The cellular protein lyric interacts with HIV-1 Gag

Comparative analysis of retroviral Gag-host cell interactions: focus on the nuclear interactome

Retroviruses exploit host proteins to assemble and release virions from infected cells. Previously, most studies focused on interacting partners of retroviral Gag proteins that localize to the cytoplasm or plasma membrane. Given that several full-length Gag proteins have been found in the nucleus, identifying the Gag-nuclear interactome has high potential for novel findings involving previously unknown host processes. Here we systematically compared nuclear factors identified in published HIV-1 proteomic studies and performed our own mass spectrometry analysis using affinity-tagged HIV-1 and RSV Gag proteins mixed with nuclear extracts. We identified 57 nuclear proteins in common between HIV-1 and RSV Gag, and a set of nuclear proteins present in our analysis and ≥ 1 of the published HIV-1 datasets. Many proteins were associated with nuclear processes which could have functional consequences for viral replication, including transcription initiation/elongation/termination, RNA processing, splicing, and chromatin remodeling. Examples include facilitating chromatin remodeling to expose the integrated provirus, promoting expression of viral genes, repressing the transcription of antagonistic cellular genes, preventing splicing of viral RNA, altering splicing of cellular RNAs, or influencing viral or host RNA folding or RNA nuclear export. Many proteins in our pulldowns common to RSV and HIV-1 Gag are critical for transcription, including PolR2B, the second largest subunit of RNA polymerase II (RNAPII), and LEO1, a PAF1C complex member that regulates transcriptional elongation, supporting the possibility that Gag influences the host transcription profile to aid the virus. Through the interaction of RSV and HIV-1 Gag with splicing-related proteins CBLL1, HNRNPH3, TRA2B, PTBP1 and U2AF1, we speculate that Gag could enhance unspliced viral RNA production for translation and packaging. To validate one putative hit, we demonstrated an interaction of RSV Gag with Mediator complex member Med26, required for RNA polymerase II-mediated transcription. Although 57 host proteins interacted with both Gag proteins, unique host proteins belonging to each interactome dataset were identified. These results provide a strong premise for future functional studies to investigate roles for these nuclear host factors that may have shared functions in the biology of both retroviruses, as well as functions specific to RSV and HIV-1, given their distinctive hosts and molecular pathology.

Comparative analysis of retroviral Gag-host cell interactions: focus on the nuclear interactome

Retroviruses exploit a variety of host proteins to assemble and release virions from infected cells. To date, most studies that examined possible interacting partners of retroviral Gag proteins focused on host proteins that localize primarily to the cytoplasm or plasma membrane. Given the recent findings that several full-length Gag proteins localize to the nucleus, identifying the Gag-nuclear interactome has high potential for novel findings that reveal previously unknown host processes. In this study, we systematically compared nuclear factors identified in published HIV-1 proteomic studies which had used a variety of experimental approaches. In addition, to contribute to this body of knowledge, we report results from a mass spectrometry approach using affinity-tagged (His6) HIV-1 and RSV Gag proteins mixed with nuclear extracts. Taken together, the previous studies-as well as our own-identified potential binding partners of HIV-1 and RSV Gag involved in several nuclear processes, including transcription, splicing, RNA modification, and chromatin remodeling. Although a subset of host proteins interacted with both Gag proteins, there were also unique host proteins belonging to each interactome dataset. To validate one of the novel findings, we demonstrated the interaction of RSV Gag with a member of the Mediator complex, Med26, which is required for RNA polymerase II-mediated transcription. These results provide a strong premise for future functional studies to investigate roles for these nuclear host factors that may have shared functions in the biology of both retroviruses, as well as functions specific to RSV and HIV-1, given their distinctive hosts and molecular pathology.

Cellular Targets of HIV-1 Protease: Just the Tip of the Iceberg?

Human immunodeficiency virus 1 (HIV-1) viral protease (PR) is one of the most studied viral enzymes and a crucial antiviral target. Despite its well-characterized role in virion maturation, an increasing body of research is starting to focus on its ability to cleave host cell proteins. Such findings are apparently in contrast with the dogma of HIV-1 PR activity being restricted to the interior of nascent virions and suggest catalytic activity within the host cell environment. Given the limited amount of PR present in the virion at the time of infection, such events mainly occur during late viral gene expression, mediated by newly synthesized Gag-Pol polyprotein precursors, rather than before proviral integration. HIV-1 PR mainly targets proteins involved in three different processes: those involved in translation, those controlling cell survival, and restriction factors responsible for innate/intrinsic antiviral responses. Indeed, by cleaving host cell translation initiation factors, HIV-1 PR can impair cap-dependent translation, thus promoting IRES-mediated translation of late viral transcripts and viral production. By targeting several apoptotic factors, it modulates cell survival, thus promoting immune evasion and viral dissemination. Additionally, HIV-1 PR counteracts restriction factors incorporated in the virion that would otherwise interfere with nascent virus vitality. Thus, HIV-1 PR appears to modulate host cell function at different times and locations during its life cycle, thereby ensuring efficient viral persistency and propagation. However, we are far from having a complete picture of PR-mediated host cell modulation, which is emerging as a field that needs further investigation.

Subtype Differences in the Interaction of HIV-1 Matrix with Calmodulin: Implications for Biological Functions

The HIV-1 Gag polyprotein plays essential roles during the late stage of the HIV-1 replication cycle, and has recently been identified as a promising therapeutic target. The N-terminal portion of the HIV-1 Gag polyprotein encodes the myristoylated matrix (MA) protein, which functions in the trafficking of the structural proteins to the plasma membrane (PM) and facilitation of envelope incorporation into budding virus. Numerous host cell proteins interact with the MA portion of the Gag polyprotein during this process. One such factor is the ubiquitous calcium-binding protein calmodulin (CaM), which interacts preferentially with myristoylated proteins, thereby regulating cell physiology. The exact role of this interaction is poorly understood to date. Atomic resolution structures revealed the nature of the CaM-MA interaction for clade B isolates. In this study, we expanded our knowledge and characterized biophysically and computationally the CaM interaction with MA from other HIV-1 clades and discovered differences in the CaM recognition as compared to the prototypical clade B MA, with significant alterations in the interaction with the MA protein from clade C. Structural investigation and in silico mutational analysis revealed that HIV-1 MA protein from clade C, which is responsible for the majority of global HIV-1 infections, interacts with lower affinity and altered kinetics as compared to the canonical clade B. This finding may have implications for additional altered interaction networks as compared to the well-studied clade B. Our analysis highlights the importance of expanding investigations of virus-host cell factor interaction networks to other HIV-1 clades.

HIV-1 Gag Forms Ribonucleoprotein Complexes with Unspliced Viral RNA at Transcription Sites

The ability of the retroviral Gag protein of Rous sarcoma virus (RSV) to transiently traffic through the nucleus is well-established and has been implicated in genomic RNA (gRNA) packaging Although other retroviral Gag proteins (human immunodeficiency virus type 1, HIV-1; feline immunodeficiency virus, FIV; Mason-Pfizer monkey virus, MPMV; mouse mammary tumor virus, MMTV; murine leukemia virus, MLV; and prototype foamy virus, PFV) have also been observed in the nucleus, little is known about what, if any, role nuclear trafficking plays in those viruses. In the case of HIV-1, the Gag protein interacts in nucleoli with the regulatory protein Rev, which facilitates nuclear export of gRNA. Based on the knowledge that RSV Gag forms viral ribonucleoprotein (RNPs) complexes with unspliced viral RNA (USvRNA) in the nucleus, we hypothesized that the interaction of HIV-1 Gag with Rev could be mediated through vRNA to form HIV-1 RNPs. Using inducible HIV-1 proviral constructs, we visualized HIV-1 Gag and USvRNA in discrete foci in the nuclei of HeLa cells by confocal microscopy. Two-dimensional co-localization and RNA-immunoprecipitation of fractionated cells revealed that interaction of nuclear HIV-1 Gag with USvRNA was specific. Interestingly, treatment of cells with transcription inhibitors reduced the number of HIV-1 Gag and USvRNA nuclear foci, yet resulted in an increase in the degree of Gag co-localization with USvRNA, suggesting that Gag accumulates on newly synthesized viral transcripts. Three-dimensional imaging analysis revealed that HIV-1 Gag localized to the perichromatin space and associated with USvRNA and Rev in a tripartite RNP complex. To examine a more biologically relevant cell, latently infected CD4+ T cells were treated with prostratin to stimulate NF-κB mediated transcription, demonstrating striking localization of full-length Gag at HIV-1 transcriptional burst site, which was labelled with USvRNA-specific riboprobes. In addition, smaller HIV-1 RNPs were observed in the nuclei of these cells. These data suggest that HIV-1 Gag binds to unspliced viral transcripts produced at the proviral integration site, forming vRNPs in the nucleus.

How HIV-1 Gag Manipulates Its Host Cell Proteins: A Focus on Interactors of the Nucleocapsid Domain

The human immunodeficiency virus (HIV-1) polyprotein Gag (Group-specific antigen) plays a central role in controlling the late phase of the viral lifecycle. Considered to be only a scaffolding protein for a long time, the structural protein Gag plays determinate and specific roles in HIV-1 replication. Indeed, via its different domains, Gag orchestrates the specific encapsidation of the genomic RNA, drives the formation of the viral particle by its auto-assembly (multimerization), binds multiple viral proteins, and interacts with a large number of cellular proteins that are needed for its functions from its translation location to the plasma membrane, where newly formed virions are released. Here, we review the interactions between HIV-1 Gag and 66 cellular proteins. Notably, we describe the techniques used to evidence these interactions, the different domains of Gag involved, and the implications of these interactions in the HIV-1 replication cycle. In the final part, we focus on the interactions involving the highly conserved nucleocapsid (NC) domain of Gag and detail the functions of the NC interactants along the viral lifecycle.

Elucidating the in vivo interactome of HIV-1 RNA by hybridization capture and mass spectrometry

HIV-1 replication requires myriad interactions between cellular proteins and the viral unspliced RNA. These interactions are important in archetypal RNA processes such as transcription and translation as well as for more specialized functions including alternative splicing and packaging of unspliced genomic RNA into virions. We present here a hybridization capture strategy for purification of unspliced full-length HIV RNA-protein complexes preserved in vivo by formaldehyde crosslinking, and coupled with mass spectrometry to identify HIV RNA-protein interactors in HIV-1 infected cells. One hundred eighty-nine proteins were identified to interact with unspliced HIV RNA including Rev and Gag/Gag-Pol, 24 host proteins previously shown to bind segments of HIV RNA, and over 90 proteins previously shown to impact HIV replication. Further analysis using siRNA knockdown techniques against several of these proteins revealed significant changes to HIV expression. These results demonstrate the utility of the approach for the discovery of host proteins involved in HIV replication. Additionally, because this strategy only requires availability of 30 nucleotides of the HIV-RNA for hybridization with a capture oligonucleotide, it is readily applicable to any HIV system of interest regardless of cell type, HIV-1 virus strain, or experimental perturbation.

AEG-1/MTDH/LYRIC: A Promiscuous Protein Partner Critical in Cancer, Obesity, and CNS Diseases

Since its original discovery in 2002, AEG-1/MTDH/LYRIC has emerged as a primary regulator of several diseases including cancer, inflammatory diseases, and neurodegenerative diseases. AEG-1/MTDH/LYRIC has emerged as a key contributory molecule in almost every aspect of cancer progression, including uncontrolled cell growth, evasion of apoptosis, increased cell migration and invasion, angiogenesis, chemoresistance, and metastasis. Additionally, recent studies highlight a seminal role of AEG-1/MTDH/LYRIC in neurodegenerative diseases and obesity. By interacting with multiple protein partners, AEG-1/MTDH/LYRIC plays multifaceted roles in the pathogenesis of a wide variety of diseases. This review discusses the current state of understanding of AEG-1/MTDH/LYRIC regulation and function in cancer and other diseases with a focus on its association/interaction with several pivotal protein partners.

Astrocyte elevated gene-1 (AEG-1) and the A(E)Ging HIV/AIDS-HAND

Recent attempts to analyze human immunodeficiency virus (HIV)-1-induced gene expression changes in astrocytes uncovered a multifunctional oncogene, astrocyte elevated gene-1 (AEG-1). Our previous studies revealed that AEG-1 regulates reactive astrocytes proliferation, migration and inflammation, hallmarks of aging and CNS injury. Moreover, the involvement of AEG-1 in neurodegenerative disorders, such as Huntington's disease and migraine, and its induction in the aged brain suggest a plausible role in regulating overall CNS homeostasis and aging. Therefore, it is important to investigate AEG-1 specifically in aging-associated cognitive decline. In this study, we decipher the common mechanistic links in cancer, aging and HIV-1-associated neurocognitive disorders that likely contribute to AEG-1-based regulation of astrocyte responses and function. Despite AEG-1 incorporation into HIV-1 virions and its induction by HIV-1, tumor necrosis factor-α and interleukin-1β, the specific role(s) of AEG-1 in astrocyte-driven HIV-1 neuropathogenesis are incompletely defined. We propose that AEG-1 plays a central role in a multitude of cellular stress responses involving mitochondria, endoplasmic reticulum and the nucleolus. It is thus important to further investigate AEG-1-based cellular and molecular regulation in order to successfully develop better therapeutic approaches that target AEG-1 to combat cancer, HIV-1 and aging.

The La-Related Proteins, a Family with Connections to Cancer

The evolutionarily-conserved La-related protein (LARP) family currently comprises Genuine La, LARP1, LARP1b, LARP4, LARP4b, LARP6 and LARP7. Emerging evidence suggests each LARP has a distinct role in transcription and/or mRNA translation that is attributable to subtle sequence variations within their La modules and specific C-terminal domains. As emerging research uncovers the function of each LARP, it is evident that La, LARP1, LARP6, LARP7 and possibly LARP4a and 4b are dysregulated in cancer. Of these, LARP1 is the first to be demonstrated to drive oncogenesis. Here, we review the role of each LARP and the evidence linking it to malignancy. We discuss a future strategy of targeting members of this protein family as cancer therapy.

Proteomic analysis of HIV-1 Gag interacting partners using proximity-dependent biotinylation

BACKGROUND

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein is necessary and sufficient to assemble non-infectious particles. Given that HIV-1 subverts many host proteins at all stages of its life cycle, it is essential to identify these interactions as potential targets for antiretroviral therapy.

FINDINGS

This work demonstrates the use of proximity-dependent biotin identification (BioID) of host proteins and complexes that are proximal to the N-terminal domains of the HIV-1 Gag polyprotein. Two of the hits identified in the BioID screen were validated by immunoprecipation and confirmed the interaction of DDX17 and RPS6 with HIV-1 Gag.

CONCLUSIONS

Our results show that BioID is both a successful and complementary method to screen for nearby interacting proteins of HIV-1 Gag during the replicative cycle in different cell lines.

Interplay between the alpharetroviral Gag protein and SR proteins SF2 and SC35 in the nucleus

Retroviruses are positive-sense, single-stranded RNA viruses that reverse transcribe their RNA genomes into double-stranded DNA for integration into the host cell chromosome. The integrated provirus is used as a template for the transcription of viral RNA. The full-length viral RNA can be used for the translation of the Gag and Gag-Pol structural proteins or as the genomic RNA (gRNA) for encapsidation into new virions by the Gag protein. The mechanism by which Gag selectively incorporates unspliced gRNA into virus particles is poorly understood. Although Gag was previously thought to localize exclusively to the cytoplasm and plasma membrane where particles are released, we found that the Gag protein of Rous sarcoma virus, an alpharetrovirus, undergoes transient nuclear trafficking. When the nuclear export signal of RSV Gag is mutated (Gag.L219A), the protein accumulates in discrete subnuclear foci reminiscent of nuclear bodies such as splicing speckles, paraspeckles, and PML bodies. In this report, we observed that RSV Gag.L219A foci appeared to be tethered in the nucleus, partially co-localizing with the splicing speckle components SC35 and SF2. Overexpression of SC35 increased the number of Gag.L219A nucleoplasmic foci, suggesting that SC35 may facilitate the formation of Gag foci. We previously reported that RSV Gag nuclear trafficking is required for efficient gRNA packaging. Together with the data presented herein, our findings raise the intriguing hypothesis that RSV Gag may co-opt splicing factors to localize near transcription sites. Because splicing occurs co-transcriptionally, we speculate that this mechanism could allow Gag to associate with unspliced viral RNA shortly after its transcription initiation in the nucleus, before the viral RNA can be spliced or exported from the nucleus as an mRNA template.

Analysis of HIV-1 Gag protein interactions via biotin ligase tagging

We have examined the interactions of wild-type (WT) and matrix protein-deleted (ΔMA) HIV-1 precursor Gag (PrGag) proteins in virus-producing cells using a biotin ligase-tagging approach. To do so, WT and ΔMA PrGag proteins were tagged with the Escherichia coli promiscuous biotin ligase (BirA*), expressed in cells, and examined. Localization patterns of PrGag proteins and biotinylated proteins overlapped, consistent with observations that BirA*-tagged proteins biotinylate neighbor proteins that are in close proximity. Results indicate that BirA*-tagged PrGag proteins biotinylated themselves as well as WT PrGag proteins in trans. Previous data have shown that the HIV-1 Envelope (Env) protein requires an interaction with MA for assembly into virions. Unexpectedly, ΔMA proteins biotinylated Env, whereas WT BirA*-tagged proteins did not, suggesting that the presence of MA made Env inaccessible to biotinylation. We also identified over 50 cellular proteins that were biotinylated by BirA*-tagged PrGag proteins. These included membrane proteins, cytoskeleton-associated proteins, nuclear transport factors, lipid metabolism regulators, translation factors, and RNA-processing proteins. The identification of these biotinylated proteins offers new insights into HIV-1 Gag protein trafficking and activities and provides new potential targets for antiviral interference.

IMPORTANCE

We have employed a novel strategy to analyze the interactions of the HIV-1 structural Gag proteins, which involved tagging wild-type and mutant Gag proteins with a biotin ligase. Expression of the tagged proteins in cells allowed us to analyze proteins that came in close proximity to the Gag proteins as they were synthesized, transported, assembled, and released from cells. The tagged proteins biotinylated proteins encoded by the HIV-1 pol gene and neighbor Gag proteins, but, surprisingly, only the mutant Gag protein biotinylated the HIV-1 Envelope protein. We also identified over 50 cellular proteins that were biotinylated, including membrane and cytoskeletal proteins and proteins involved in lipid metabolism, nuclear import, translation, and RNA processing. Our results offer new insights into HIV-1 Gag protein trafficking and activities and provide new potential targets for antiviral interference.

Proteomics in the investigation of HIV-1 interactions with host proteins

Productive HIV-1 infection depends on host machinery, including a broad array of cellular proteins. Proteomics has played a significant role in the discovery of HIV-1 host proteins. In this review, after a brief survey of the HIV-1 host proteins that were discovered by proteomic analyses, I focus on analyzing the interactions between the virion and host proteins, as well as the technologies and strategies used in those proteomic studies. With the help of proteomics, the identification and characterization of HIV-1 host proteins can be translated into novel antiretroviral therapeutics.

Proteome analysis of the HIV-1 Gag interactome

Human immunodeficiency virus Gag drives assembly of virions in infected cells and interacts with host factors which facilitate or restrict viral replication. Although several Gag-binding proteins have been characterized, understanding of virus-host interactions remains incomplete. In a series of six affinity purification screens, we have identified protein candidates for interaction with HIV-1 Gag. Proteins previously found in virions or identified in siRNA screens for host factors influencing HIV-1 replication were recovered. Helicases, translation factors, cytoskeletal and motor proteins, factors involved in RNA degradation and RNA interference were enriched in the interaction data. Cellular networks of cytoskeleton, SR proteins and tRNA synthetases were identified. Most prominently, components of cytoplasmic RNA transport granules were co-purified with Gag. This study provides a survey of known Gag-host interactions and identifies novel Gag binding candidates. These factors are associated with distinct molecular functions and cellular pathways relevant in host-pathogen interactions.

The role of AEG-1/MTDH/LYRIC in the pathogenesis of central nervous system disease

Astrocyte-elevated gene-1 (AEG-1/MTDH/LYRIC) is a potent oncogene that regulates key cellular processes underlying disease of the central nervous system (CNS). From its involvement in human immunodeficiency virus (HIV)-1 infection to its role in neurodegenerative disease and malignant brain tumors, AEG-1/MTDH/LYRIC facilitates cellular survival and proliferation through the control of a multitude of molecular signaling cascades. AEG-1/MTDH/LYRIC induction by HIV-1 and TNF highlights its importance in viral infection, and its incorporation into viral vesicles supports its potential role in active viral replication. Overexpression of AEG-1/MTDH/LYRIC in the brains of Huntington's disease patients suggests its function in neurodegenerative disease, and its association with genetic polymorphisms in large genome-wide association studies of migraine patients suggests a possible role in the pathogenesis of migraine headaches. In the field of cancer, AEG-1/MTDH/LYRIC promotes angiogenesis, migration, invasion, and enhanced tumor metabolism through key oncogenic signaling cascades. In response to external stress cues and cellular mechanisms to inhibit further growth, AEG-1/MTDH/LYRIC activates pathways that bypass cell checkpoints and potentiates signals to enhance survival and tumorigenesis. As an oncogene that promotes aberrant cellular processes within the CNS, AEG-1/MTDH/LYRIC represents an important therapeutic target for the treatment of neurological disease.

AEG-1/MTDH/LYRIC: signaling pathways, downstream genes, interacting proteins, and regulation of tumor angiogenesis

Astrocyte elevated gene-1 (AEG-1), also known as metadherin (MTDH) and lysine-rich CEACAM1 coisolated (LYRIC), was initially cloned in 2002. AEG-1/MTDH/LYRIC has emerged as an important oncogene that is overexpressed in multiple types of human cancer. Expanded research on AEG-1/MTDH/LYRIC has established a functional role of this molecule in several crucial aspects of tumor progression, including transformation, proliferation, cell survival, evasion of apoptosis, migration and invasion, metastasis, angiogenesis, and chemoresistance. The multifunctional role of AEG-1/MTDH/LYRIC in tumor development and progression is associated with a number of signaling cascades, and recent studies identified several important interacting partners of AEG-1/MTDH/LYRIC in regulating cancer promotion and other biological functions. This review evaluates the current literature on AEG-1/MTDH/LYRIC function relative to signaling changes, interacting partners, and angiogenesis and highlights new perspectives of this molecule, indicating its potential as a significant target for the clinical treatment of various cancers and other diseases.

Solution structure of calmodulin bound to the binding domain of the HIV-1 matrix protein

Subcellular distribution of calmodulin (CaM) in human immunodeficiency virus type-1 (HIV-1)-infected cells is distinct from that observed in uninfected cells. CaM co-localizes and interacts with the HIV-1 Gag protein in the cytosol of infected cells. Although it has been shown that binding of Gag to CaM is mediated by the matrix (MA) domain, the structural details of this interaction are not known. We have recently shown that binding of CaM to MA induces a conformational change that triggers myristate exposure, and that the CaM-binding domain of MA is confined to a region spanning residues 8-43 (MA-(8-43)). Here, we present the NMR structure of CaM bound to MA-(8-43). Our data revealed that MA-(8-43), which contains a novel CaM-binding motif, binds to CaM in an antiparallel mode with the N-terminal helix (α1) anchored to the CaM C-terminal lobe, and the C-terminal helix (α2) of MA-(8-43) bound to the N-terminal lobe of CaM. The CaM protein preserves a semiextended conformation. Binding of MA-(8-43) to CaM is mediated by numerous hydrophobic interactions and stabilized by favorable electrostatic contacts. Our structural data are consistent with the findings that CaM induces unfolding of the MA protein to have access to helices α1 and α2. It is noteworthy that several MA residues involved in CaM binding have been previously implicated in membrane binding, envelope incorporation, and particle production. The present findings may ultimately help in identification of the functional role of CaM in HIV-1 replication.

Role of the HIV-1 Matrix Protein in Gag Intracellular Trafficking and Targeting to the Plasma Membrane for Virus Assembly

Human immunodeficiency virus type-1 (HIV-1) encodes a polypeptide called Gag that is able to form virus-like particles in vitro in the absence of any cellular or viral constituents. During the late phase of the HIV-1 infection, Gag polyproteins are transported to the plasma membrane (PM) for assembly. In the past two decades, in vivo, in vitro, and structural studies have shown that Gag trafficking and targeting to the PM are orchestrated events that are dependent on multiple factors including cellular proteins and specific membrane lipids. The matrix (MA) domain of Gag has been the focus of these studies as it appears to be engaged in multiple intracellular interactions that are suggested to be critical for virus assembly and replication. The interaction between Gag and the PM is perhaps the most understood. It is now established that the ultimate localization of Gag on punctate sites on the PM is mediated by specific interactions between the MA domain of Gag and phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)], a minor lipid localized on the inner leaflet of the PM. Structure-based studies revealed that binding of PI(4,5)P(2) to MA induces minor conformational changes, leading to exposure of the myristyl (myr) group. Exposure of the myr group is also triggered by binding of calmodulin, enhanced by factors that promote protein self-association like the capsid domain of Gag, and is modulated by pH. Despite the steady progress in defining both the viral and cellular determinants of retroviral assembly and release, Gag's intracellular interactions and trafficking to its assembly sites in the infected cell are poorly understood. In this review, we summarize the current understanding of the structural and functional role of MA in HIV replication.

Budding of retroviruses utilizing divergent L domains requires nucleocapsid

We recently reported that human immunodeficiency virus type 1 (HIV-1) carrying PTAP and LYPX(n)L L domains ceased budding when the nucleocapsid (NC) domain was mutated, suggesting a role for NC in HIV-1 release. Here we investigated whether NC involvement in virus release is a property specific to HIV-1 or a general requirement of retroviruses. Specifically, we examined a possible role for NC in the budding of retroviruses relying on divergent L domains and structurally homologous NC domains that harbor diverse protein sequences. We found that NC is critical for the release of viruses utilizing the PTAP motif whether it functions within its native Gag in simian immunodeficiency virus cpzGAB2 (SIVcpzGAB2) or SIVsmmE543 or when it is transplanted into the heterologous Gag protein of equine infectious anemia virus (EIAV). In both cases, virus release was severely diminished even though NC mutant Gag proteins retained the ability to assemble spherical particles. Moreover, budding-defective NC mutants, which displayed particles tethered to the plasma membrane, were triggered to release virus when access to the cell endocytic sorting complex required for transport pathway was restored (i.e., in trans expression of Nedd4.2s). We also examined the role of NC in the budding of EIAV, a retrovirus relying exclusively on the (L)YPX(n)L-type L domain. We found that EIAV late budding defects were rescued by overexpression of the isolated Alix Bro1 domain (Bro1). Bro1-mediated rescue of EIAV release required the wild-type NC. EIAV NC mutants lost interactions with Bro1 and failed to produce viruses despite retaining the ability to self-assemble. Together, our studies establish a role for NC in the budding of retroviruses harboring divergent L domains and evolutionarily diverse NC sequences, suggesting the utilization of a common conserved mechanism and/or cellular factor rather than a specific motif.