Pierre Jalinot

The C-terminus of the HTLV-1 Tax oncoprotein mediates interaction with the PDZ domain of cellular proteins.

Infection by HTLV-1 has been correlated with the appearance of various proliferative or degenerative diseases. Some of these disorders have been observed in transgenic mice expressing the Tax protein, which is known to transactivate various viral and cellular promoters through interactions with several transcription factors. In this study we show that the C-terminus of this viral oncoprotein represents a motif permitting binding of Tax to the PDZ domains of several cellular proteins. A two-hybrid screen with Tax as bait indeed yielded complementary DNAs coding for six proteins including PDZ domains. Two of them correspond to truncated forms of the PSD-95 and β1-syntrophin proteins, another clone codes for a protein homologous to the product of the C. elegans gene lin-7. The other three clones code for new human members of the PDZ family of cellular proteins. The interaction of Tax with the products of these clones was confirmed by immunoprecipitation assays in mammalian cells, and analysis of various mutants of Tax established the importance of the C-terminal amino acids for several of these interactions. These data suggest that Tax could perturb the normal function of targeted cellular proteins by strongly interacting with their PDZ domains.

Exclusion of Int-6 from PML nuclear bodies by binding to the HTLV-I Tax oncoprotein

The Tax transactivator of the human T cell leukemia virus type I (HTLV-I) exhibits oncogenic properties. A screen for proteins interacting with Tax yielded a complementary DNA (cDNA) encoding the human Int-6 protein. In mice, the Int-6 gene can be converted into a putative dominant negative oncogene after retroviral insertion. Here, Int-6 was localized in the cell nucleus to give a speckled staining pattern superposed to that of the promyelocytic leukemia (PML) protein. The binding of Tax to Int-6 caused its redistribution from the nuclear domains to the cytoplasm. Thus, Int-6 is a component of the PML nuclear bodies and Tax disrupts its normal cellular localization by binding to it.

Photoswitchable Inhibitors of Microtubule Dynamics Optically Control Mitosis and Cell Death

Small molecules that interfere with microtubule dynamics, such as Taxol and the Vinca alkaloids, are widely used in cell biology research and as clinical anticancer drugs. However, their activity cannot be restricted to specific target cells, which also causes severe side effects in chemotherapy. Here, we introduce the photostatins, inhibitors that can be switched on and off in vivo by visible light, to optically control microtubule dynamics. Photostatins modulate microtubule dynamics with a subsecond response time and control mitosis in living organisms with single-cell spatial precision. In longer-term applications in cell culture, photostatins are up to 250 times more cytotoxic when switched on with blue light than when kept in the dark. Therefore, photostatins are both valuable tools for cell biology, and are promising as a new class of precision chemotherapeutics whose toxicity may be spatiotemporally constrained using light.

The PDZ Protein TIP-1 Interacts with the Rho Effector Rhotekin and Is Involved in Rho Signaling to the Serum Response Element

The human T-cell lymphotrophic virus, type 1 Tax protein can interact via its C terminus with various proteins including a PDZ domain. In this work, one of them, TIP-1, is characterized as a cytoplasmic 14-kDa protein mainly corresponding to one PDZ domain. A two-hybrid screen performed with TIP-1 as bait showed that it interacts with the human homologue of rhotekin that was previously identified in mice as a Rho effector. Both human and mouse rhotekins exhibit at their C termini the sequence QSPV-COOH that matches theX(S/T)XV-COOH consensus known for proteins recognizing PDZ domains. Mutation of the serine and valine residues to alanine impairs interaction of rhotekin with TIP-1. Transient expression experiments with a reporter construct including the c-Fos serum response element (SRE) showed that coexpression of TIP-1 with the constitutively active RhoA.V14 mutant and human rhotekin caused a strong activation of the SRE. A negative mutant of Rho, RhoA.N19, was unable to cooperate with TIP-1 and rhotekin. The positive effect of TIP-1 was also lost when the C terminus of rhotekin was mutated. These data show that the complex of active Rho with its effector rhotekin bound to TIP-1 produces in the cytoplasm a signal that triggers strong activation of the SRE.

Association of the mammalian proto-oncoprotein Int-6 with the three protein complexes eIF3, COP9 signalosome and 26S proteasome.

The mammalian Int‐6 protein has been characterized as a subunit of the eIF3 translation initiation factor and also as a transforming protein when its C‐terminal part is deleted. It includes a protein domain, which also exists in various subunits of eIF3, of the 26S proteasome and of the COP9 signalosome (CSN). By performing a two‐hybrid screen with Int‐6 as bait, we have isolated subunits belonging to all three complexes, namely eIF3‐p110, Rpt4, CSN3 and CSN6. The results of transient expression experiments in COS7 cells confirmed the interaction of Int‐6 with Rpt4, CSN3 and CSN6, but also showed that Int‐6 is able to bind another subunit of the CSN: CSN7a. Immunoprecipitation experiments performed with the endogenous proteins showed that Int‐6 binds the entire CSN, but in low amount, and also that Int‐6 is associated with the 26S proteasome. Taken together these results show that the Int‐6 protein can bind the three complexes with various efficiencies, possibly exerting a regulatory activity in both protein translation and degradation.

Genetic Characterization of Transactivation of the Human T-Cell Leukemia Virus Type 1 Promoter: Binding of Tax to Tax-Responsive Element 1 Is Mediated by the Cyclic AMP-Responsive Members of the CREB/ATF Family of Transcription Factors

To achieve a better understanding of the mechanism of transactivation by Tax of human T-cell leukemia virus type 1 Tax-responsive element 1 (TRE-1), we developed a genetic approach with Saccharomyces cerevisiae. We constructed a yeast reporter strain containing the lacZ gene under the control of the CYC1 promoter associated with three copies of TRE-1. Expression of either the cyclic AMP response element-binding protein (CREB) or CREB fused to the GAL4 activation domain (GAD) in this strain did not modify the expression of the reporter gene. Tax alone was also inactive. However, expression of the reporter gene was induced by coexpression of Tax and CREB. This effect was stronger with the GAD-CREB fusion protein. Analysis of different CREB mutants with this genetic system indicated that the C-terminal 92 amino acid residues, which include the basic domain and the leucine zipper, are necessary and sufficient to mediate transactivation by Tax. To identify cellular proteins binding to TRE-1 in a Tax-dependent manner, this strain was also used to screen a library of human cDNAs fused to GAD. Of five positive clones isolated from 0.75 x 10(6) yeast colonies, four were members of the CREB/activating transcription factor (ATF) family: CREB, two isoforms of the cyclic AMP-responsive element modulator (CREM), and ATF-1. Interestingly, these three proteins can be phosphorylated by protein kinase A and thus form a particular subgroup within the CREB/ATF family. Expression of ATF-2 in S. cerevisiae did not activate TRE-1 in the presence of Tax. This shows that in a eukaryotic nucleus, Tax specifically interacts with the basic domain-leucine zipper region of ATF-1, CREB, and CREM. The fifth clone identified in this screening corresponded to the Ku autoantigen p70 subunit. When fused to GAD, the C-terminal region of Ku was able to activate transcription via TRE-1 but this activation was not dependent on Tax.

Human Papillomavirus Type 18 E6 Protein Binds the Cellular PDZ Protein TIP-2/GIPC, Which Is Involved in Transforming Growth Factor β Signaling and Triggers Its Degradation by the Proteasome

ABSTRACT Several viral proteins expressed by DNA or RNA transforming viruses have the particular property of binding via their C-terminal end to various cellular proteins with PDZ domains. This study is focused on the PDZ protein TIP-2/GIPC, which was originally identified in two-hybrid screens performed with two different baits: the human T-cell leukemia virus type 1 Tax oncoprotein and the regulator of G signaling RGS-GAIP. Further studies have shown that TIP-2/GIPC is also able to associate with the cytoplasmic domains of various transmembrane proteins. In this report we show that TIP-2/GIPC interacts with the E6 protein of human papillomavirus type 18 (HPV-18). This event triggers polyubiquitination and proteasome-mediated degradation of the cellular protein. In agreement with this observation, silencing of E6 by RNA interference in HeLa cells causes an increase in the intracellular TIP-2/GIPC level. This PDZ protein has been previously found to be involved in transforming growth factor β (TGF-β) signaling by favoring expression of the TGF-β type III receptor at the cell membrane. In line with this activity of TIP-2/GIPC, we observed that depletion of this protein in HeLa cells hampers induction of the Id3 gene by TGF-β treatment and also diminishes the antiproliferative effect of this cytokine. Conversely, silencing of E6 increases the expression of Id3 and blocks proliferation of HeLa cells. These results support the notion that HPV-18 E6 renders cells less sensitive to the cytostatic effect of TGF-β by lowering the intracellular amount of TIP-2/GIPC.

Human INT6/eIF3e is required for nonsense‐mediated mRNA decay

The mammalian integration site 6 (INT6) protein has been implicated in breast carcinogenesis and characterized as the eIF3e non‐core subunit of the translation initiation factor eIF3, but its role in this complex is not known. Here, we show that INT6 knockdown by RNA interference strongly inhibits nonsense‐mediated messenger RNA decay (NMD), which triggers degradation of mRNAs with premature stop codons. In contrast to the eIF3b core subunit, which is required for both NMD and general translation, INT6 is only necessary for the former process. Consistent with such a role, immunoprecipitation experiments showed that INT6 co‐purifies with CBP80 and the NMD factor UPF2. In addition, several transcripts known to be upregulated by UPF1 or UPF2 depletion were also found to be sensitive to INT6 suppression. From these observations, we propose that INT6, in association with eIF3, is involved in routing specific mRNAs for degradation.

The Human T-Lymphotropic Virus Type 1 Tax Protein Inhibits Nonsense-Mediated mRNA Decay by Interacting with INT6/EIF3E and UPF1

ABSTRACT In this report, we analyzed whether the degradation of mRNAs by the nonsense-mediated mRNA decay (NMD) pathway was affected in human T-lymphotropic virus type 1 (HTLV-1)-infected cells. This pathway was indeed strongly inhibited in C91PL, HUT102, and MT2 cells, and such an effect was also observed by the sole expression of the Tax protein in Jurkat and HeLa cells. In line with this activity, Tax binds INT6/EIF3E (here called INT6), which is a subunit of the translation initiation factor eukaryotic initiation factor 3 (eIF3) required for efficient NMD, as well as the NMD core factor upstream frameshift protein 1 (UPF1). It was also observed that Tax expression alters the morphology of processing bodies (P-bodies), the cytoplasmic structures which concentrate RNA degradation factors. The presence of UPF1 in these subcellular compartments was increased by Tax, whereas that of INT6 was decreased. In line with these effects, the level of the phosphorylated form of UPF1 was increased in the presence of Tax. Analysis of several mutants of the viral protein showed that the interaction with INT6 is necessary for NMD inhibition. The alteration of mRNA stability was observed to affect viral transcripts, such as that coding for the HTLV-1 basic leucine zipper factor (HBZ), and also several cellular mRNAs sensitive to the NMD pathway. Our data indicate that the effect of Tax on viral and cellular gene expression is not restricted to transcriptional control but can also involve posttranscriptional regulation.

Silencing of human Int-6 impairs mitosis progression and inhibits cyclin B-Cdk1 activation

The Int-6 protein has been originally identified as the product of a mouse gene being a frequent integration site of the mouse mammary tumour virus. Here, we show that reducing Int-6 expression by RNA interference in HeLa cells markedly alters mitosis progression. Defects in spindle formation, chromosome segregation and cytokinesis were observed. These abnormalities of mitosis completion are correlated with an inhibition of cyclin B–Cdk1 kinase activity, due to a prolonged inhibitory phosphorylated state of Cdk1. In line with this observation, the Wee1 tyrosine kinase that negatively controls Cdk1 was less efficiently inactivated during G2 in Int-6-depleted cells. These findings support the notion that the oncogenic properties associated with alteration of Int-6 originate from chromosomal instability.