The LMP2A protein of Epstein-Barr virus regulates phosphorylation of ITSN1 and Shb adaptors by tyrosine kinases

E3 Ubiquitin Ligases in Gammaherpesviruses and HIV: A Review of Virus Adaptation and Exploitation

For productive infection and replication to occur, viruses must control cellular machinery and counteract restriction factors and antiviral proteins. Viruses can accomplish this, in part, via the regulation of cellular gene expression and post-transcriptional and post-translational control. Many viruses co-opt and counteract cellular processes via modulation of the host post-translational modification machinery and encoding or hijacking kinases, SUMO ligases, deubiquitinases, and ubiquitin ligases, in addition to other modifiers. In this review, we focus on three oncoviruses, Epstein-Barr virus (EBV), Kaposi's sarcoma herpesvirus (KSHV), and human immunodeficiency virus (HIV) and their interactions with the ubiquitin-proteasome system via viral-encoded or cellular E3 ubiquitin ligase activity.

Molecular Interactions between Two LMP2A PY Motifs of EBV and WW Domains of E3 Ubiquitin Ligase AIP4

Interactions involving Epstein–Barr virus (EBV) LMP2A and Nedd4 family E3 ubiquitin–protein ligases promote the ubiquitination of LMP2A-associated proteins, which results in the perturbation of normal B-cell signaling. Here, we solved the solution structure of the WW2 domain of hAIP4 and investigated the binding mode involving the N-terminal domain of LMP2A and the WW2 domain. The WW2 domain presented a conserved WW domain scaffold with a three-stranded anti-parallel β-sheet and bound two PY motifs via different binding mechanisms. Our NMR titration and ITC data demonstrated that the PY motifs of LMP2A can recognize and interact weakly with the XP groove of the WW2 domain (residues located around the third β-strand), and then residues between two PY motifs optimize the binding by interacting with the loop 1 region of the WW2 domain. In particular, the residue Val15 in the hairpin loop region between β1 and β2 of the WW2 domain exhibited unique changes depending on the terminal residues of the PY motif. This result suggested that the hairpin loop is responsible for additional interactions outside the XP groove, and this hypothesis was confirmed in a deuterium exchange experiment. These weak but wide interactions can stabilize the complex formed between the PY and WW domains.

Evidence of a dysregulated vitamin D endocrine system in SARS-CoV-2 infected patient's lung cells

Although a defective vitamin D endocrine system has been widely suspected to be associated in SARS-CoV-2 pathobiology, the status of the vitamin D endocrine system and vitamin D-modulated genes in lung cells of patients infected with SARS-CoV-2 remains unknown. To understand the significance of the vitamin D endocrine system in SARS-CoV-2 pathobiology, computational approaches were applied to transcriptomic datasets from bronchoalveolar lavage fluid (BALF) cells of such patients or healthy individuals. Levels of vitamin D receptor, retinoid X receptor, and CYP27A1 in BALF cells of patients infected with SARS-CoV-2 were found to be reduced. Additionally, 107 differentially expressed, predominantly downregulated genes, as potentially modulated by vitamin D endocrine system, were identified in transcriptomic datasets from patient's cells. Further analysis of differentially expressed genes provided eight novel genes with a conserved motif with vitamin D-responsive elements, implying the role of both direct and indirect mechanisms of gene expression by the dysregulated vitamin D endocrine system in SARS-CoV-2-infected cells. Protein-protein interaction network of differentially expressed vitamin D-modulated genes were enriched in the immune system, NF-κB/cytokine signaling, and cell cycle regulation as top predicted pathways that might be affected in the cells of such patients. In brief, the results presented here povide computational evidence to implicate a dysregulated vitamin D endocrine system in the pathobiology of SARS-CoV-2 infection.

Quantitative proteomic analysis in symptomatic and asymptomatic apical periodontitis

AIM

To quantitatively and qualitatively compare the host proteomic profile in samples of symptomatic and asymptomatic apical periodontitis (AP) using nano-liquid chromatography-electron spray tandem mass spectrometry.

METHODOLOGY

Samples were obtained from 18 patients with radiographically evident AP, divided into symptomatic and asymptomatic groups (nine per group) according to clinical characteristics. After sample collection, protein extraction, purification and quantification of the samples were performed, which were analysed by reverse-phase liquid chromatography coupled to mass spectrometry. Label-free quantitative proteomic analysis was performed by Protein Lynx Global Service software. Differences in expression of proteins between the groups were calculated using the Monte Carlo algorithm, considering P < 0.05 for down-regulated proteins and 1 - P > 0.95 for up-regulated proteins. Proteins were identified with the embedded ion accounting algorithm in the software and a search of the Homo sapiens UniProt database.

RESULTS

A total of 853 individual human proteins were identified. In the quantitative analysis, common proteins to both groups accounted for 143 proteins. Differences in expression between groups resulted in 51 up-regulated proteins (1 - P > 0.95) in the symptomatic group, including alpha-1-antitrypsin, protein S100-A8, myeloperoxidase, peroxiredoxin and lactotransferrin. This group also had 43 down-regulated proteins (P < 0.05), comprising immunoglobulin, neutrophil defensin, pyruvate kinase and alpha-enolase. The qualitative analysis considered only the exclusive proteins of each group. For the symptomatic group, 318 complete proteins and 29 fragments were identified, such as dedicator of cytokinesis protein, intersectin, prostaglandin, phospholipase DDHD2 and superoxide dismutase. For the asymptomatic group, 326 complete proteins and 37 fragments were identified, including azurocidin, C-reactive protein, collagen alpha, cathepsin, heat shock and laminin.

CONCLUSIONS

Quantitative differences in the expression of common proteins in cases of symptomatic and asymptomatic AP were found, which were mostly related to host immune response in both groups. Exclusive proteins in the symptomatic group were mainly related to the host response to the presence of viruses in endodontic infections, oxidative stress and proteolytic enzymes. The results provide a basis for a better understanding of cellular and molecular pathways involved in AP, establishing specific proteomic profiles for symptomatic and asymptomatic conditions.

Intersectin goes nuclear: secret life of an endocytic protein

Intersectin 1-short (ITSN1-s) is a 1220 amino acid ubiquitously expressed scaffold protein presenting a multidomain structure that allows to spatiotemporally regulate the functional interaction of a plethora of proteins. Besides its well-established role in endocytosis, ITSN1-s is involved in the regulation of cell signaling and is implicated in tumorigenesis processes, although the signaling pathways involved are still poorly understood. Here, we identify ITSN1-s as a nucleocytoplasmic trafficking protein. We show that, by binding to importin (IMP)α, a small fraction of ITSN1-s localizes in the cell nucleus at the steady state, where it preferentially associates with the nuclear envelope and interacts with lamin A/C. However, upon pharmacological ablation of chromosome region maintenance 1 (CRM-1)-dependent nuclear export pathway, the protein accumulates into the nucleus, thus revealing its moonlighting nature. Analysis of deletion mutants revealed that the coiled coil (CC) and Src homology (SH3) regions play the major role in its nucleocytoplasmic shuttling. While no evidence of nuclear localization signal (NLS) was detected in the CC region, a functional bipartite NLS was identified within the SH3D region of ITSN1-s (RKKNPGGWWEGELQARGKKRQIGW-1127), capable of conferring energy-dependent nuclear accumulation to reporter proteins and whose mutational ablation affects nuclear import of the whole SH3 region. Thus, ITSN1-s is an endocytic protein, which shuttles between the nucleus and the cytoplasm in a CRM-1- and IMPα-dependent fashion.

Intersectin scaffold proteins and their role in cell signaling and endocytosis

Intersectins (ITSNs) are a family of multi-domain proteins involved in regulation of diverse cellular pathways. These scaffold proteins are well known for regulating endocytosis but also play important roles in cell signaling pathways including kinase regulation and Ras activation. ITSNs participate in several human cancers, such as neuroblastomas and glioblastomas, while their downregulation is associated with lung injury. Alterations in ITSN expression have been found in neurodegenerative diseases such as Down Syndrome and Alzheimer's disease. Binding proteins for ITSNs include endocytic regulatory factors, cytoskeleton related proteins (i.e. actin or dynamin), signaling proteins as well as herpes virus proteins. This review will summarize recent studies on ITSNs, highlighting the importance of these scaffold proteins in the aforementioned processes.

The role of the Src Homology-2 domain containing protein B (SHB) in β cells

This review will describe the SH2-domain signaling protein Src Homology-2 domain containing protein B (SHB) and its role in various physiological processes relating in particular to glucose homeostasis and β cell function. SHB operates downstream of several tyrosine kinase receptors and assembles signaling complexes in response to receptor activation by interacting with other signaling proteins via its other domains (proline-rich, phosphotyrosine-binding and tyrosine-phosphorylation sites). The subsequent responses are context-dependent. Absence of Shb in mice has been found to exert effects on hematopoiesis, angiogenesis and glucose metabolism. Specifically, first-phase insulin secretion in response to glucose was impaired and this effect was related to altered characteristics of focal adhesion kinase activation modulating signaling through Akt, ERK, β catenin and cAMP. It is believed that SHB plays a role in integrating adaptive responses to various stimuli by simultaneously modulating cellular responses in different cell-types, thus playing a role in maintaining physiological homeostasis.

Intersectin adaptor proteins are associated with actin-regulating protein WIP in invadopodia

Invasive cancer cells form actin-rich membrane protrusions called invadopodia that degrade extracellular matrix and facilitate cell invasion and metastasis. WIP (WASP-interacting protein) together with N-WASP (neural Wiskott-Aldrich syndrome protein) are localized in invadopodia and play a crucial role in their formation. Here we show that WIP interacts with endocytic adaptor proteins of the intersectin (ITSN) family, ITSN1 and ITSN2. The interaction is mediated by the SH3 domains of ITSNs and the middle part of the WIP proline-rich motifs. We have also demonstrated that ITSN1, WIP and N-WASP can form a complex in cells. Endogenous ITSN1 and ITSN2 are located in invasive protrusions of MDA-MB-231 breast cancer cell line. Moreover, data from immunofluorescent analysis revealed co-localization of ITSN1 and WIP at sites of invadopodia formation and in clathrin-coated pits. Together, these findings provide insights into the molecular mechanisms of invadopodia formation and identify ITSNs as scaffold proteins involved in this process.

Latent Membrane Protein 2 (LMP2)

LMP2A is an EBV-encoded protein with three domains: (a) an N-terminal cytoplasmic domain, which has PY motifs that bind to WW domain-containing E3 ubiquitin ligases and an ITAM that binds to SH2 domain-containing proteins, (b) a transmembrane domain with 12 transmembrane segments that localizes LMP2A in cellular membranes, and (c) a 27-amino acid C-terminal domain which mediates homodimerization and heterodimerization of LMP2 protein isoforms. The most prominent two isoforms of the protein are LMP2A and LMP2B. The LMP2B isoform lacks the 19-amino acid N-terminal domain found in LMP2A, which modulates cellular signaling resulting in a baseline activation of B cells and degradation of cellular kinases leading to the downregulation of normal B cell signaling pathways. These two seemingly contradictory processes allow EBV to establish and maintain latency. LMP2 is expressed in many EBV-associated malignancies. While its antigenic properties may be useful in developing LMP2-specific immunity, the LMP2A N-terminal motifs also provide a basis to target LMP2A-modulated cellular kinases for the development of treatment strategies.

Adaptor proteins intersectin 1 and 2 bind similar proline-rich ligands but are differentially recognized by SH2 domain-containing proteins

Background

Scaffolding proteins of the intersectin (ITSN) family, ITSN1 and ITSN2, are crucial for the initiation stage of clathrin-mediated endocytosis. These proteins are closely related but have implications in distinct pathologies. To determine how these proteins could be separated in certain cell pathways we performed a comparative study of ITSNs.

Methodology/Principal Findings

We have shown that endogenous ITSN1 and ITSN2 colocalize and form a complex in cells. A structural comparison of five SH3 domains, which mediated most ITSNs protein-protein interactions, demonstrated a similarity of their ligand-binding sites. We showed that the SH3 domains of ITSN2 bound well-established interactors of ITSN1 as well as newly identified ITSNs protein partners. A search for a novel interacting interface revealed multiple tyrosines that could be phosphorylated in ITSN2. Phosphorylation of ITSN2 isoforms but not ITSN1 short isoform was observed in various cell lines. EGF stimulation of HeLa cells enhanced tyrosine phosphorylation of ITSN2 isoforms and enabled their recognition by the SH2 domains of the Fyn, Fgr and Abl1 kinases, the regulatory subunit of PI3K, the adaptor proteins Grb2 and Crk, and phospholipase C gamma. The SH2 domains mentioned were unable to bind ITSN1 short isoform.

Conclusions/Significance

Our results indicate that during evolution of vertebrates ITSN2 acquired a novel protein-interaction interface that allows its specific recognition by the SH2 domains of signaling proteins. We propose that these data could be important to understand the functional diversity of paralogous ITSN proteins.

Viral proteins and Src family kinases: Mechanisms of pathogenicity from a “liaison dangereuse”

To complete their life cycle and spread, viruses interfere with and gain control of diverse cellular processes, this most often occurring through interaction between viral proteins (VPs) and resident protein partners. Among the latter, Src family kinases (SFKs), a class of non-receptor tyrosine kinases that contributes to the conversion of extracellular signals into intracellular signaling cascades and is involved in virtually all cellular processes, have recently emerged as critical mediators between the cell’s infrastructure and the viral demands. In this scenario, structural or ex novo synthesized VPs are able to bind to the different domains of these enzymes through specific short linear motifs present along their sequences. Proline-rich motifs displaying the conserved minimal consensus PxxP and recognizing the SFK Src homology (SH)3 domain constitute a cardinal signature for the formation of multiprotein complexes and this interaction may promote phosphorylation of VPs by SFKs, thus creating phosphotyrosine motifs that become a docking site for the SH2 domains of SFKs or other SH2 domain-bearing signaling molecules. Importantly, the formation of these assemblies also results in a change in the activity and/or location of SFKs, and these events are critical in perturbing key signaling pathways so that viruses can utilize the cell’s machinery to their own benefit. In the light of these observations, although VPs as such, especially those with enzyme activity, are still regarded as valuable targets for therapeutic strategies, multiprotein complexes composed of viral and host cell proteins are increasingly becoming objects of investigation with a view to deeply characterize the structural aspects that favor their formation and to develop new compounds able to contrast viral diseases in an alternative manner.

Emerging roles for intersectin (ITSN) in regulating signaling and disease pathways

Intersectins (ITSNs) represent a family of multi-domain adaptor proteins that regulate endocytosis and cell signaling. ITSN genes are highly conserved and present in all metazoan genomes examined thus far. Lower eukaryotes have only one ITSN gene, whereas higher eukaryotes have two ITSN genes. ITSN was first identified as an endocytic scaffold protein, and numerous studies reveal a conserved role for ITSN in endocytosis. Subsequently, ITSNs were found to regulate multiple signaling pathways including receptor tyrosine kinases (RTKs), GTPases, and phosphatidylinositol 3-kinase Class 2beta (PI3KC2β). ITSN has also been implicated in diseases such as Down Syndrome (DS), Alzheimer Disease (AD), and other neurodegenerative disorders. This review summarizes the evolutionary conservation of ITSN, the latest research on the role of ITSN in endocytosis, the emerging roles of ITSN in regulating cell signaling pathways, and the involvement of ITSN in human diseases such as DS, AD, and cancer.