Youjia Cao

Pleiotropic defects in TCR signaling in a Vav-1-null Jurkat T-cell line

The Rac/Rho‐specific guanine nucleotide exchange factor, Vav‐1, is a key component of the T‐cell antigen receptor (TCR)‐linked signaling machinery. Here we have used somatic cell gene‐targeting technology to generate a Vav‐1‐deficient Jurkat T‐cell line. The J.Vav1 cell line exhibits dramatic defects in TCR‐dependent interleukin (IL)‐2 promoter activation, accompanied by significant reductions in the activities of the NFAT(IL‐2), NFκB, AP‐1 and REAP transcription factors that bind to the IL‐2 promoter region. In contrast, loss of Vav‐1 had variable effects on early TCR‐stimulated signaling events. J.Vav1 cells display a selective defect in sustained Ca2+ signaling during TCR stimulation, and complementation of this abnormality by exogenously introduced Vav‐1 is dependent on the Vav‐1 calponin homology domain. While JNK activation was severely impaired, the stimulation of Ras, ERK and protein kinase C‐θ activities, as well as the mobilization of lipid rafts, appeared normal in the J.Vav1 cells. Finally, evidence is presented to suggest that the alternative Vav family members, Vav‐2 and Vav‐3, are activated during TCR ligation, and partially compensate for the loss of Vav‐1 in Jurkat T cells.

ICP34.5 Protein of Herpes Simplex Virus Facilitates the Initiation of Protein Translation by Bridging Eukaryotic Initiation Factor 2α (eIF2α) and Protein Phosphatase 1

The ICP34.5 protein of herpes simplex virus type 1 is a neurovirulence factor that plays critical roles in viral replication and anti-host responses. One of its functions is to recruit protein phosphatase 1 (PP1) that leads to the dephosphorylation of the α subunit of translation initiation factor eIF2 (eIF2α), which is inactivated by infection-induced phosphorylation. As PP1 is a protein phosphatase with a wide range of substrates, the question remains to be answered how ICP34.5 directs PP1 to specifically dephosphorylate eIF2α. Here we report that ICP34.5 not only binds PP1 but also associates with eIF2α by in vitro and in vivo assays. The binding site of eIF2α is identified at amino acids 233–248 of ICP34.5, which falls in the highly homologous region with human gene growth arrest and DNA damage 34. The interaction between ICP34.5 and eIF2α is independent of the phosphorylation status of eIF2α at serine 51. Deletion mutation of this region results in the failure of dephosphorylation of eIF2α by PP1 and, consequently, interrupts viral protein synthesis and replication. Our data illustrated that the binding between viral protein ICP34.5 and the host eIF2α is crucial for the specific dephosphorylation of eIF2α by PP1. We propose that herpes simplex virus protein ICP34.5 bridges PP1 and eIF2α via their binding motifs and thereby facilitates the protein synthesis and viral replication.

Inhibition of TANK binding kinase 1 by herpes simplex virus 1 facilitates productive infection.

ABSTRACT The γ134.5 protein of herpes simplex viruses (HSV) is essential for viral pathogenesis, where it precludes translational arrest mediated by double-stranded-RNA-dependent protein kinase (PKR). Paradoxically, inhibition of PKR alone is not sufficient for HSV to exhibit viral virulence. Here we report that γ134.5 inhibits TANK binding kinase 1 (TBK1) through its amino-terminal sequences, which facilitates viral replication and neuroinvasion. Compared to wild-type virus, the γ134.5 mutant lacking the amino terminus induces stronger antiviral immunity. This parallels a defect of γ134.5 for interacting with TBK1 and reducing phosphorylation of interferon (IFN) regulatory factor 3. This activity is independent of PKR. Although resistant to IFN treatment, the γ134.5 amino-terminal deletion mutant replicates at an intermediate level between replication of wild-type virus and that of the γ134.5 null mutant in TBK1+/+ cells. However, such impaired viral growth is not observed in TBK1−/− cells, indicating that the interaction of γ134.5 with TBK1 dictates HSV infection. Upon corneal infection, this mutant replicates transiently but barely invades the trigeminal ganglia or brain, which is a difference from wild-type virus and the γ134.5 null mutant. Therefore, in addition to PKR, γ134.5 negatively regulates TBK1, which contributes viral replication and spread in vivo.

The γ134.5 Protein of Herpes Simplex Virus 1 Is Required To Interfere with Dendritic Cell Maturation during Productive Infection

ABSTRACT The γ134.5 protein of herpes simplex virus 1 is an essential factor for viral virulence. In infected cells, this viral protein prevents the translation arrest mediated by double-stranded RNA-dependent protein kinase R. Additionally, it associates with and inhibits TANK-binding kinase 1, an essential component of Toll-like receptor-dependent and -independent pathways that activate interferon regulatory factor 3 and cytokine expression. Here, we show that γ134.5 is required to block the maturation of conventional dendritic cells (DCs) that initiate adaptive immune responses. Unlike wild-type virus, the γ134.5 null mutant stimulates the expression of CD86, major histocompatibility complex class II (MHC-II), and cytokines such as alpha/beta interferon in immature DCs. Viral replication in DCs inversely correlates with interferon production. These phenotypes are also mirrored in a mouse ocular infection model. Further, DCs infected with the γ134.5 null mutant effectively activate naïve T cells whereas DCs infected with wild-type virus fail to do so. Type I interferon-neutralizing antibodies partially reverse virus-induced upregulation of CD86 and MHC-II, suggesting that γ134.5 acts through interferon-dependent and -independent mechanisms. These data indicate that γ134.5 is involved in the impairment of innate immunity by inhibiting both type I interferon production and DC maturation, leading to defective T-cell activation.

PRL-1 Protein Promotes ERK1/2 and RhoA Protein Activation through a Non-canonical Interaction with the Src Homology 3 Domain of p115 Rho GTPase-activating Protein

Background: The mechanism for the oncogenic phosphatase PRL-1 remains undefined. Results: We identified and characterized a novel PRL-1-binding protein, p115 RhoGAP. Conclusion: PRL-1 activates the ERK1/2 pathway by displacing MEKK1 from p115 RhoGAP and RhoA by preventing its interaction with p115 RhoGAP. Significance: This study offers a novel strategy for anticancer therapeutics by blocking the interaction between PRL-1 and p115 RhoGAP. Phosphatases of the regenerating liver (PRL) play oncogenic roles in cancer development and metastasis. Although previous studies indicate that PRL-1 promotes cell growth and migration by activating both the ERK1/2 and RhoA pathways, the mechanism by which it activates these signaling events remains unclear. We have identified a PRL-1-binding peptide (Peptide 1) that shares high sequence identity with a conserved motif in the Src homology 3 (SH3) domain of p115 Rho GTPase-activating protein (GAP). p115 RhoGAP directly binds PRL-1 in vitro and in cells via its SH3 domain. Structural analyses of the PRL-1·Peptide 1 complex revealed a novel protein-protein interaction whereby a sequence motif within the PxxP ligand-binding site of the p115 RhoGAP SH3 domain occupies a folded groove within PRL-1. This prevents the canonical interaction between the SH3 domain of p115 RhoGAP and MEKK1 and results in activation of ERK1/2. Furthermore, PRL-1 binding activates RhoA signaling by inhibiting the catalytic activity of p115 RhoGAP. The results demonstrate that PRL-1 binding to p115 RhoGAP provides a coordinated mechanism underlying ERK1/2 and RhoA activation.

The calponin homology domain of Vav1 associates with calmodulin and is prerequisite to T cell antigen receptor-induced calcium release in Jurkat T lymphocytes.

Vav1 is a guanine nucleotide exchange factor that is expressed specifically in hematopoietic cells and plays important roles in T cell development and activation. Vav1 consists of multiple structural domains so as to facilitate both its guanine nucleotide exchange activity and scaffold function following T cell antigen receptor (TCR) engagement. Previous studies demonstrated that the calponin homology (CH) domain of Vav1 is required for TCR-stimulated calcium mobilization and thus downstream activation of nuclear factor of activated T cells. However, it remained obscure how Vav1 functions in regulating calcium flux. In an effort to explore molecules interacting with Vav1, we found that calmodulin bound to Vav1 in a calcium-dependent and TCR activation-independent manner. The binding site was mapped to the CH domain of Vav1. Reconstitution of vav1-null Jurkat T cells (J.Vav1) with CH-deleted Vav1 exhibited a severe deficiency in calcium release to the same extent as that of Jurkat cells treated with the calmodulin inhibitor or J.Vav1 cells. The defect persisted even when phospholipase-Cγ1 was fully activated, indicating a prerequisite role of Vav1 CH domain in calcium signaling. The results suggest that Vav1 and calmodulin function cooperatively to potentiate TCR-induced calcium release. This study unveiled a mechanism by which the Vav1 CH domain is involved in calcium signaling and provides insight into our understanding of the role of Vav1 in T cell activation.

A Herpesvirus Virulence Factor Inhibits Dendritic Cell Maturation through Protein Phosphatase 1 and IκB Kinase

ABSTRACT Dendritic cells are sentinels in innate and adaptive immunity. Upon virus infection, a complex program is in operation, which activates IκB kinase (IKK), a key regulator of inflammatory cytokines and costimulatory molecules. Here we show that the γ134.5 protein, a virulence factor of herpes simplex viruses, blocks Toll-like receptor-mediated dendritic cell maturation. While the wild-type virus inhibits the induction of major histocompatibility complex (MHC) class II, CD86, interleukin-6 (IL-6), and IL-12, the γ134.5-null mutant does not. Notably, γ134.5 works in the absence of any other viral proteins. When expressed in mammalian cells, including dendritic cells, γ134.5 associates with IKKα/β and inhibits NF-κB activation. This is mirrored by the inhibition of IKKα/β phosphorylation, p65/RelA phosphorylation, and nuclear translocation in response to lipopolysaccharide or poly(I:C) stimulation. Importantly, γ134.5 recruits both IKKα/β and protein phosphatase 1, forming a complex that dephosphorylates two serine residues within the catalytic domains of IκB kinase. The amino-terminal domain of γ134.5 interacts with IKKα/β, whereas the carboxyl-terminal domain binds to protein phosphatase 1. Deletions or mutations in either domain abolish the activity of γ134.5. These results suggest that the control of IκB kinase dephosphorylation by γ134.5 represents a critical viral mechanism to disrupt dendritic cell functions.

Structure of human cytosolic X-prolyl aminopeptidase: A double MN(II) dependent dimeric enzyme with a novel three-domain subunit

X-prolyl aminopeptidases catalyze the removal of a penultimate prolyl residue from the N termini of peptides. Mammalian X-prolyl aminopeptidases are shown to be responsible for the degradation of bradykinin, a blood pressure regulator peptide, and have been linked to myocardial infarction. The x-ray crystal structure of human cytosolic X-prolyl aminopeptidase (XPN-PEP1) was solved at a resolution of 1.6Å. The structure reveals a dimer with a unique three-domain organization in each subunit, rather than the two domains common to all other known structures of X-prolyl aminopeptidase and prolidases. The C-terminal catalytic domain of XPNPEP1 coordinates two metal ions and shares a similar fold with other prolyl aminopeptidases. Metal content analysis and activity assays confirm that the enzyme is double Mn(II) dependent for its activity, which contrasts with the previous notion that each XPNPEP1 subunit contains only one Mn(II) ion. Activity assays on an E41A mutant demonstrate that the acidic residue, which was considered as a stabilizing factor in the protonation of catalytic residue His498, plays only a marginal role in catalysis. Further mutagenesis reveals the significance of the N-terminal domain and dimerization for the activity of XPNPEP1, and we provide putative structural explanations for their functional roles. Structural comparisons further suggest mechanisms for substrate selectivity in different X-prolyl peptidases.

PDlim2 Selectively Interacts with the PDZ Binding Motif of Highly Pathogenic Avian H5N1 Influenza A Virus NS1

The multi-functional NS1 protein of influenza A virus is a viral virulence determining factor. The last four residues at the C-terminus of NS1 constitute a type I PDZ domain binding motif (PBM). Avian influenza viruses currently in circulation carry an NS1 PBM with consensus sequence ESEV, whereas human influenza viruses bear an NS1 PBM with consensus sequence RSKV or RSEV. The PBM sequence of the influenza A virus NS1 is reported to contribute to high viral pathogenicity in animal studies. Here, we report the identification of PDlim2 as a novel binding target of the highly pathogenic avian influenza virus H5N1 strain with an NS1 PBM of ESEV (A/Chicken/Henan/12/2004/H5N1, HN12-NS1) by yeast two-hybrid screening. The interaction was confirmed by in vitro GST pull-down assays, as well as by in vivo mammalian two-hybrid assays and bimolecular fluorescence complementation assays. The binding was also confirmed to be mediated by the interaction of the PDlim2 PDZ domain with the NS1 PBM motif. Interestingly, our assays showed that PDlim2 bound specifically with HN12-NS1, but exhibited no binding to NS1 from a human influenza H1N1 virus bearing an RSEV PBM (A/Puerto Rico/8/34/H1N1, PR8-NS1). A crystal structure of the PDlim2 PDZ domain fused with the C-terminal hexapeptide from HN12-NS1, together with GST pull-down assays on PDlim2 mutants, reveals that residues Arg16 and Lys31 of PDlim2 are critical for the binding between PDlim2 and HN12-NS1. The identification of a selective binding target of HN12-NS1 (ESEV), but not PR8-NS1 (RSEV), enables us to propose a structural mechanism for the interaction between NS1 PBM and PDlim2 or other PDZ-containing proteins.

Dephosphorylation of eIF2α Mediated by the γ134.5 Protein of Herpes Simplex Virus 1 Facilitates Viral Neuroinvasion

ABSTRACT The γ134.5 protein, a virulence factor of herpes simplex viruses, redirects protein phosphatase 1 to dephosphorylate the α subunit of translation initiation factor 2 (eIF2α). Additionally, it inhibits the induction of antiviral genes by TANK-binding kinase 1. Nevertheless, its precise role in vivo remains to be established. Here we show that eIF2α dephosphorylation by γ134.5 is crucial for viral neuroinvasion. V193E and F195L substitutions in γ134.5 abrogate viral replication in the eye and spread to the trigeminal ganglia and brain. Intriguingly, inhibition of antiviral gene induction by γ134.5 is not sufficient to exhibit viral virulence.