Ami Aronheim

Identification and analysis of PH domain-containing targets of phosphatidylinositol 3-kinase using a novel in vivo assay in yeast.

Phosphatidylinositol 3‐kinase (PI3K) mediates a variety of cellular responses by generating PtdIns(3,4)P2 and PtdIns(3,4,5)P3. These 3‐phosphoinositides then function directly as second messengers to activate downstream signaling molecules by binding pleckstrin homology (PH) domains in these signaling molecules. We have established a novel assay in the yeast Saccharomyces cerevisiae to identify proteins that bind PtdIns(3,4)P2 and PtdIns(3,4,5)P3 in vivo which we have called TOPIS (Targets of PI3K Identification System). The assay uses a plasma membrane‐targeted Ras to complement a temperature‐sensitive CDC25 Ras exchange factor in yeast. Coexpression of PI3K and a fusion protein of activated Ras joined to a PH domain known to bind PtdIns(3,4)P2 (AKT) or PtdIns(3,4,5)P3 (BTK) rescues yeast growth at the non‐permissive temperature of 37°C. Using this assay, we have identified several amino acids in the β1–β2 region of PH domains that are critical for high affinity binding to PtdIns(3,4)P2 and/or PtdIns(3,4,5)P3, and we have proposed a structural model for how these PH domains might bind PI3K products with high affinity. From these data, we derived a consensus sequence which predicts high‐affinity binding to PtdIns(3,4)P2 and/or PtdIns(3,4,5)P3, and we have identified several new PH domain‐containing proteins that bind PI3K products, including Gab1, Dos, myosinX, and Sbf1. Use of this assay to screen for novel cDNAs which rescue yeast at the non‐permissive temperature should provide a powerful approach for uncovering additional targets of PI3K.

The Ras recruitment system, a novel approach to the study of protein–protein interactions

The yeast two-hybrid system represents one of the most efficient approaches currently available for identifying and characterizing protein-protein interactions [1-4]. Although very powerful, this procedure exhibits several problems and inherent limitations [5]. A new system, the Sos recruitment system (SRS), was developed recently [6] based on a different readout from that of the two-hybrid system [6-8]. SRS overcomes several of the limitations of the two-hybrid system and thus serves as an attractive alternative for studying protein-protein interactions between known and novel proteins. Nevertheless, we encountered a number of problems using SRS and so have developed an improved protein recruitment system, designated the Ras recruitment system (RRS), based on the absolute requirement that Ras be localized to the plasma membrane for its function [9-10]. Ras membrane localization and activation can be achieved through interaction between two hybrid proteins. We have demonstrated the effectiveness of the novel RRS system using five different known protein-protein interactions and have identified two previously unknown protein-protein interactions through a library screening protocol. The first interaction (detailed here) is between JDP2, a member of the basic leucine zipper (bZIP) family, and C/EBPgamma, a member of the CCAAT/enhancer-binding protein (C/EBP) family. The second interaction is between the p21-activated protein kinase Pak65 and a small G protein (described in the accompanying paper by Aronheim et al. [11]). The RRS system significantly extends the usefulness of the previously described SRS system and overcomes several of its limitations.

Chp, a homologue of the GTPase Cdc42Hs, activates the JNK pathway and is implicated in reorganizing the actin cytoskeleton

The p21-activated protein kinases (PAKs) are activated through direct interaction with the GTPases Rac and Cdc42Hs, which are implicated in the control of the mitogen-activated protein kinase (MAP kinase) c-Jun N-terminal kinase (JNK) and the reorganization of the actin cytoskeleton [1-3]. The exact role of the PAK proteins in these signaling pathways is not entirely clear. To elucidate the biological function of Pak2 and to identify its molecular targets, we used a novel two-hybrid system, the Ras recruitment system (RRS), that aims to detect protein-protein interactions at the inner surface of the plasma membrane (described in the accompanying paper by Broder et al. [4]). The Pak2 regulatory domain (PakR) was fused at the carboxyl terminus of a RasL61 mutant protein and screened against a myristoylated rat pituitary cDNA library. Four clones were identified that interact specifically with PakR and three were subsequently shown to encode a previously unknown homologue of the GTPase Cdc42Hs. This approximately 36 kDa protein, designated Chp, exhibits an overall sequence identity to Cdc42Hs of approximately 52%. Chp contains two additional sequences at the amino and carboxyl termini that are not found in any known GTPase. The amino terminus contains a polyproline sequence, typically found in Src homology 3 (SH3)-binding domains, and the carboxyl terminus appears to be important for Pak2 binding. Results from the microinjection of Chp into cells implicated Chp in the induction of lamellipodia and showed that Chp activates the JNK MAP kinase cascade.

A Novel Specific Role for IκB Kinase Complex-associated Protein in Cytosolic Stress Signaling

We demonstrate here a novel role for the IκB kinase complex-associated protein (IKAP) in the regulation of activation of the mammalian stress response via the c-Jun N-terminal kinase (JNK)-signaling pathway. We cloned IKAP as a JNK-associating protein using the Ras recruitment yeast two-hybrid system. IKAP efficiently and specifically enhanced JNK activation induced by ectopic expression of MEKK1 and ASK1, upstream activators of JNK. Importantly, IKAP also enhanced JNK activation induced by ultraviolet light irradiation as well as treatments with tumor necrosis factor or epidermal growth factor. The JNK association site in IKAP was mapped to the C-terminal part of IKAP. Interestingly, this region is deleted from IKAP expressed in the autonomous nervous system of the patients affected by familial dysautonomia. Ectopic expression of this C-terminal fragment of IKAP was sufficient to support JNK activation. Taken together, our data demonstrate a novel role for IKAP in the regulation of the JNK-mediated stress signaling. Additionally, our results point to a role of JNK signaling in familial dysautonomia and, thus, further support the involvement of JNK signaling in the development, survival, and degeneration of the sensory and autonomic nervous system.

RalA interacts with ZONAB in a cell density-dependent manner and regulates its transcriptional activity

Ral proteins are members of the Ras superfamily of small GTPases and are involved in signalling pathways for actin cytoskeleton remodelling, cell cycle control, cellular transformation and vesicle transport. To identify novel RalA effector proteins, we used the reverse Ras recruitment system and found that RalA interacts with a Y‐box transcription factor, ZO‐1‐associated nucleic acid‐binding protein (ZONAB), in a GTP‐dependent manner. The amount of the RalA–ZONAB complex increases as epithelial cells become more dense and increase cell contacts. The RalA–ZONAB interaction results in a relief of transcriptional repression of a ZONAB‐regulated promoter. Additionally, expression of oncogenic Ras alleviates transcriptional repression by ZONAB in a RalA‐dependent manner. The data presented here implicate the RalA/ZONAB interaction in the regulation of ZONAB function.

Jun Dimerization Protein 2 Functions as a Progesterone Receptor N-Terminal Domain Coactivator

ABSTRACT The progesterone receptor (PR) contains two transcription activation function (AF) domains, constitutive AF-1 in the N terminus and AF-2 in the C terminus. AF-2 activity is mediated by a hormone-dependent interaction with a family of steroid receptor coactivators (SRCs). SRC-1 can also stimulate AF-1 activity through a secondary domain that interacts simultaneously with the primary AF-2 interaction site. Other protein interactions and mechanisms that mediate AF-1 activity are not well defined. By interaction cloning, we identified an AP-1 family member, Jun dimerization protein 2 (JDP-2), as a novel PR-interacting protein. JDP-2 was first defined as a c-Jun interacting protein that functions as an AP-1 repressor. PR and JDP-2 interact directly in vitro through the DNA binding domain (DBD) of PR and the basic leucine zipper (bZIP) region of JDP-2. The two proteins also physically associate in mammalian cells, as detected by coimmunoprecipitation, and are recruited in vivo to a progesterone-inducible target gene promoter, as detected by a chromatin immunoprecipitation (ChIP) assay. In cell transfection assays, JDP-2 substantially increased hormone-dependent PR-mediated transactivation and worked primarily by stimulating AF-1 activity. JDP-2 is a substantially stronger coactivator of AF-1 than SRC-1 and stimulates AF-1 independent of SRC-1 pathways. The PR DBD is necessary but not sufficient for JDP-2 stimulation of PR activity; the DBD and AF-1 are required together. JDP-2 lacks an intrinsic activation domain and makes direct protein interactions with other coactivators, including CBP and p300 CBP-associated factor (pCAF), but not with SRCs. These results indicate that JDP-2 stimulates AF-1 activity by the novel mechanism of docking to the DBD and recruiting or stabilizing N-terminal PR interactions with other general coactivators. JDP-2 has preferential activity on PR among the nuclear receptors tested and is expressed in progesterone target cells and tissues, suggesting that it has a physiological role in PR function.

The c-Jun Dimerization Protein 2 Inhibits Cell Transformation and Acts as a Tumor Suppressor Gene

The c-Jun dimerization protein, JDP2, is a member of the AP-1 (activating protein-1) family of the basic leucine zipper transcription factors. JDP2 can bind 12-O-tetradecanoylphorbol-13-acetate (TPA)-responsive element and cAMP-responsive element DNA response elements, resulting in the inhibition of transcription. Although the role of AP-1 in cell proliferation and malignant transformation is well established, the role of JDP2 in this process is of subject to debate. On the one hand, JDP2 was shown to inhibit cyclin D transcription and promote differentiation of skeletal muscle and osteoclast cells. On the other hand, JDP2 was shown to partially transform chicken embryo fibroblast and was identified in a screen for oncogenes able to collaborate with the loss of p27kip cyclin-dependent inhibitor to induce lymphomas. Using cell transformation assays in NIH3T3 cells and injection of prostate cancer cell lines overexpressing JDP2 into severe combined immuno-deficient (SCID) mice, we show for the first time the potential role of JDP2 in inhibition of cell transformation and tumor suppression. The mechanism of tumor suppressor action of JDP2 can be partially explained by the generation of inhibitory AP-1 complexes via the increase of JunB, JunD, and Fra2 expression and decrease of c-Jun expression.

A novel c-Jun N-terminal kinase (JNK)-binding protein WDR62 is recruited to stress granules and mediates a nonclassical JNK activation.

WDR62 is a novel JNK-binding protein. WDR62 potentiates JNK kinase activity but inhibits AP-1 transcription. Cells transfected with WDR62 display cytoplasmic granular localization. WDR62 is localized to stress granule and activated JNK to processing bodies following arsenite treatment. WDR62 may mediate mRNA fate following stress.

Induction of Terminal Differentiation by the c-Jun Dimerization Protein JDP2 in C2 Myoblasts and Rhabdomyosarcoma Cells

Muscle cell differentiation is a result of a complex interplay between transcription factors and cell signaling proteins. Proliferating myoblasts must exit from the cell cycle prior to their differentiation. The muscle regulatory factor and myocyte enhancer factor-2 protein families play a major role in promoting muscle cell differentiation. Conversely, members of the AP-1 family of transcription factors that promote cell proliferation antagonize muscle cell differentiation. Here we tested the role of the c-Jun dimerization protein JDP2 in muscle cell differentiation. Endogenous expression of JDP2 was induced in both C2C12 myoblast and rhabdomyosarcoma (RD) cells programmed to differentiate. Ectopic expression of JDP2 in C2C12 myoblast cells inhibited cell cycle progression and induced spontaneous muscle cell differentiation. Likewise, constitutive expression of JDP2 in RD cells reduced their tumorigenic characteristics and restored their ability to differentiate into myotubes. JDP2 potentiated and synergized with 12-O-tetradecanoylphorbol-13-acetate to induce muscle cell differentiation of RD cells. In addition, JDP2 induced p38 activity in both C2 and RD cells programmed to differentiate. This is the first demonstration of a single transcription factor that rescues the myogenic program in an otherwise non-differentiating cancer cell line. Our results indicate that the JDP2 protein plays a major role in promoting skeletal muscle differentiation via its involvement in cell cycle arrest and activation of the myogenic program.

AP-1 Repressor Protein JDP-2: Inhibition of UV-Mediated Apoptosis through p53 Down-Regulation

ABSTRACT Members of the AP-1 transcription factor family, especially c-Jun and c-Fos, have long been known to mediate critical steps in the cellular response to ultraviolet (UV) irradiation. We sought to examine whether two newly discovered members of the AP-1 family, JDP-1 and JDP-2, also participate in the mammalian UV response. Here we report that JDP-2, but not JDP-1, is transiently induced upon UV challenge and that elevated levels of JDP-2 increase cell survival following UV exposure. This protective function of JDP-2 appears to be mediated through repression of p53 expression at the transcriptional level, via a conserved atypical AP-1 site in the p53promoter.