The reaper-binding protein scythe modulates apoptosis and proliferation during mammalian development

A novel interaction between CCaMK and a protein containing the Scythe_N ubiquitin-like domain in Lotus japonicus

In the Rhizobium-legume symbiosis, calcium/calmodulin-dependent protein kinase (CCaMK) is a key regulator for both rhizobial infection and nodule organogenesis. Deregulation of CCaMK by either a point mutation in the autophosphorylation site or the deletion of the carboxyl-terminal regulatory domain results in spontaneous nodule formation without rhizobia. However, the underlying biochemical mechanisms are poorly understood. Here, using the kinase domain of CCaMK as a bait in yeast two-hybrid screening, we identify a novel protein, CIP73 (for CCaMK-interacting protein of approximately 73 kD), that interacts with CCaMK. CIP73 contains a Scythe_N ubiquitin-like domain and belongs to the large ubiquitin superfamily. Deletion and mutagenesis analysis demonstrate that CIP73 could only interact with CCaMK when the calmodulin-binding domain and three EF-hand motifs are removed from the kinase domain. The amino-terminal 80 amino acid residues (80-160) of CCaMK are required for interacting with CIP73 in yeast cells. On the other hand, protein pull-down assay and bimolecular fluorescence complementation assay in Nicotiana benthamiana show that the full-length CCaMK could interact with CIP73 in vitro and in planta. Importantly, CCaMK phosphorylates the amino terminus of CIP73 in a Ca2+/calmodulin-dependent manner in vitro. CIP73 transcripts are preferentially expressed in roots, and very low expression is detected in leaves, stems, and nodules. The expression in roots is significantly decreased after inoculation of Mesorhizobium loti. RNA interference knockdown of CIP73 expression by hairy root transformation in Lotus japonicus led to decreased nodule formation, suggesting that CIP73 performed an essential role in nodulation.

BAG-6 is essential for selective elimination of defective proteasomal substrates

BAG-6/Scythe/BAT3 is a ubiquitin-like protein that was originally reported to be the product of a novel gene located within the human major histocompatibility complex, although the mechanisms of its function remain largely obscure. Here, we demonstrate the involvement of BAG-6 in the degradation of a CL1 model defective protein substrate in mammalian cells. We show that BAG-6 is essential for not only model substrate degradation but also the ubiquitin-mediated metabolism of newly synthesized defective polypeptides. Furthermore, our in vivo and in vitro analysis shows that BAG-6 interacts physically with puromycin-labeled nascent chain polypeptides and regulates their proteasome-mediated degradation. Finally, we show that knockdown of BAG-6 results in the suppressed presentation of MHC class I on the cell surface, a procedure known to be affected by the efficiency of metabolism of defective ribosomal products. Therefore, we propose that BAG-6 is necessary for ubiquitin-mediated degradation of newly synthesized defective polypeptides.

A ribosome-associating factor chaperones tail-anchored membrane proteins

Hundreds of proteins are inserted post-translationally into the endoplasmic reticulum (ER) membrane by a single carboxy-terminal transmembrane domain (TMD). During targeting through the cytosol, the hydrophobic TMD of these tail-anchored (TA) proteins requires constant chaperoning to prevent aggregation or inappropriate interactions. A central component of this targeting system is TRC40, a conserved cytosolic factor that recognizes the TMD of TA proteins and delivers them to the ER for insertion. The mechanism that permits TRC40 to find and capture its TA protein cargos effectively in a highly crowded cytosol is unknown. Here we identify a conserved three-protein complex composed of Bat3, TRC35 and Ubl4A that facilitates TA protein capture by TRC40. This Bat3 complex is recruited to ribosomes synthesizing membrane proteins, interacts with the TMDs of newly released TA proteins, and transfers them to TRC40 for targeting. Depletion of the Bat3 complex allows non-TRC40 factors to compete for TA proteins, explaining their mislocalization in the analogous yeast deletion strains. Thus, the Bat3 complex acts as a TMD-selective chaperone that effectively channels TA proteins to the TRC40 insertion pathway.

E3 ubiquitin ligase activity and targeting of BAT3 by multiple Legionella pneumophila translocated substrates

The intracellular bacterial pathogen Legionella pneumophila modulates a number of host processes during intracellular growth, including the eukaryotic ubiquitination machinery, which dictates the stability, activity, and/or localization of a large number of proteins. A number of L. pneumophila proteins contain eukaryotic-like motifs typically associated with ubiquitination. Central among these is a family of five F-box-domain-containing proteins of Legionella pneumophila. Each of these five proteins is translocated to the host cytosol by the Dot/Icm type IV protein translocation system during infection. We show that three of these proteins, LegU1, LegAU13, and LicA, interact with components of the host ubiquitination machinery in vivo. In addition, LegU1 and LegAU13 are integrated into functional Skp-Cullin-F-box (SCF) complexes that confer E3 ubiquitin ligase activity. LegU1 specifically interacts with and can direct the ubiquitination of the host chaperone protein BAT3. In a screen for additional L. pneumophila proteins that associate with LegU1 in mammalian cells, we identified the bacterial protein Lpg2160. We demonstrate that Lpg2160 also associates with BAT3 independently of LegU1. We show that Lpg2160 is a translocated substrate of the Dot/Icm system and contains a C-terminal translocation signal. We propose a model in which LegU1 and Lpg2160 may function redundantly or in concert to modulate BAT3 activity during the course of infection.

Bat3 promotes the membrane integration of tail-anchored proteins

The membrane integration of tail-anchored proteins at the endoplasmic reticulum (ER) is post-translational, with different tail-anchored proteins exploiting distinct cytosolic factors. For example, mammalian TRC40 has a well-defined role during delivery of tail-anchored proteins to the ER. Although its Saccharomyces cerevisiae equivalent, Get3, is known to function in concert with at least four other components, Get1, Get2, Get4 and Get5 (Mdy2), the role of additional mammalian proteins during tail-anchored protein biogenesis is unclear. To this end, we analysed the cytosolic binding partners of Sec61beta, a well-defined substrate of TRC40, and identified Bat3 as a previously unknown interacting partner. Depletion of Bat3 inhibits the membrane integration of Sec61beta, but not of a second, TRC40-independent, tail-anchored protein, cytochrome b5. Thus, Bat3 influences the in vitro membrane integration of tail-anchored proteins using the TRC40 pathway. When expressed in Saccharomyces cerevisiae lacking a functional GET pathway for tail-anchored protein biogenesis, Bat3 associates with the resulting cytosolic pool of non-targeted chains and diverts it to the nucleus. This Bat3-mediated mislocalisation is not dependent upon Sgt2, a recently identified component of the yeast GET pathway, and we propose that Bat3 either modulates the TRC40 pathway in higher eukaryotes or provides an alternative fate for newly synthesised tail-anchored proteins.

The intestinal trefoil factor (Tff3), also expressed in the inner ear, interacts with peptides contributing to apoptosis

The 3 members of the mammalian trefoil factor family (TFF) are expressed and secreted as cytoprotective peptides along the entire length of the normal gastrointestinal tract. More recently, they were shown to display multifunctional properties. Goblet cells of the small and large intestine constitute a major source for the synthesis of the third family member, TFF3 (formerly intestinal trefoil factor, ITF). TFF3, like the other family members, is rapidly up-regulated in response to physical wounding of the digestive tract. In addition, Tff3 was also detected in the posterior pituitary gland. Apart from this Tff3 function as a neuropeptide, also presence of Tff3 in the mouse cochlea was noted and Tff3-deficient animals display hearing impairment and accelerated presbyacusis. To elucidate Tff3's mode of function and its unexpected contribution to the hearing process, we strived to determine Tff3's interacting partners and to establish the functional network. To this end, we used a protein-protein binding assay based on a specific transcriptional regulation in yeast cells (the yeast-two-hybrid assay). We looked for interacting partners of Tff3 in a mouse cochlea cDNA library (from donors aged 3-15 days, P3-P15). Our data show that several binding candidates exist and that they could contribute to the known involvement of the trefoil peptides to apoptosis.

A multiplex human syndrome implicates a key role for intestinal cell kinase in development of central nervous, skeletal, and endocrine systems

Six infants in an Old Order Amish pedigree were observed to be affected with endocrine-cerebro-osteodysplasia (ECO). ECO is a previously unidentified neonatal lethal recessive disorder with multiple anomalies involving the endocrine, cerebral, and skeletal systems. Autozygosity mapping and sequencing identified a previously unknown missense mutation, R272Q, in ICK, encoding intestinal cell kinase (ICK). Our results established that R272 is conserved across species and among ethnicities, and three-dimensional analysis of the protein structure suggests protein instability due to the R272Q mutation. We also demonstrate that the R272Q mutant fails to localize at the nucleus and has diminished kinase activity. These findings suggest that ICK plays a key role in the development of multiple organ systems.

Scythe/BAT3 regulates apoptotic cell death induced by papillomavirus binding factor in human osteosarcoma

Papillomavirus binding factor (PBF) was first identified as a transcription factor regulating the promoter activity of human papillomavirus. We previously demonstrated that PBF is an osteosarcoma-associated antigen and 92% of osteosarcoma tissues express PBF in the nucleus. Moreover, PBF-positive osteosarcoma has a significantly poorer prognosis than that with negative expression of PBF. In the present study, we assessed the biological role of PBF in cell survival. Overexpression of PBF induced cell death-mediated lactate dehydrase (LDH) release from 293EBNA cells. Cleaved poly(ADP-ribose) polymerase and active caspase-3 were also detected. However, PBF-induced apoptosis did not affect caspase-9 activity. Next, to identify the apoptosis regulator of PBF, we screened a cDNA library constructed from mRNA of the osteosarcoma cell line OS2000 using a yeast two-hybrid system and isolated Scythe/BAT3. Scythe/BAT3 mRNA was detected in 56% of osteosarcoma tissues and ubiquitously in various normal tissues. Although Scythe/BAT3 was localized to the cytoplasm in normal tissue, it was localized to the nucleus in osteosarcoma tissue. PBF and Scythe/BAT3 also colocalized to the cytoplasm in 293T cells and the nucleus in OS2000. Furthermore, overexpression of Scythe/BAT3 suppressed cell death events that resulted from overexpression of PBF in OS2000, but not in 293EBNA cells. Thus, our results support the ideas that: (i) PBF could induce apoptotic cell death via a caspase-9-independent pathway; (ii) the apoptosis regulator Scythe/BAT3 is a PBF-associated molecule acting as a nucleus-cytoplasm shuttling protein; and (iii) colocalization of PBF and Scythe/BAT3 in the nucleus might be an important factor for survival of osteosarcoma cells.

Dendritic cells release HLA-B-associated transcript-3 positive exosomes to regulate natural killer function

NKp30, a natural cytotoxicity receptor expressed on NK cells is critically involved in direct cytotoxicity against various tumor cells and directs both maturation and selective killing of dendritic cells. Recently the intracellular protein BAT3, which is involved in DNA damage induced apoptosis, was identified as a ligand for NKp30. However, the mechanisms underlying the exposure of the intracellular ligand BAT3 to surface NKp30 and its role in NK-DC cross talk remained elusive. Electron microscopy and flow cytometry demonstrate that exosomes released from 293T cells and iDCs express BAT3 on the surface and are recognized by NKp30-Ig. Overexpression and depletion of BAT3 in 293T cells directly correlates with the exosomal expression level and the activation of NK cell-mediated cytokine release. Furthermore, the NKp30-mediated NK/DC cross talk resulting either in iDC killing or maturation was BAT3-dependent. Taken together this puts forward a new model for the activation of NK cells through intracellular signals that are released via exosomes from accessory cells. The manipulation of the exosomal regulation may offer a novel strategy to induce tumor immunity or inhibit autoimmune diseases caused by NK cell-activation.

UNDERSTANDING NITROGEN LIMITATION IN AUREOCOCCUS ANOPHAGEFFERENS (PELAGOPHYCEAE) THROUGH cDNA AND qRT-PCR ANALYSIS(1)

Brown tides of the marine pelagophyte Aureococcus anophagefferens Hargraves et Sieburth have been investigated extensively for the past two decades. Its growth is fueled by a variety of nitrogen (N) compounds, with dissolved organic nitrogen (DON) being particularly important during blooms. Characterization of a cDNA library suggests that A. anophagefferens can assimilate eight different forms of N. Expression of genes related to the sensing, uptake, and assimilation of inorganic and organic N, as well as the catabolic process of autophagy, was assayed in cells grown on different N sources and in N-limited cells. Growth on nitrate elicited an increase in the relative expression of nitrate and ammonium transporters, a nutrient stress-induced transporter, and a sensory kinase. Growth on urea increased the relative expression of a urea and a formate/nitrite transporter, while growth on ammonium resulted in an increase in the relative expression of an ammonium transporter, a novel ATP-binding cassette (ABC) transporter and a putative high-affinity phosphate transporter. N limitation resulted in a 30- to 110-fold increase in the relative expression of nitrate, ammonium, urea, amino acid/polyamine, and formate/nitrite transporters. A. anophagefferens demonstrated the highest relative accumulation of a transcript encoding a novel purine transporter, which was highly expressed across all N sources. This finding suggests that purines are an important source of N for the growth of this organism and could possibly contribute to the initiation and maintenance of blooms in the natural environment.

BAT3 and SET1A form a complex with CTCFL/BORIS to modulate H3K4 histone dimethylation and gene expression

Chromatin status is characterized in part by covalent posttranslational modifications of histones that regulate chromatin dynamics and direct gene expression. BORIS (brother of the regulator of imprinted sites) is an insulator DNA-binding protein that is thought to play a role in chromatin organization and gene expression. BORIS is a cancer-germ line gene; these are genes normally present in male germ cells (testis) that are also expressed in cancer cell lines as well as primary tumors. This work identifies SET1A, an H3K4 methyltransferase, and BAT3, a cochaperone recruiter, as binding partners for BORIS, and these proteins bind to the upstream promoter regions of two well-characterized procarcinogenic genes, Myc and BRCA1. RNA interference (RNAi) knockdown of BAT3, as well as SET1A, decreased Myc and BRCA1 gene expression but did not affect the binding properties of BORIS, but RNAi knockdown of BORIS prevented the assembly of BAT3 and SET1A at the Myc and BRCA1 promoters. Finally, chromatin analysis suggested that BORIS and BAT3 exert their effects on gene expression by recruiting proteins such as SET1A that are linked to changes in H3K4 dimethylation. Thus, we propose that BORIS acts as a platform upon which BAT3 and SET1A assemble and exert effects upon chromatin structure and gene expression.

Bat3 deficiency accelerates the degradation of Hsp70-2/HspA2 during spermatogenesis

Meiosis is critical for sexual reproduction. During meiosis, the dynamics and integrity of homologous chromosomes are tightly regulated. The genetic and molecular mechanisms governing these processes in vivo, however, remain largely unknown. In this study, we demonstrate that Bat3/Scythe is essential for survival and maintenance of male germ cells (GCs). Targeted inactivation of Bat3/Scythe in mice results in widespread apoptosis of meiotic male GCs and complete male infertility. Pachytene spermatocytes exhibit abnormal assembly and disassembly of synaptonemal complexes as demonstrated by abnormal SYCP3 staining and sustained γ-H2AX and Rad51/replication protein A foci. Further investigation revealed that a testis-specific protein, Hsp70-2/HspA2, is absent in Bat3-deficient male GCs at any stage of spermatogenesis; however, Hsp70-2 transcripts are expressed at normal levels. We found that Bat3 deficiency induces polyubiquitylation and subsequent degradation of Hsp70-2. Inhibition of proteasomal degradation restores Hsp70-2 protein levels. Our findings identify Bat3 as a critical regulator of Hsp70-2 in spermatogenesis, thereby providing a possible molecular target in idiopathic male infertility.

BAT3 interacts with transforming growth factor-beta (TGF-beta) receptors and enhances TGF-beta1-induced type I collagen expression in mesangial cells

Transforming growth factor-beta1 (TGF-beta1) plays essential roles in a wide array of cellular processes, such as in development and the pathogenesis of tissue fibrosis, including that associated with progressive kidney diseases. Tight regulation of its signaling pathways is critical, and proteins that associate with the TGF-beta receptors may exert positive or negative regulatory effects on TGF-beta signaling. In the present study we employed a yeast-based two-hybrid screening system to identify BAT3 (HLA-B-associated transcript 3) as a TGF-beta receptor-interacting protein. Analysis of endogenously expressed BAT3 in various tissues including the kidney reveals the existence of approximately 140-kDa full-length protein as well as truncated forms of BAT3 whose expression is developmentally regulated. Endogenous BAT3 protein interacts with TGF-beta receptors type I and type II in renal mesangial cells. Functional assays show that expression of full-length BAT3 results in enhancement of TGF-beta1-stimulated transcriptional activation of p3TP-Lux reporter, and these effects require the presence of functional TGF-beta signaling receptors as demonstrated in R-1B and DR-26 mutant cells. Moreover, expression of full-length BAT3, but not C-terminal truncated mutant of BAT3, enhanced TGF-beta1-induced type I collagen expression in mesangial cells, whereas knock down of BAT3 protein expression by small interfering RNA suppressed the expression of type I collagen induced by TGF-beta1. Our findings suggest that BAT3, a TGF-beta receptor-interacting protein, is capable of modulating TGF-beta signaling and acts as a positive regulator of TGF-beta1 stimulation of type I collagen expression in mesangial cells.

Human leukocyte antigen-B-associated transcript 3 is released from tumor cells and engages the NKp30 receptor on natural killer cells

The activity of natural killer (NK) cells is regulated by surface receptors, which direct target cell recognition. NKp30 (Natural Cytotoxicity Receptor 3) induces target cell lysis and is also crucial for the interaction with dendritic cells. So far, the cellular ligands for NKp30 have remained elusive. Here we show that the nuclear factor HLA-B-associated transcript 3 (BAT3) was released from tumor cells, bound directly to NKp30, and engaged NKp30 on NK cells. BAT3 triggered NKp30-mediated cytotoxicity and was necessary for tumor rejection in a multiple myeloma model. These data identify BAT3 as a cellular ligand for NKp30. We propose a concept for target cell recognition by NK cells beyond "missing self" and "induced self," mediated through a tumor cell-derived extracellular factor.

Scythe regulates apoptosis-inducing factor stability during endoplasmic reticulum stress-induced apoptosis

Scythe (BAT3; HLA-B associated transcript 3, Bag 6) is a protein that has been implicated in apoptosis because it can modulate the Drosophila melanogaster apoptotic regulator, Reaper. Mice lacking Scythe show pronounced defects in organogenesis and in the regulation of apoptosis and proliferation during mammalian development. However, the biochemical pathways important for Scythe function are unknown. We report here multiple levels of interaction between Scythe and the apoptogenic mitochondrial intermembrane protein AIF (apoptosis-inducing factor). Scythe physically interacts with AIF and regulates its stability. AIF stability is markedly reduced in Scythe(-/-) cells, which are more resistant to endoplasmic reticulum stress induced by thapsigargin. Reintroduction of Scythe or overexpression of AIF in Scythe(-/-) cells restores their sensitivity to apoptosis. Together, these data implicate Scythe as a regulator of AIF.

Screening of AMP-activated protein kinase alpha2 subunit interacting proteins by bacterial two-hybrid system

AMP-activated protein kinase (AMPK) is a ubiquitous eukaryotic protein kinase regulating cellular metabolism and energy demand. In brain AMPK plays a role as a multidimensional energy sensor and AMPK alpha2 subunit is expressed at higher levels than AMPK alpha1 subunit. In order to identify potential targets of AMPK in brain, we perform bacterial two-hybrid screening of a rat fetal brain cDNA library using AMPK alpha2 subunit as bait. Here, we present seven potential AMPK alpha2 subunit interacting proteins, including 6-phosphofructo-1-kinase (PFK-1), polyubiquitin, cytochrome c oxidase subunit I (COX I), heat shock protein 8 (HSP8), HLA-B-associated transcript 3 (BAT3) isoform 1, protein tyrosine phosphatase receptor type D (PTPRD) and islet-brain 1 (IB1). They are involved in glycolysis, protein degradation, mitochondrial electron transport and apoptosis pathways participating in energy regulation directly or indirectly. These data may provide new insight into further studying the pathways of AMPK energy regulation in brain and possible mechanisms of AMPK-mediated neuroprotective effect.

Scythe regulates apoptosis through modulating ubiquitin-mediated proteolysis of the Xenopus elongation factor XEF1AO

Scythe was originally identified as a novel Reaper-binding anti-apoptotic protein, although the mechanisms of its functions remain largely obscure. Our previous analysis revealed that Scythe can bind to a proteasomal subunit via N-terminal domains and that the domains are required for appropriate development of Xenopus embryos. In the present study, we show evidence that the N-terminus of Scythe interacts with XEF1AO, a maternal form of Xenopus laevis EF1A that was suggested to be a potential inducer of apoptosis in vertebrates, and that the binding enhances the poly-ubiquitin modification and subsequent degradation of XEF1AO. Scythe is required for degradation of XEF1AO, since immunodepletion of Scythe from embryonic extracts stabilized XEF1AO significantly. Furthermore, we show that apoptosis induced by accumulation of XEF1AO can be suppressed by co-expression of the full-length form of Scythe. These observations indicate that the proteolytic regulation of XEF1AO, mediated through Scythe, is essential to prevent inappropriate accumulation of XEF1AO and resulting apoptotic events during the course of Xenopus development.

HLA-B-associated transcript 3 (Bat3)/Scythe is essential for p300-mediated acetylation of p53

In response to DNA damage, p53 undergoes post-translational modifications (including acetylation) that are critical for its transcriptional activity. However, the mechanism by which p53 acetylation is regulated is still unclear. Here, we describe an essential role for HLA-B-associated transcript 3 (Bat3)/Scythe in controlling the acetylation of p53 required for DNA damage responses. Depletion of Bat3 from human and mouse cells markedly impairs p53-mediated transactivation of its target genes Puma and p21. Although DNA damage-induced phosphorylation, stabilization, and nuclear accumulation of p53 are not significantly affected by Bat3 depletion, p53 acetylation is almost completely abolished. Bat3 forms a complex with p300, and an increased amount of Bat3 enhances the recruitment of p53 to p300 and facilitates subsequent p53 acetylation. In contrast, Bat3-depleted cells show reduced p53-p300 complex formation and decreased p53 acetylation. Furthermore, consistent with our in vitro findings, thymocytes from Bat3-deficient mice exhibit reduced induction of puma and p21, and are resistant to DNA damage-induced apoptosis in vivo. Our data indicate that Bat3 is a novel and essential regulator of p53-mediated responses to genotoxic stress, and that Bat3 controls DNA damage-induced acetylation of p53.

Phenotype-based identification of host genes required for replication of African swine fever virus

African swine fever virus (ASFV) produces a fatal acute hemorrhagic fever in domesticated pigs that potentially is a worldwide economic threat. Using an expressed sequence tag (EST) library-based antisense method of random gene inactivation and a phenotypic screen for limitation of ASFV replication in cultured human cells, we identified six host genes whose cellular functions are required by ASFV. These included three loci, BAT3 (HLA-B-associated transcript 3), C1qTNF (C1q and tumor necrosis factor-related protein 6), and TOM40 (translocase of outer mitochondrial membrane 40), for which antisense expression from a tetracycline-regulated promoter resulted in reversible inhibition of ASFV production by >99%. The effects of antisense transcription of the BAT3 EST and also of expression in the sense orientation of this EST, which encodes amino acid residues 450 to 518 of the mature BAT3 protein, were investigated more extensively. Sense expression of the BAT3 peptide, which appears to reversibly interfere with BAT3 function by a dominant negative mechanism, resulted in decreased synthesis of viral DNA and proteins early after ASFV infection, altered transcription of apoptosis-related genes as determined by cDNA microarray analysis, and increased cellular sensitivity to staurosporine-induced apoptosis. Antisense transcription of BAT3 reduced ASFV production without affecting abundance of the virus macromolecules we assayed. Our results, which demonstrate the utility of EST-based functional screens for the detection of host genes exploited by pathogenic viruses, reveal a novel collection of cellular genes previously not known to be required for ASFV infection.

Identification of yin-yang regulators and a phosphorylation consensus for male germ cell-associated kinase (MAK)-related kinase

MAK (male germ cell-associated protein kinase) and MRK/ICK (MAK-related kinase/intestinal cell kinase) are human homologs of Ime2p in Saccharomyces cerevisiae and of Mde3 and Pit1 in Schizosaccharomyces pombe and are similar to human cyclin-dependent kinase 2 (CDK2) and extracellular signal-regulated kinase 2 (ERK2). MAK and MRK require dual phosphorylation in a TDY motif catalyzed by an unidentified human threonine kinase and tyrosine autophosphorylation. Herein, we establish that human CDK-related kinase CCRK (cell cycle-related kinase) is an activating T157 kinase for MRK, whereas active CDK7/cyclin H/MAT1 complexes phosphorylate CDK2 but not MRK. Protein phosphatase 5 (PP5) interacts with MRK in a complex and dephosphorylates MRK at T157 in vitro and in situ. Thus, CCRK and PP5 are yin-yang regulators of T157 phosphorylation. To determine a substrate consensus, we screened a combinatorial peptide library with active MRK. MRK preferentially phosphorylates R-P-X-S/T-P sites, with the preference for arginine at position -3 (P-3) being more stringent than for prolines at P-2 and P+1. Using the consensus, we identified a putative phosphorylation site (RPLT(1080)S) for MRK in human Scythe, an antiapoptotic protein that interacts with MRK. MRK phosphorylates Scythe at T1080 in vitro as determined by site-directed mutagenesis and mass spectrometry, supporting the consensus and suggesting Scythe as a physiological substrate for MRK.

Human SGT interacts with Bag-6/Bat-3/Scythe and cells with reduced levels of either protein display persistence of few misaligned chromosomes and mitotic arrest

The human small glutamine-rich TPR-containing protein (hSGT) is essential for cell division since RNA-interference-mediated strong reduction of hSGT protein levels causes mitotic arrest (M. Winnefeld, J. Rommelaere, and C. Cziepluch, The human small glutamine-rich TPR-containing protein is required for progress through cell division, Exp. Cell Res. 293 (2004), 43-57). Analysis of HeLa cells expressing a histone 2A-YFP fusion protein revealed the continuous presence of few mislocalized chromosomes close to the spindle poles as possible cause for hSGT depletion-dependent prometaphase arrest. Cells unable to rescue these mislocalized chromosomes into the metaphase plate died at this stage through apoptosis. In order to address hSGT function at the molecular level, mass spectrometry analysis of proteins which co-immunoprecipitated with Flag-tagged hSGT was performed. Thereby, Hsp70 and Bag-6/Bat-3/Scythe were identified as novel hSGT interaction partners while interaction with Hsc70 was confirmed. Results obtained with truncated versions of the hSGT protein revealed that Bag-6/Bat-3/Scythe and Hsp70 or Hsc70 were independently able to form complexes with hSGT. Interaction of hSGT with Hsc70, Hsp70 or Bag-6/Bat-3/Scythe was demonstrated in prometaphase, thereby suggesting a possible role for complexes containing hSGT and distinct (co)-chaperones during mitosis. Finally, cells from populations with reduced levels of Bag-6/Bat-3/Scythe also displayed persistence of mislocalized chromosomes and mitotic arrest, which strongly indicated that hSGT-Bag-6/Bat-3/Scythe complexes could be directly or indirectly required for complete chromosome congression.