Evidence for the interaction of the regulatory protein Ki-1/57 with p53 and its interacting proteins

Clinical and immunohistochemical effects of OncoTherad (MRB-CFI-1) nanoimmunotherapy on SERBP1, HABP4, CD44 and Ki-67 in BCG-unresponsive non-muscle invasive bladder cancer

Non-muscle-invasive bladder cancer (NMIBC) is a malignancy with a high recurrence and progression rate, particularly in patients who fail to respond to standard Bacillus Calmette-Guérin (BCG) therapy. OncoTherad (MRB-CFI-1) nanoimmunotherapy has emerged as a promising therapeutic option, with potential to modulate immune responses and inhibit tumor progression. This study evaluated the clinical efficacy of OncoTherad (MRB-CFI-1) nanoimmunotherapy in patients with BCG-unresponsive NMIBC and investigated correlations between therapeutic outcomes and histopathological and molecular findings. In this retrospective cross-sectional study, 20 patients with BCG-unresponsive NMIBC were treated with OncoTherad (MRB-CFI-1) across two clinical centers. Bladder tissue samples were collected before and after treatment, and immunohistochemical analyses were performed to assess the expression of SERBP1, HABP4, CD44, and Ki-67. Primary endpoints included pathological complete response (pCR), recurrence-free survival (RFS), and duration of response (DoR), which were analyzed in relation to immunohistochemical biomarker findings. Our results demonstrated that high Ki-67 proliferative index and elevated immunoreactivity for CD44 and SERBP1 were associated with shorter RFS. Treatment with OncoTherad (MRB-CFI-1) significantly reduced (p < 0.05) the immunoreactivity of SERBP1 and CD44, which was accompanied by a marked decrease in Ki-67 proliferative index, indicating effective suppression of tumor activity. Conversely, a significant increase (p < 0.05) in HABP4 immunoreactivity was observed, suggesting a protective role against NMIBC recurrence and progression. A pCR was achieved in 65 % of patients, with a median RFS of 21.1 months and a median DoR of 15.7 months, underscoring the clinical efficacy of OncoTherad (MRB-CFI-1). These findings suggest that OncoTherad (MRB-CFI-1) nanoimmunotherapy offers a novel and effective treatment strategy for patients with BCG-unresponsive NMIBC, providing a promising alternative to radical cystectomy and significantly improving patient outcomes.

Avian leukosis virus usurps the cellular SERBP1 protein to enhance its transcription and promote productive infections in avian cells

Avian leukosis virus subgroup K (ALV-K) is composed of newly emerging isolates, which cluster separately from the well-characterized subgroups A, B, C, D, E, and J in sequence analysis, and exhibits a specific host range and a unique pattern of superinfection interference. Avian leukosis virus subgroup K replicate more slowly in avian cells than other ALV strains, leading to escaped detection during ALV eradication, but the underlying mechanism are largely unknown. In our previous study, we have reported that JS11C1 and most of other suspected ALV-K strains possessed unique mutations in the U3 region. Here, we selected 5 mutations in some important transcriptional regulation elements to explore the possible factor contributing for the lower activity of LTR, including CA-TG mutation in the CAAT box, 21 nt deletion in the CAAT box, A-G and A-T mutations in the CArG boxes, 11 nt insertion in the PRE boxes, and C-T mutation in the TATA box. On the basis of infectious clone of JS11C1, we demonstrated that the 11 nt fragment in the PRE boxes was associated with the transcription activity of LTR, the enhancer ability of U3, and the replication capacity of the virus. Notably, we determined the differential U3-protein interaction profile of ALVs and found that the 11 nt fragment specifically binds to cellular SERPINE1 mRNA binding protein 1 (SERBP1) to increase the LTR activity and enhance virus replication. Collectively, these findings reveal that a 11 nt fragment in the U3 gene contributed to its binding ability to the cellular SERBP1 to enhance its transcription and the infectious virus productions in avian cells. This study highlighted the vital role of host factor in retrovirus replication and thus provides a new perspective to elucidate the interaction between retrovirus and its host and a molecular basis to develop efficient strategies against retroviruses.

HABP4 overexpression promotes apoptosis in goat turbinate bone cells

Hyaluronic acid-binding protein (HABP4) plays important roles in regulating cell cycle and apoptosis. However, its functions in regulating cell apoptosis remain unclear. To reveal the effects of HABP4 on cell proliferation, cell cycle and apoptosis, the HABP4 sequence was cloned, and we investigated the gain and loss functions of HABP4 in goat turbinate bone cells. Our results showed that a 1,496-bp HABP4 sequence was cloned successfully. The interference effect of siRNA1 on HABP4 was the strongest, reducing its mRNA expression level by 83%, decreasing the cells in the G0/G1 and S phases of the cell cycle and inhibiting cell growth and apoptosis. The overexpression of HABP4 produced contrasting results. Furthermore, an HABP4 knockdown caused the up-regulated expression of genes associated with apoptosis, including Bcl-2 and BCL2L11, but the down-regulation of Caspase3, Caspase7, Bax, PARP1, SOCS2 and P53 mRNA levels. Additionally, HABP4 overexpression significantly up-regulated the expression levels of Bax, Caspase3, Caspase7, BCL2L11, P53, SOCS2 and PARP1. However, the expression of Bcl-2 was down-regulated. These data provide an important foundation for further in-depth studies of HABP4 functions.

Intracellular hyaluronic acid-binding protein 4 (HABP4): a candidate tumor suppressor in colorectal cancer

Hyaluronic Acid-binding protein 4 (HABP4) is a regulatory protein of 57 kDa that is functionally involved in transcription regulation and RNA metabolism and shows several characteristics common to oncoproteins or tumor suppressors, including altered expression in cancer tissues, nucleus/cytoplasm shuttling, intrinsic lack of protein structure, complex interactomes and post translational modifications. Its gene has been found in a region on chromosome 9q22.3-31, which contains SNP haplotypes occurring in individuals with a high risk for familial colon cancer. To test a possible role of HABP4 in tumorigenesis we generated knockout mice by the CRISPR/Cas9 method and treated the animals with azoxymethane (AOM)/dextran sodium sulfate (DSS) for induction of colon tumors. HABP4-/- mice, compared to wild type mice, had more and larger tumors, and expressed more of the proliferation marker proteins Cyclin-D1, CDK4 and PCNA. Furthermore, the cells of the bottom of the colon crypts in the HABP4-/- mice divided more rapidly. Next, we generated also HABP4-/- HCT 116 cells, in cell culture and found again an increased proliferation in clonogenic assays in comparison to wild-type cells. Our study of the protein expression levels of HABP4 in human colon cancer samples, through immunohistochemistry assays, showed, that 30% of the tumors analyzed had low expression of HABP4. Our data suggest that HABP4 is involved in proliferation regulation of colon cells in vitro and in vivo and that it is a promising new candidate for a tumor suppressor protein that can be explored both in the diagnosis and possibly therapy of colon cancer.

Complex interactomes and post-translational modifications of the regulatory proteins HABP4 and SERBP1 suggest pleiotropic cellular functions

The 57 kDa antigen recognized by the Ki-1 antibody, is also known as intracellular hyaluronic acid binding protein 4 and shares 40.7% identity and 67.4% similarity with serpin mRNA binding protein 1, which is also named CGI-55, or plasminogen activator inhibitor type-1-RNA binding protein-1, indicating that they might be paralog proteins, possibly with similar or redundant functions in human cells. Through the identification of their protein interactomes, both regulatory proteins have been functionally implicated in transcriptional regulation, mRNA metabolism, specifically RNA splicing, the regulation of mRNA stability, especially, in the context of the progesterone hormone response, and the DNA damage response. Both proteins also show a complex pattern of post-translational modifications, involving Ser/Thr phosphorylation, mainly through protein kinase C, arginine methylation and SUMOylation, suggesting that their functions and locations are highly regulated. Furthermore, they show a highly dynamic cellular localization pattern with localizations in both the cytoplasm and nucleus as well as punctuated localizations in both granular cytoplasmic protein bodies, upon stress, and nuclear splicing speckles. Several reports in the literature show altered expressions of both regulatory proteins in a series of cancers as well as mutations in their genes that may contribute to tumorigenesis. This review highlights important aspects of the structure, interactome, post-translational modifications, sub-cellular localization and function of both regulatory proteins and further discusses their possible functions and their potential as tumor markers in different cancer settings.

Functional Analysis of Human Hub Proteins and Their Interactors Involved in the Intrinsic Disorder-Enriched Interactions

Some of the intrinsically disordered proteins and protein regions are promiscuous interactors that are involved in one-to-many and many-to-one binding. Several studies have analyzed enrichment of intrinsic disorder among the promiscuous hub proteins. We extended these works by providing a detailed functional characterization of the disorder-enriched hub protein-protein interactions (PPIs), including both hubs and their interactors, and by analyzing their enrichment among disease-associated proteins. We focused on the human interactome, given its high degree of completeness and relevance to the analysis of the disease-linked proteins. We quantified and investigated numerous functional and structural characteristics of the disorder-enriched hub PPIs, including protein binding, structural stability, evolutionary conservation, several categories of functional sites, and presence of over twenty types of posttranslational modifications (PTMs). We showed that the disorder-enriched hub PPIs have a significantly enlarged number of disordered protein binding regions and long intrinsically disordered regions. They also include high numbers of targeting, catalytic, and many types of PTM sites. We empirically demonstrated that these hub PPIs are significantly enriched among 11 out of 18 considered classes of human diseases that are associated with at least 100 human proteins. Finally, we also illustrated how over a dozen specific human hubs utilize intrinsic disorder for their promiscuous PPIs.

Human Regulatory Protein Ki-1/57 Is a Target of SUMOylation and Affects PML Nuclear Body Formation

Ki-1/57 is a nuclear and cytoplasmic regulatory protein first identified in malignant cells from Hodgkin's lymphoma. It is involved in gene expression regulation on both transcriptional and mRNA metabolism levels. Ki-1/57 belongs to the family of intrinsically unstructured proteins and undergoes phosphorylation by PKC and methylation by PRMT1. Previous characterization of its protein interaction profile by yeast two-hybrid screening showed that Ki-1/57 interacts with proteins of the SUMOylation machinery, the SUMO E2 conjugating enzyme UBC9 and the SUMO E3 ligase PIAS3, which suggested that Ki-1/57 could be involved with this process. Here we identified seven potential SUMO target sites (lysine residues) on Ki-1/57 sequence and observed that Ki-1/57 is modified by SUMO proteins in vitro and in vivo. We showed that SUMOylation of Ki-1/57 occurred on lysines 213, 276, and 336. In transfected cells expressing FLAG-Ki-1/57 wild-type, its paralog FLAG-CGI-55 wild-type, or their non-SUMOylated triple mutants, the number of PML-nuclear bodies (PML-NBs) is reduced compared with the control cells not expressing the constructs. More interestingly, after treating cells with arsenic trioxide (As₂O₃), the number of PML-NBs is no longer reduced when the non-SUMOylated triple mutant Ki-1/57 is expressed, suggesting that the SUMOylation of Ki-1/57 has a role in the control of As₂O₃-induced PML-NB formation. A proteome-wide analysis of Ki-1/57 partners in the presence of either SUMO-1 or SUMO-2 suggests that the involvement of Ki-1/57 with the regulation of gene expression is independent of the presence of either SUMO-1 or SUMO-2; however, the presence of SUMO-1 strongly influences the interaction of Ki-1/57 with proteins associated with cellular metabolism, maintenance, and cell cycle.

p53, p63 and p73 in the wonderland of S. cerevisiae

Since its discovery in 1979, p53 has been on the forefront of cancer research. It is considered a master gene of cancer suppression and is found mutated in around 50% of all human tumors. In addition, the progressive identification of p53-related transcription factors p63 and p73 as well as their multiple isoforms have added further layers of complexity to an already dense network. Among the numerous models used to unravel the p53 family mysteries, S. cerevisiae has been particularly useful. This seemingly naive model allows the expression of a functional human p53 and thus the assessment of p53 intrinsic transcriptional activity. The aim of this article is to review the various contributions that the budding yeast has made to the understanding of p53, p63 and p73 biology and to envision new possible directions for yeast-based assays in the field of cancer as well as other p53-family-related diseases.

Sugarcane transcriptome analysis in response to infection caused by Acidovorax avenae subsp. avenae

Sugarcane is an important tropical crop mainly cultivated to produce ethanol and sugar. Crop productivity is negatively affected by Acidovorax avenae subsp avenae (Aaa), which causes the red stripe disease. Little is known about the molecular mechanisms triggered in response to the infection. We have investigated the molecular mechanism activated in sugarcane using a RNA-seq approach. We have produced a de novo transcriptome assembly (TR7) from sugarcane RNA-seq libraries submitted to drought and infection with Aaa. Together, these libraries present 247 million of raw reads and resulted in 168,767 reference transcripts. Mapping in TR7 of reads obtained from infected libraries, revealed 798 differentially expressed transcripts, of which 723 were annotated, corresponding to 467 genes. GO and KEGG enrichment analysis showed that several metabolic pathways, such as code for proteins response to stress, metabolism of carbohydrates, processes of transcription and translation of proteins, amino acid metabolism and biosynthesis of secondary metabolites were significantly regulated in sugarcane. Differential analysis revealed that genes in the biosynthetic pathways of ET and JA PRRs, oxidative burst genes, NBS-LRR genes, cell wall fortification genes, SAR induced genes and pathogenesis-related genes (PR) were upregulated. In addition, 20 genes were validated by RT-qPCR. Together, these data contribute to a better understanding of the molecular mechanisms triggered by the Aaa in sugarcane and opens the opportunity for the development of molecular markers associated with disease tolerance in breeding programs.

Ki-1/57 and CGI-55 ectopic expression impact cellular pathways involved in proliferation and stress response regulation

Ki-1/57 (HABP4) and CGI-55 (SERBP1) are regulatory proteins and paralogs with 40.7% amino acid sequence identity and 67.4% similarity. Functionally, they have been implicated in the regulation of gene expression on both the transcriptional and mRNA metabolism levels. A link with tumorigenesis is suggested, since both paralogs show altered expression levels in tumor cells and the Ki-1/57 gene is found in a region of chromosome 9q that represents a haplotype for familiar colon cancer. However, the target genes regulated by Ki-1/57 and CGI-55 are unknown. Here, we analyzed the alterations of the global transcriptome profile after Ki-1/57 or CGI-55 overexpression in HEK293T cells by DNA microchip technology. We were able to identify 363 or 190 down-regulated and 50 or 27 up-regulated genes for Ki-1/57 and CGI-55, respectively, of which 20 were shared between both proteins. Expression levels of selected genes were confirmed by qRT-PCR both after protein overexpression and siRNA knockdown. The majority of the genes with altered expression were associated to proliferation, apoptosis and cell cycle control processes, prompting us to further explore these contexts experimentally. We observed that overexpression of Ki-1/57 or CGI-55 results in reduced cell proliferation, mainly due to a G1 phase arrest, whereas siRNA knockdown of CGI-55 caused an increase in proliferation. In the case of Ki-1/57 overexpression, we found protection from apoptosis after treatment with the ER-stress inducer thapsigargin. Together, our data give important new insights that may help to explain these proteins putative involvement in tumorigenic events.

CHD chromatin remodelling enzymes and the DNA damage response

The protein and DNA complex known as chromatin is a dynamic structure, adapting to alter the spatial arrangement of genetic information within the nucleus to meet the ever changing demands of life. Following decades of research, a dizzying array of regulatory factors is now known to control the architecture of chromatin at nearly every level. Amongst these, ATP-dependent chromatin remodelling enzymes play a key role, required for the establishment, maintenance and re-organization of chromatin through their ability to adjust the contact points between DNA and histones, the spacing between individual nucleosomes and the over-arching chromatin superstructure. Utilizing energy from ATP hydrolysis, these enzymes serve as the gatekeepers of genomic access and are essential for transcriptional regulation, DNA replication and cell division. In recent years, a vital role in DNA Double Strand Break (DSB) repair has emerged, particularly within complex chromatin environments such as heterochromatin, or regions undergoing energetic transactions such as transcription or DNA replication. Here, we will provide an overview of what is understood about ATP-dependent chromatin remodelling enzymes in the context of the DNA damage response. We will first touch upon all four major chromatin remodelling enzyme families and then focus chiefly on the nine members of the Chromodomain, Helicase, DNA-binding (CHD) family, particularly CHD3, CHD4, CHD5 and CHD6. These four proteins have established and emerging roles in DNA repair, the oxidative stress response, the maintenance of genomic stability and/or cancer prevention.

Evidence for the association of the human regulatory protein Ki-1/57 with the translational machinery

Ki-1/57 is a cytoplasmic and nuclear protein of 57 kDa first identified in malignant cells from Hodgkin's lymphoma. Based on yeast-two hybrid protein interaction we found out that Ki-1/57 interacts with adaptor protein RACK1 (receptor of activated kinase 1), CIRP (cold-inducible RNA-binding protein), RPL38 (ribosomal protein L38) and FXR1 (fragile X mental retardation-related protein 1). Since these proteins are involved in the regulation of translation we suspected that Ki-1/57 may have a role in it. We show by immunoprecipitation the association of Ki-1/57 with FMRP. Confocal microscopy revealed that Ki-1/57 colocalizes with FMRP/FXR1/2 to stress granules. Furthermore Ki-1/57 cosediments with free ribosomal particles and enhances translation, when tethered to a reporter mRNA, suggesting that Ki-1/57 may be involved in translational regulation.

Solution structure of the human signaling protein RACK1

Background

The adaptor protein RACK1 (receptor of activated kinase 1) was originally identified as an anchoring protein for protein kinase C. RACK1 is a 36 kDa protein, and is composed of seven WD repeats which mediate its protein-protein interactions. RACK1 is ubiquitously expressed and has been implicated in diverse cellular processes involving: protein translation regulation, neuropathological processes, cellular stress, and tissue development.

Results

In this study we performed a biophysical analysis of human RACK1 with the aim of obtaining low resolution structural information. Small angle X-ray scattering (SAXS) experiments demonstrated that human RACK1 is globular and monomeric in solution and its low resolution structure is strikingly similar to that of an homology model previously calculated by us and to the crystallographic structure of RACK1 isoform A from Arabidopsis thaliana. Both sedimentation velocity and sedimentation equilibrium analytical ultracentrifugation techniques showed that RACK1 is predominantly a monomer of around 37 kDa in solution, but also presents small amounts of oligomeric species. Moreover, hydrodynamic data suggested that RACK1 has a slightly asymmetric shape. The interaction of RACK1 and Ki-1/57 was tested by sedimentation equilibrium. The results suggested that the association between RACK1 and Ki-1/57(122-413) follows a stoichiometry of 1:1. The binding constant (KB) observed for RACK1-Ki-1/57(122-413) interaction was of around (1.5 ± 0.2) × 10⁶ M^-1 and resulted in a dissociation constant (KD) of (0.7 ± 0.1) × 10^-6 M. Moreover, the fluorescence data also suggests that the interaction may occur in a cooperative fashion.

Conclusion

Our SAXS and analytical ultracentrifugation experiments indicated that RACK1 is predominantly a monomer in solution. RACK1 and Ki-1/57(122-413) interact strongly under the tested conditions.

Repression of smooth muscle differentiation by a novel high mobility group box-containing protein, HMG2L1

The molecular mechanisms regulating smooth muscle-specific gene expression during smooth muscle development are poorly understood. Myocardin is an extraordinarily powerful cofactor of serum response factor (SRF) that stimulates expression of smooth muscle-specific genes. In an effort to search for proteins that regulate myocardin function, we identified a novel HMG box-containing protein HMG2L1 (high mobility group 2 like 1). We found that HMG2L1 expression is correlated with the smooth muscle cell (SMC) synthetic phenotype. Overexpression of HMG2L1 in SMCs down-regulated smooth muscle marker expression. Conversely, depletion of endogenous HMG2L1 in SMCs increases smooth muscle-specific gene expression. Furthermore, we found HMG2L1 specifically abrogates myocardin-induced activation of smooth muscle-specific genes. By GST pulldown assays, the interaction domains between HMG2L1 and myocardin were mapped to the N termini of each of the proteins. Finally, we demonstrated that HMG2L1 abrogates myocardin function through disrupting its binding to SRF and abolishing SRF-myocardin complex binding to the promoters of smooth muscle-specific genes. This study provides the first evidence of this novel HMG2L1 molecule playing an important role in attenuating smooth muscle differentiation.

Arginine methylation analysis of the splicing-associated SR protein SFRS9/SRP30C

The human SFRS9/SRp30c belongs to the SR family of splicing regulators. Despite evidence that members of this protein family may be targeted by arginine methylation, this has yet to be experimentally addressed. In this study, we found that SFRS9 is a target for PRMT1-mediated arginine methylation in vitro, and that it is immunoprecipitated from HEK-293 lysates by antibodies that recognize both mono- and dimethylated arginines. We further observed that upon treatment with the methylation inhibitor Adox, the fluorescent EGFP-SFRS9 re-localizes to dot-like structures in the cell nucleus. In subsequent confocal analyses, we found that EGFP-SFRS9 localizes to nucleoli in Adox-treated cells. Our findings indicate the importance of arginine methylation for the subnuclear localization of SFRS9.

Functional association of human Ki-1/57 with pre-mRNA splicing events

The cytoplasmic and nuclear protein Ki-1/57 was first identified in malignant cells from Hodgkin's lymphoma. Despite studies showing its phosphorylation, arginine methylation, and interaction with several regulatory proteins, the functional role of Ki-1/57 in human cells remains to be determined. Here, we investigated the relationship of Ki-1/57 with RNA functions. Through immunoprecipitation assays, we verified the association of Ki-1/57 with the endogenous splicing proteins hnRNPQ and SFRS9 in HeLa cell extracts. We also found that recombinant Ki-1/57 was able to bind to a poly-U RNA probe in electrophoretic mobility shift assays. In a classic splicing test, we showed that Ki-1/57 can modify the splicing site selection of the adenoviral E1A minigene in a dose-dependent manner. Further confocal and fluorescence microscopy analysis revealed the localization of enhanced green fluorescent proteinKi-1/57 to nuclear bodies involved in RNA processing and or small nuclear ribonucleoprotein assembly, depending on the cellular methylation status and its N-terminal region. In summary, our findings suggest that Ki-1/57 is probably involved in cellular events related to RNA functions, such as pre-mRNA splicing.

Human regulatory protein Ki-1/57 has characteristics of an intrinsically unstructured protein

The human protein Ki-1/57 was first identified through the cross reactivity of the anti-CD30 monoclonal antibody Ki-1, in Hodgkin lymphoma cells. The expression of Ki-1/57 in diverse cancer cells and its phosphorylation in peripheral blood leukocytes after mitogenic activation suggested its possible role in cell signaling. Ki-1/57 interacts with several other regulatory proteins involved in cellular signaling, transcriptional regulation and RNA metabolism, suggesting it may have pleiotropic functions. In a previous spectroscopic analysis, we observed a low content of secondary structure for Ki-1/57 constructs. Here, Circular dichroism experiments, in vitro RNA binding analysis, and limited proteolysis assays of recombinant Ki-1/57(122-413) and proteolysis assays of endogenous full length protein from human HEK293 cells suggested that Ki-1/57 has characteristics of an intrinsically unstructured protein. Small-angle X-ray scattering (SAXS) experiments were performed with the C-terminal fragment Ki-1/57(122-413). These results indicated an elongated shape and a partially unstructured conformation of the molecule in solution, confirming the characteristics of an intrinsically unstructured protein. Experimental curves together with ab initio modeling approaches revealed an extended and flexible molecule in solution. An elongated shape was also observed by analytical gel filtration. Furthermore, sedimentation velocity analysis suggested that Ki-1/57 is a highly asymmetric protein. These findings may explain the functional plasticity of Ki-1/57, as suggested by the wide array of proteins with which it is capable of interacting in yeast two-hybrid interaction assays.

Drinking-water arsenic exposure modulates gene expression in human lymphocytes from a U.S. population

BACKGROUND

Arsenic exposure impairs development and can lead to cancer, cardiovascular disease, and diabetes. The mechanism underlying these effects remains unknown. Primarily because of geologic sources of contamination, drinking-water arsenic levels are above the current recommended maximum contaminant level of 10 microg/L in the northeastern, western, and north central regions of the United States.

OBJECTIVES

We investigated the effects of arsenic exposure, defined by internal biomarkers at levels relevant to the United States and similarly exposed populations, on gene expression.

METHODS

We conducted separate Affymetrix microarray-based genomewide analyses of expression patterns. Peripheral blood lymphocyte samples from 21 controls interviewed (1999-2002) as part of a case-control study in New Hampshire were selected based on high- versus low-level arsenic exposure levels.

RESULTS

The biologic functions of the transcripts that showed statistically significant abundance differences between high- and low-arsenic exposure groups included an overrepresentation of genes involved in defense response, immune function, cell growth, apoptosis, regulation of cell cycle, T-cell receptor signaling pathway, and diabetes. Notably, the high-arsenic exposure group exhibited higher levels of several killer cell immunoglobulin-like receptors that inhibit natural killer cell activity.

CONCLUSIONS

These findings define biologic changes that occur with chronic arsenic exposure in humans and provide leads and potential targets for understanding and monitoring the pathogenesis of arsenic-induced diseases.

A spectroscopic analysis of the interaction between the human regulatory proteins RACK1 and Ki-1/57

Ki-1/57 is a 57-kDa cytoplasmic and nuclear protein associated with protein kinase activity and is hyper-phosphorylated on Ser/Thr residues upon cellular activation. In previous studies we identified the receptor of activated kinase-1 (RACK1), a signaling adaptor protein that binds activated PKC, as a protein that interacts with Ki-1/57. Here we demonstrate that the far-UV circular dichroism spectrum of the WD repeat-containing RACK1 protein shows an unusual positive ellipticity at 229 nm, which in other proteins of the WD family has been attributed to surface tryptophans that are quenchable by N-bromosuccinimide (NBS). As well as NBS, in vitro binding of 6xHis-Ki-1/57(122-413) and 6xHis-Ki-1/57(264-413) can also quench the positive ellipticity of the RACK1 spectrum. We generated a model of RACK1 by homology modeling using a G protein beta subunit as template. Our model suggests the family-typical seven-bladed beta-propeller, with an aromatic cluster around the central tunnel that contains four Trp residues (17, 83, 150, 170), which are likely involved in the interaction with Ki-1/57.

Ki-1/57 interacts with PRMT1 and is a substrate for arginine methylation

The human 57 kDa Ki-1 antigen (Ki-1/57) is a cytoplasmic and nuclear protein, associated with Ser/Thr protein kinase activity, and phosphorylated at the serine and threonine residues upon cellular activation. We have shown that Ki-1/57 interacts with chromo-helicase DNA-binding domain protein 3 and with the adaptor/signaling protein receptor of activated kinase 1 in the nucleus. Among the identified proteins that interacted with Ki-1/57 in a yeast two-hybrid system was the protein arginine-methyltransferase-1 (PRMT1). Most interestingly, when PRMT1 was used as bait in a yeast two-hybrid system we were able to identify Ki-1/57 as prey among 14 other interacting proteins, the majority of which are involved in RNA metabolism or in the regulation of transcription. We found that Ki-1/57 and its putative paralog CGI-55 have two conserved Gly/Arg-rich motif clusters (RGG/RXR box, where X is any amino acid) that may be substrates for arginine-methylation by PRMT1. We observed that all Ki-1/57 protein fragments containing RGG/RXR box clusters interact with PRMT1 and are targets for methylation in vitro. Furthermore, we found that Ki-1/57 is a target for methylation in vivo. Using immunofluorescence experiments we observed that treatment of HeLa cells with an inhibitor of methylation, adenosine-2',3'-dialdehyde (Adox), led to a reduction in the cytoplasmic immunostaining of Ki-1/57, whereas its paralog CGI-55 was partially redistributed from the nucleus to the cytoplasm upon Adox treatment. In summary, our data show that the yeast two-hybrid assay is an effective system for identifying novel PRMT arginine-methylation substrates and may be successfully applied to other members of the growing family of PRMTs.

The methylation of the C-terminal region of hnRNPQ (NSAP1) is important for its nuclear localization

Protein arginine methylation is an irreversible post-translational protein modification catalyzed by a family of at least nine different enzymes entitled PRMTs (protein arginine methyl transferases). Although PRMT1 is responsible for 85% of the protein methylation in human cells, its substrate spectrum has not yet been fully characterized nor are the functional consequences of methylation for the protein substrates well understood. Therefore, we set out to employ the yeast two-hybrid system in order to identify new substrate proteins for human PRMT1. We were able to identify nine different PRMT1 interacting proteins involved in different aspects of RNA metabolism, five of which had been previously described either as substrates for PRMT1 or as functionally associated with PRMT1. Among the four new identified possible protein substrates was hnRNPQ3 (NSAP1), a protein whose function has been implicated in diverse steps of mRNA maturation, including splicing, editing, and degradation. By in vitro methylation assays we were able to show that hnRNPQ3 is a substrate for PRMT1 and that its C-terminal RGG box domain is the sole target for methylation. By further studies with the inhibitor of methylation Adox we provide evidence that hnRNPQ1-3 are methylated in vivo. Finally, we demonstrate by immunofluorescence analysis of HeLa cells that the methylation of hnRNPQ is important for its nuclear localization, since Adox treatment causes its re-distribution from the nucleus to the cytoplasm.