, SERINC3 and SERINC5 are taken up into viral particles, which reduces their infectivity. Surprisingly, in a T cell line called MOLT-3, Nef remains crucial for HIV-1 spreading in the absence of SERINC3 and SERINC5. We now show that this effect of Nef also does not depend on the cellular signaling molecules and Nef interaction partners LCK and PAK2. Nef was required for efficient HIV-1 spreading even in triple-knockout cells that completely lacked Nef/AP-2-sensitive CD4, in addition to the Nef/AP-2 targets SERINC3 and SERINC5. Nevertheless, our results indicate that the enhancement of HIV-1 spreading by Nef in the triple-knockout cells remained AP-2 dependent, which suggests the presence of an unknown antiviral factor that is sensitive to Nef/AP-2-mediated downregulation.

Flow cytometry based-FRET: basics, novel developments and future perspectives

Förster resonance energy transfer (FRET) is a widespread technology used to analyze and quantify protein interactions in multiple settings. While FRET is traditionally measured by microscopy, flow cytometry based-FRET is becoming popular within the last decade and more commonly used. Flow cytometry based-FRET offers the possibility to assess FRET in a short time-frame in a high number of cells thereby allowing stringent and statistically robust quantification of FRET in multiple samples. Furthermore, established, simple and easy to implement gating strategies facilitate the adaptation of flow cytometry based-FRET measurements to most common flow cytometers. We here summarize the basics of flow cytometry based-FRET, highlight recent novel developments in this field and emphasize on exciting future perspectives.

An Amino Acid Polymorphism within the HIV-1 Nef Dileucine Motif Functionally Uncouples Cell Surface CD4 and SERINC5 Downregulation

Serine incorporator 5 (SERINC5) reduces the infectivity of progeny HIV-1 virions by incorporating into the outer host-derived viral membrane during egress. To counter SERINC5, the HIV-1 accessory protein Nef triggers SERINC5 internalization by engaging the adaptor protein 2 (AP-2) complex using the [D/E]xxxL[L/I]₁₆₇ Nef dileucine motif. Nef also engages AP-2 via its dileucine motif to downregulate the CD4 receptor. Although these two Nef functions are related, the mechanisms governing SERINC5 downregulation are incompletely understood. Here, we demonstrate that two primary Nef isolates, referred to as 2410 and 2391 Nef, acquired from acutely HIV-1 infected women from Zimbabwe, both downregulate CD4 from the cell surface. However, only 2410 Nef retains the ability to downregulate cell surface SERINC5. Using a series of Nef chimeras, we mapped the region of 2391 Nef responsible for the functional uncoupling of these two antagonistic pathways to the dileucine motif. Modifications of the first and second x positions of the 2410 Nef dileucine motif to asparagine and aspartic acid residues, respectively (ND₁₆₄), impaired cell surface SERINC5 downregulation, which resulted in reduced infectious virus yield in the presence of SERINC5. The ND₁₆₄ mutation additionally partially impaired, but did not completely abrogate, Nef-mediated cell surface CD4 downregulation. Furthermore, the patient infected with HIV-1 encoding 2391 Nef had stable CD4⁺ T cell counts, whereas infection with HIV-1 encoding 2410 Nef resulted in CD4⁺ T cell decline and disease progression. IMPORTANCE A contributing factor to HIV-1 persistence is evasion of the host immune response. HIV-1 uses the Nef accessory protein to evade the antiviral roles of the adaptive and intrinsic innate immune responses. Nef targets SERINC5, a restriction factor which potently impairs HIV-1 infection by triggering SERINC5 removal from the cell surface. The molecular determinants underlying this Nef function remain incompletely understood. Recent studies have found a correlation between the extent of Nef-mediated SERINC5 downregulation and the rate of disease progression. Furthermore, single-residue polymorphisms outside the known Nef functional motifs can modulate SERINC5 downregulation. The identification of a naturally occurring Nef polymorphism impairing SERINC5 downregulation in this study supports a link between Nef downregulation of SERINC5 and the rate of plasma CD4⁺ T cell decline. Moreover, the observed functional impairments of this polymorphism could provide clues to further elucidate unknown aspects of the SERINC5 antagonistic pathway via Nef.

Viral Interactions with Adaptor-Protein Complexes: A Ubiquitous Trait among Viral Species

Numerous viruses hijack cellular protein trafficking pathways to mediate cell entry or to rearrange membrane structures thereby promoting viral replication and antagonizing the immune response. Adaptor protein complexes (AP), which mediate protein sorting in endocytic and secretory transport pathways, are one of the conserved viral targets with many viruses possessing AP-interacting motifs. We present here different mechanisms of viral interference with AP complexes and the functional consequences that allow for efficient viral propagation and evasion of host immune defense. The ubiquity of this phenomenon is evidenced by the fact that there are representatives for AP interference in all major viral families, covered in this review. The best described examples are interactions of human immunodeficiency virus and human herpesviruses with AP complexes. Several other viruses, like Ebola, Nipah, and SARS-CoV-2, are pointed out as high priority disease-causative agents supporting the need for deeper understanding of virus-AP interplay which can be exploited in the design of novel antiviral therapies.

Nef homodimers down-regulate SERINC5 by AP-2-mediated endocytosis to promote HIV-1 infectivity

SERINC5 is a multipass intrinsic membrane protein that suppresses HIV-1 infectivity when incorporated into budding virions. The HIV-1 Nef virulence factor prevents viral incorporation of SERINC5 by triggering its down-regulation from the producer cell membrane through an AP-2-dependent endolysosomal pathway. However, the mechanistic basis for SERINC5 down-regulation by Nef remains elusive. Here we demonstrate that Nef homodimers are important for SERINC5 down-regulation, trafficking to late endosomes, and exclusion from newly synthesized viral particles. Based on previous X-ray crystal structures, we mutated three conserved residues in the Nef dimer interface (Leu¹¹², Tyr¹¹⁵, and Phe¹²¹) and demonstrated attenuated homodimer formation in a cell-based fluorescence complementation assay. Point mutations at each position reduced the infectivity of HIV-1 produced from transfected 293T cells, the Jurkat TAg T-cell line, and donor mononuclear cells in a SERINC5-dependent manner. In SERINC5-transfected 293T cells, virion incorporation of SERINC5 was increased by dimerization-defective Nef mutants, whereas down-regulation of SERINC5 from the membrane of transfected Jurkat cells by these mutants was significantly reduced. Nef dimer interface mutants also failed to trigger internalization of SERINC5 and localization to Rab7⁺ late endosomes in T cells. Importantly, fluorescence complementation assays demonstrated that dimerization-defective Nef mutants retained interaction with both SERINC5 and AP-2. These results show that down-regulation of SERINC5 and subsequent enhancement of viral infectivity require Nef homodimers and support a mechanism by which the Nef dimer bridges SERINC5 to AP-2 for endocytosis. Pharmacological disruption of Nef homodimers may control HIV-1 infectivity and viral spread by enhancing virion incorporation of SERINC5.

How HIV Nef Proteins Hijack Membrane Traffic To Promote Infection

The accessory protein Nef of human immunodeficiency virus (HIV) is a primary determinant of viral pathogenesis. Nef is abundantly expressed during infection and reroutes a variety of cell surface proteins to disrupt host immunity and promote the viral replication cycle. Nef counteracts host defenses by sequestering and/or degrading its targets via the endocytic and secretory pathways. Nef does this by physically engaging a number of host trafficking proteins. Substantial progress has been achieved in identifying the targets of Nef, and a structural and mechanistic understanding of Nef's ability to command the protein trafficking machinery has recently started to coalesce. Comparative analysis of HIV and simian immunodeficiency virus (SIV) Nef proteins in the context of recent structural advances sheds further light on both viral evolution and the mechanisms whereby trafficking is hijacked. This review describes how advances in cell and structural biology are uncovering in growing detail how Nef subverts the host immune system, facilitates virus release, and enhances viral infectivity.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

Analysis of IFITM-IFITM Interactions by a Flow Cytometry-Based FRET Assay

The interferon-induced transmembrane proteins 1–3 (IFITM1–3) inhibit host cell entry of several viruses. However, it is incompletely understood how IFITM1–3 exert antiviral activity. Two phenylalanine residues, F75 and F78, within the intramembrane domain 1 (IM1) were previously shown to be required for IFITM3/IFITM3 interactions and for inhibition of viral entry, suggesting that IFITM/IFITM interactions might be pivotal to antiviral activity. Here, we employed a fluorescence resonance energy transfer (FRET) assay to analyze IFITM/IFITM interactions. For assay calibration, we equipped two cytosolic, non-interacting proteins, super yellow fluorescent protein (SYFP) and super cyan fluorescent protein (SCFP), with signals that target proteins to membrane rafts and also analyzed a SCFP-SYFP fusion protein. This strategy allowed us to discriminate background signals resulting from colocalization of proteins at membrane subdomains from signals elicited by protein–protein interactions. Coexpression of IFITM1–3 and IFITM5 fused to fluorescent proteins elicited strong FRET signals, and mutation of F75 and F78 in IFITM3 (mutant IFITM3-FF) abrogated antiviral activity, as expected, but did not alter cellular localization and FRET signals. Moreover, IFITM3-FF co-immunoprecipitated efficiently with wild type (wt) IFITM3, lending further support to the finding that lack of antiviral activity of IFITM3-FF was not due to altered membrane targeting or abrogated IFITM3-IFITM3 interactions. Collectively, we report an assay that allows quantifying IFITM/IFITM interactions. Moreover, we confirm residues F75 and F78 as critical for antiviral activity but also show that these residues are dispensable for IFITM3 membrane localization and IFITM3/IFITM3 interactions.

Physiological relevance of ACOT8-Nef interaction in HIV infection

During human immunodeficiency virus (HIV) infection, Nef viral protein plays a crucial role in viral pathogenesis and progression of acquired immunodeficiency syndrome. Nef is expressed in the early stages of infection and alters the cellular environment increasing infectivity, viral replication, and the evasion of host immune response through several mechanisms. Nef has numerous functional domains that allow it to interact with a number of proteins, interfering with intracellular traffic. Among these proteins, human peroxisomal thioesterase 8, ACOT8, has been shown to be an important cellular partner of Nef. It has been suggested that this interaction may be involved in Nef-dependent endocytosis and also in the modulation of lipid composition in membrane rafts. However, the actual role of this interaction, as well as the mechanisms involved, has not yet been fully elucidated. In this review, we focused on the interplay between Nef and ACOT8 proteins, highlighting the possible physiological relevance in HIV infection.

Endocytic sorting motif interactions involved in Nef-mediated downmodulation of CD4 and CD3

Lentiviral Nefs recruit assembly polypeptide complexes and target sorting motifs in cellular receptors to induce their internalization. While Nef-mediated CD4 downmodulation is conserved, the ability to internalize CD3 was lost in HIV-1 and its precursors. Although both functions play key roles in lentiviral replication and pathogenicity, the underlying structural requirements are poorly defined. Here, we determine the structure of SIV_mac239 Nef bound to the ExxxLM motif of another Nef molecule at 2.5 Å resolution. This provides a basis for a structural model, where a hydrophobic crevice in simian immunodeficiency virus (SIV) Nef targets a dileucine motif in CD4 and a tyrosine-based motif in CD3. Introducing key residues into this crevice of HIV-1 Nef enables CD3 binding but an additional N-terminal tyrosine motif is required for internalization. Our resolution of the CD4/Nef/AP2 complex and generation of HIV-1 Nefs capable of CD3 downregulation provide insights into sorting motif interactions and target discrimination of Nef.HIV and simian immunodeficiency virus (SIV) Nef proteins both stimulate the clathrin-mediated endocytosis of CD4 but differ in downmodulation of the immune receptor CD3. Here, the authors present the structure of SIV Nef bound to the ExxxLM motif of another Nef molecule, which allows them to propose a model how Nef recognizes these motifs in CD3 and CD4.

The interaction between HIV-1 Nef and adaptor protein-2 reduces Nef-mediated CD4+ T cell apoptosis

Acquired Immune Deficiency Syndrome is characterized by a decline in CD4⁺ T cells. Here, we elucidated the mechanism underlying apoptosis in Human Immunodeficiency Virus-1 (HIV-1) infection by examining host apoptotic pathways hijacked by the HIV-1 Nef protein in the CD4⁺ T-cell line Sup-T1. Using a panel of Nef mutants unable to bind specific host proteins we uncovered that Nef generates pro- and anti-apoptotic signals. Apoptosis increased upon mutating the motifs involved in the interaction of Nef:AP-1 (Nef_M20A or Nef_{EEEE62-65AAAA}) or Nef:AP-2 (Nef_LL164/165AA), implying these interactions limit Nef-mediated apoptosis. In contrast, disrupting the Nef:PAK2 interaction motifs (Nef_H89A or Nef_F191A) reduced apoptosis. To validate further, apoptosis was measured after short-hairpin RNA knock-down of AP-1, AP-2 and PAK2. AP-2α depletion enhanced apoptosis, demonstrating that disrupting the Nef:AP-2α interaction limits Nef-mediated apoptosis. Collectively, we describe a mechanism by which HIV-1 regulates cell survival and demonstrate the consequence of interfering with Nef:host protein interactions.

Cell-based Fluorescence Complementation Reveals a Role for HIV-1 Nef Protein Dimerization in AP-2 Adaptor Recruitment and CD4 Co-receptor Down-regulation

The HIV-1 Nef accessory factor enhances viral infectivity, immune evasion, and AIDS progression. Nef triggers rapid down-regulation of CD4 via the endocytic adaptor protein 2 (AP-2) complex, a process linked to enhanced viral infectivity and immune escape. Here, we describe a bimolecular fluorescence complementation (BiFC) assay to visualize the interaction of Nef with AP-2 and CD4 in living cells. Interacting protein pairs were fused to complementary non-fluorescent fragments of YFP and co-expressed in 293T cells. Nef interactions with both CD4 and AP-2 resulted in complementation of YFP and a bright fluorescent signal by confocal microcopy that localized to the cell periphery. Co-expression of the AP-2 α subunit enhanced the Nef·AP-2 σ2 subunit BiFC signal and vice versa, suggesting that the AP-2 α-σ2 hemicomplex interacts cooperatively with Nef. Mutagenesis of Nef amino acids Arg-134, Glu-174, and Asp-175, which stabilize Nef for AP-2 α-σ2 binding in a recent co-crystal structure, substantially reduced AP-2 interaction without affecting CD4 binding. A dimerization-defective mutant of Nef failed to interact with either CD4 or AP-2 in the BiFC assay, indicating that Nef quaternary structure is required for CD4 and AP-2 recruitment as well as CD4 down-regulation. A small molecule previously shown to bind the Nef dimerization interface also reduced Nef interactions with AP-2 and CD4 and restored CD4 expression to the surface of HIV-infected cells. Our findings provide a mechanistic explanation for previous observations that dimerization-defective Nef mutants fail to down-regulate CD4 and validate the Nef dimerization interface as a target site for antiretroviral drug development.

HIV Genome-Wide Protein Associations: a Review of 30 Years of Research

The HIV genome encodes a small number of viral proteins (i.e., 16), invariably establishing cooperative associations among HIV proteins and between HIV and host proteins, to invade host cells and hijack their internal machineries. As a known example, the HIV envelope glycoprotein GP120 is closely associated with GP41 for viral entry. From a genome-wide perspective, a hypothesis can be worked out to determine whether 16 HIV proteins could develop 120 possible pairwise associations either by physical interactions or by functional associations mediated via HIV or host molecules. Here, we present the first systematic review of experimental evidence on HIV genome-wide protein associations using a large body of publications accumulated over the past 3 decades. Of 120 possible pairwise associations between 16 HIV proteins, at least 34 physical interactions and 17 functional associations have been identified. To achieve efficient viral replication and infection, HIV protein associations play essential roles (e.g., cleavage, inhibition, and activation) during the HIV life cycle. In either a dispensable or an indispensable manner, each HIV protein collaborates with another viral protein to accomplish specific activities that precisely take place at the proper stages of the HIV life cycle. In addition, HIV genome-wide protein associations have an impact on anti-HIV inhibitors due to the extensive cross talk between drug-inhibited proteins and other HIV proteins. Overall, this study presents for the first time a comprehensive overview of HIV genome-wide protein associations, highlighting meticulous collaborations between all viral proteins during the HIV life cycle.

HIV Genome-Wide Protein Associations: a Review of 30 Years of Research

The HIV genome encodes a small number of viral proteins (i.e., 16), invariably establishing cooperative associations among HIV proteins and between HIV and host proteins, to invade host cells and hijack their internal machineries. As a known example, the HIV envelope glycoprotein GP120 is closely associated with GP41 for viral entry. From a genome-wide perspective, a hypothesis can be worked out to determine whether 16 HIV proteins could develop 120 possible pairwise associations either by physical interactions or by functional associations mediated via HIV or host molecules. Here, we present the first systematic review of experimental evidence on HIV genome-wide protein associations using a large body of publications accumulated over the past 3 decades. Of 120 possible pairwise associations between 16 HIV proteins, at least 34 physical interactions and 17 functional associations have been identified. To achieve efficient viral replication and infection, HIV protein associations play essential roles (e.g., cleavage, inhibition, and activation) during the HIV life cycle. In either a dispensable or an indispensable manner, each HIV protein collaborates with another viral protein to accomplish specific activities that precisely take place at the proper stages of the HIV life cycle. In addition, HIV genome-wide protein associations have an impact on anti-HIV inhibitors due to the extensive cross talk between drug-inhibited proteins and other HIV proteins. Overall, this study presents for the first time a comprehensive overview of HIV genome-wide protein associations, highlighting meticulous collaborations between all viral proteins during the HIV life cycle.

HIV-1 Nef: Taking Control of Protein Trafficking

The Nef protein of the human immunodeficiency virus is a crucial determinant of viral pathogenesis and disease progression. Nef is abundantly expressed early in infection and is thought to optimize the cellular environment for viral replication. Nef controls expression levels of various cell surface molecules that play important roles in immunity and virus life cycle, by directly interfering with the itinerary of these proteins within the endocytic and late secretory pathways. To exert these functions, Nef physically interacts with host proteins that regulate protein trafficking. In recent years, considerable progress was made in identifying host-cell-interacting partners for Nef, and the molecular machinery used by Nef to interfere with protein trafficking has started to be unraveled. Here, we briefly review the knowledge gained and discuss new findings regarding the mechanisms by which Nef modifies the intracellular trafficking pathways to prevent antigen presentation, facilitate viral particle release and enhance the infectivity of HIV-1 virions.

Molecular characterization of HIV-1 Nef and ACOT8 interaction: insights from in silico structural predictions and in vitro functional assays

HIV-1 Nef interacts with several cellular proteins, among which the human peroxisomal thioesterase 8 (ACOT8). This interaction may be involved in the endocytosis regulation of membrane proteins and might modulate lipid composition in membrane rafts. Nef regions involved in the interaction have been experimentally characterized, whereas structural details of the ACOT8 protein are unknown. The lack of structural information hampers the comprehension of the functional consequences of the complex formation during HIV-1 infection. We modelled, through in silico predictions, the ACOT8 structure and we observed a high charge complementarity between Nef and ACOT8 surfaces, which allowed the identification of the ACOT8 putative contact points involved in the interaction. The predictions were validated by in vitro assays through the development of ACOT8 deletion mutants. Coimmunoprecipitation and immunofluorescence analyses showed that ACOT8 Arg⁴⁵-Phe⁵⁵ and Arg⁸⁶-Pro⁹³ regions are involved in Nef association. In addition, K91S mutation abrogated the interaction with Nef, indicating that Lys⁹¹ plays a key role in the interaction. Finally, when associated with ACOT8, Nef may be preserved from degradation. These findings improve the comprehension of the association between HIV-1 Nef and ACOT8, helping elucidating the biological effect of their interaction.