Dual expression system suitable for high-throughput fluorescence-based screening and production of soluble proteins

Tagging Recombinant Proteins to Enhance Solubility and Aid Purification

Protein fusion technology has had a major impact on the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has a long history, and there is a considerable repertoire of these that can be used to address issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. In this chapter, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags is described.

Tagging Recombinant Proteins to Enhance Solubility and Aid Purification

Protein fusion technology has had a major impact on the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has increased greatly in recent years and there now exists a considerable repertoire of these that can be used to solve issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have therefore become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. Here, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags is described.

Synonymous rare arginine codons and tRNA abundance affect protein production and quality of TEV protease variant

It has been identified that a TEV protease (TEVp) variant, TEVp^5M, displays improved solubility. Here, we constructed fifteen TEVp^5M variants with one or more of six rare arginine codons in the coding sequence replaced with abundant E. coli arginine codons. These codon variants expressed in either E. coli BL21 (DE3) or Rossetta (DE3) showed different solubility and activity. Supply of rare tRNAs increased the tendency of certain codon variants to form insoluble aggregates at early induction stage, as determined by the fused S-tag. About 32% increase in soluble protein production of M5 variant with four synonymously mutated arginine codons was identified in Rossetta (DE3) cells using GFP fusion reporter, comparable to that of TEVp^5M. After purification, two other codon variants from both E. coli strains exhibited less activity than TEVp^5M on cleaving the native or modified recognition sequence incorporated between GST and E. coli diaminopropionate ammonialyase by enzyme-coupled assay, whereas purified M5 variant showed activity similar to the TEVp^5M. Supply of rare tRNAs caused the decrease of activity of TEVp^5M and M5 by about 21%. Our results revealed that engineering of highly soluble TEVp variants can be achieved by the combined mutations of amino acid residues and optimization of specific rare codons, whereas simple augment of rare tRNAs abundance resulted in partial loss of activity.

Expression and solubility testing in a high-throughput environment

The expression and screening of the solubility of recombinant proteins is an important step in the high-throughput (HT) production of target proteins. For many applications, E. coli remains the most widely used expression system due to the relative ease of adapting it to HT pipelines. Herein is described a platform using a 96-well format for efficient expression and solubility screening of target proteins.

High throughput quantitative expression screening and purification applied to recombinant disulfide-rich venom proteins produced in E. coli

Escherichia coli (E. coli) is the most widely used expression system for the production of recombinant proteins for structural and functional studies. However, purifying proteins is sometimes challenging since many proteins are expressed in an insoluble form. When working with difficult or multiple targets it is therefore recommended to use high throughput (HTP) protein expression screening on a small scale (1-4 ml cultures) to quickly identify conditions for soluble expression. To cope with the various structural genomics programs of the lab, a quantitative (within a range of 0.1-100 mg/L culture of recombinant protein) and HTP protein expression screening protocol was implemented and validated on thousands of proteins. The protocols were automated with the use of a liquid handling robot but can also be performed manually without specialized equipment.

Disulfide-rich venom proteins are gaining increasing recognition for their potential as therapeutic drug leads. They can be highly potent and selective, but their complex disulfide bond networks make them challenging to produce. As a member of the FP7 European Venomics project (www.venomics.eu), our challenge is to develop successful production strategies with the aim of producing thousands of novel venom proteins for functional characterization. Aided by the redox properties of disulfide bond isomerase DsbC, we adapted our HTP production pipeline for the expression of oxidized, functional venom peptides in the E. coli cytoplasm. The protocols are also applicable to the production of diverse disulfide-rich proteins. Here we demonstrate our pipeline applied to the production of animal venom proteins. With the protocols described herein it is likely that soluble disulfide-rich proteins will be obtained in as little as a week. Even from a small scale, there is the potential to use the purified proteins for validating the oxidation state by mass spectrometry, for characterization in pilot studies, or for sensitive micro-assays.

The capsids of HIV-1 and HIV-2 determine immune detection of the viral cDNA by the innate sensor cGAS in dendritic cells

HIV-2 is less pathogenic for humans than HIV-1 and might provide partial cross-protection from HIV-1-induced pathology. Although both viruses replicate in the T cells of infected patients, only HIV-2 replicates efficiently in dendritic cells (DCs) and activates innate immune pathways. How HIV is sensed in DC is unknown. Capsid-mutated HIV-2 revealed that sensing by the host requires viral cDNA synthesis, but not nuclear entry or genome integration. The HIV-1 capsid prevented viral cDNA sensing up to integration, allowing the virus to escape innate recognition. In contrast, DCs sensed capsid-mutated HIV-1 and enhanced stimulation of T cells in the absence of productive infection. Finally, we found that DC sensing of HIV-1 and HIV-2 required the DNA sensor cGAS. Thus, the HIV capsid is a determinant of innate sensing of the viral cDNA by cGAS in dendritic cells. This pathway might potentially be harnessed to develop effective vaccines against HIV-1.

Engineering soluble tobacco etch virus protease accompanies the loss of stability

Tobacco etch virus protease (TEVp) is a widely used tool enzyme in biological studies. To improve the solubility of recombinant TEVp, three variants, including the double mutant (L56V/S135G), the triple mutant (T17S/N68D/I77V), and the quintuple mutant (T17S/L56V/N68D/I77V/S135G), have been developed, however, with little information on functional stability. Here we investigated the solubility and stability of the three TEVp mutants under different temperature and denaturants, and in Escherichiacoli with different cultural conditions. The quintuple mutant showed the highest solubility and thermostablity, and the double mutant was most resistant to the denaturants. The double mutant folded best in E. coli cells at 37°C with or without the co-expressed molecular chaperones GroEL, GroES and GrpE. The least soluble wild type TEVp displayed better tolerance to denaturants than the triple and the quintuple mutants. All results demonstrated that TEVp is not engineered to embody the most desirable solubility and stability by the current mutations.

High-throughput instant quantification of protein expression and purity based on photoactive yellow protein turn off/on label

Quantifying the concentration and purity of a target protein is essential for high-throughput protein expression test and rapid screening of highly soluble proteins. However, conventional methods such as PAGE and dot blot assay generally involve multiple time-consuming tasks requiring hours or do not allow instant quantification. Here, we demonstrate a new method based on the Photoactive yellow protein turn Off/On Label (POOL) system that can instantly quantify the concentration and purity of a target protein. The main idea of POOL is to use Photoactive Yellow Protein (PYP), or its miniaturized version, as a fusion partner of the target protein. The characteristic blue light absorption and the consequent yellow color of PYP is absent when initially expressed without its chromophore, but can be turned on by binding its chromophore, p-coumaric acid. The appearance of yellow color upon adding a precursor of chromophore to the co-expressed PYP can be used to check the expression amount of the target protein via visual inspection within a few seconds as well as to quantify its concentration and purity with the aid of a spectrometer within a few minutes. The concentrations measured by the POOL method, which usually takes a few minutes, show excellent agreement with those by the BCA Kit, which usually takes ∼1 h. We demonstrate the applicability of POOL in E. coli, insect, and mammalian cells, and for high-throughput protein expression screening.

High throughput screening identifies disulfide isomerase DsbC as a very efficient partner for recombinant expression of small disulfide-rich proteins in E. coli

Background

Disulfide-rich proteins or DRPs are versatile bioactive compounds that encompass a wide variety of pharmacological, therapeutic, and/or biotechnological applications. Still, the production of DRPs in sufficient quantities is a major bottleneck for their complete structural or functional characterization. Recombinant expression of such small proteins containing multiple disulfide bonds in the bacteria E. coli is considered difficult and general methods and protocols, particularly on a high throughput scale, are limited.

Results

Here we report a high throughput screening approach that allowed the systematic investigation of the solubilizing and folding influence of twelve cytoplasmic partners on 28 DRPs in the strains BL21 (DE3) pLysS, Origami B (DE3) pLysS and SHuffle® T7 Express lysY (1008 conditions). The screening identified the conditions leading to the successful soluble expression of the 28 DRPs selected for the study. Amongst 336 conditions tested per bacterial strain, soluble expression was detected in 196 conditions using the strain BL21 (DE3) pLysS, whereas only 44 and 50 conditions for soluble expression were identified for the strains Origami B (DE3) pLysS and SHuffle® T7 Express lysY respectively. To assess the redox states of the DRPs, the solubility screen was coupled with mass spectrometry (MS) to determine the exact masses of the produced DRPs or fusion proteins. To validate the results obtained at analytical scale, several examples of proteins expressed and purified to a larger scale are presented along with their MS and functional characterization.

Conclusions

Our results show that the production of soluble and functional DRPs with cytoplasmic partners is possible in E. coli. In spite of its reducing cytoplasm, BL21 (DE3) pLysS is more efficient than the Origami B (DE3) pLysS and SHuffle® T7 Express lysY trxB^-/gor^- strains for the production of DRPs in fusion with solubilizing partners. However, our data suggest that oxidation of the proteins occurs ex vivo. Our protocols allow the production of a large diversity of DRPs using DsbC as a fusion partner, leading to pure active DRPs at milligram scale in many cases. These results open up new possibilities for the study and development of DRPs with therapeutic or biotechnological interest whose production was previously a limitation.

Structural and functional analysis of the pore-forming toxin NetB from Clostridium perfringens

Clostridium perfringens is an anaerobic bacterium that causes numerous important human and animal diseases, primarily as a result of its ability to produce many different protein toxins. In chickens, C. perfringens causes necrotic enteritis, a disease of economic importance to the worldwide poultry industry. The secreted pore-forming toxin NetB is a key virulence factor in the pathogenesis of avian necrotic enteritis and is similar to alpha-hemolysin, a β-barrel pore-forming toxin from Staphylococcus aureus. To address the molecular mechanisms underlying NetB-mediated tissue damage, we determined the crystal structure of the monomeric form of NetB to 1.8 Å. Structural comparisons with other members of the alpha-hemolysin family revealed significant differences in the conformation of the membrane binding domain. These data suggested that NetB may recognize different membrane receptors or use a different mechanism for membrane-protein interactions. Consistent with this idea, electrophysiological experiments with planar lipid bilayers revealed that NetB formed pores with much larger single-channel conductance than alpha-hemolysin. Channel conductance varied with phospholipid net charge. Furthermore, NetB differed in its ion selectivity, preferring cations over anions. Using hemolysis as a screen, we carried out a random-mutagenesis study that identified several residues that are critical for NetB-induced cell lysis. Mapping of these residues onto the crystal structure revealed that they were clustered in regions predicted to be required for oligomerization or membrane binding. Together these data provide an insight into the mechanism of NetB-mediated pore formation and will contribute to our understanding of the mode of action of this important toxin.

Necrotic enteritis is an economically important disease of the worldwide poultry industry and is mediated by Clostridium perfringens strains that produce NetB, a β-pore-forming toxin. We carried out structural and functional studies of NetB to provide a mechanistic insight into its mode of action and to assist in the development of a necrotic enteritis vaccine. We determined the structure of the monomeric form of NetB to 1.8 Å, used both site-directed and random mutagenesis to identify key residues that are required for its biological activity, and analyzed pore formation by NetB and its substitution-containing derivatives in planar lipid bilayers.

Enhanced soluble expression of recombinant Flavobacterium heparinum heparinase I in Escherichia coli by fusing it with various soluble partners

Heparinase I (HepA) was originally isolated from Flavobacterium heparinum (F. heparinum) and specifically cleaves heparin/heparan sulfate in a site-dependent manner, showing great promise for producing low molecular weight heparin (LMWH). However, expressing recombinant HepA is extremely difficult in Escherichia coli because it suffers from low yields, insufficient purity and insolubility. In this paper, we systematically cloned and fused the HepA gene to the C-terminus of five soluble partners, including translation initiation factor 2 domain I (IF2), glutathione S-transferase (GST), maltose-binding protein (MBP), small ubiquitin modifying protein (SUMO) and N-utilization substance A (NusA), to screen for their abilities to improve the solubility of recombinant HepA when expressed in E. coli. A convenient two-step immobilized metal affinity chromatography (IMAC) method was utilized to purify these fused HepA hybrids. We show that, except for NusA, the fusion partners dramatically improved the soluble expression of recombinant HepA, with IF2-HepA and SUMO-HepA creating almost completely soluble HepA (98% and 94% of expressed HepA fusions are soluble, respectively), which is the highest yield rate published to the best of our knowledge. Moreover, all of the fusion proteins show comparable biological activity to their unfused counterparts and could be used directly without removing the fusion tags. Together, our results provide a viable option to produce large amounts of soluble and active recombinant HepA for manufacturing.

Nonribosomal peptide synthesis in animals: the cyclodipeptide synthase of Nematostella

Cyclodipeptide synthases (CDPSs) are small enzymes structurally related to class-I aminoacyl-tRNA synthetases (aaRSs). They divert aminoacylated tRNAs from their canonical role in ribosomal protein synthesis, for cyclodipeptide formation. All the CDPSs experimentally characterized to date are bacterial. We show here that a predicted CDPS from the sea anemone Nematostella vectensis is an active CDPS catalyzing the formation of various cyclodipeptides, preferentially containing tryptophan. Our findings demonstrate that eukaryotes encode active CDPSs and suggest that all CDPSs have a similar aminoacyl-tRNA synthetase-like architecture and ping-pong mechanism. They also raise questions about the biological roles of the cyclodipeptides produced in bacteria and eukaryotes.

High-throughput protein expression using a combination of ligation-independent cloning (LIC) and infrared fluorescent protein (IFP) detection

Protein expression in heterologous hosts for functional studies is a cumbersome effort. Here, we report a superior platform for parallel protein expression in vivo and in vitro. The platform combines highly efficient ligation-independent cloning (LIC) with instantaneous detection of expressed proteins through N- or C-terminal fusions to infrared fluorescent protein (IFP). For each open reading frame, only two PCR fragments are generated (with three PCR primers) and inserted by LIC into ten expression vectors suitable for protein expression in microbial hosts, including Escherichia coli, Kluyveromyces lactis, Pichia pastoris, the protozoon Leishmania tarentolae, and an in vitro transcription/translation system. Accumulation of IFP-fusion proteins is detected by infrared imaging of living cells or crude protein extracts directly after SDS-PAGE without additional processing. We successfully employed the LIC-IFP platform for in vivo and in vitro expression of ten plant and fungal proteins, including transcription factors and enzymes. Using the IFP reporter, we additionally established facile methods for the visualisation of protein-protein interactions and the detection of DNA-transcription factor interactions in microtiter and gel-free format. We conclude that IFP represents an excellent reporter for high-throughput protein expression and analysis, which can be easily extended to numerous other expression hosts using the setup reported here.

Cyclodipeptide synthases, a family of class-I aminoacyl-tRNA synthetase-like enzymes involved in non-ribosomal peptide synthesis

Cyclodipeptide synthases (CDPSs) belong to a newly defined family of enzymes that use aminoacyl-tRNAs (aa-tRNAs) as substrates to synthesize the two peptide bonds of various cyclodipeptides, which are the precursors of many natural products with noteworthy biological activities. Here, we describe the crystal structure of AlbC, a CDPS from Streptomyces noursei. The AlbC structure consists of a monomer containing a Rossmann-fold domain. Strikingly, it is highly similar to the catalytic domain of class-I aminoacyl-tRNA synthetases (aaRSs), especially class-Ic TyrRSs and TrpRSs. AlbC contains a deep pocket, highly conserved among CDPSs. Site-directed mutagenesis studies indicate that this pocket accommodates the aminoacyl moiety of the aa-tRNA substrate in a way similar to that used by TyrRSs to recognize their tyrosine substrates. These studies also suggest that the tRNA moiety of the aa-tRNA interacts with AlbC via at least one patch of basic residues, which is conserved among CDPSs but not present in class-Ic aaRSs. AlbC catalyses its two-substrate reaction via a ping-pong mechanism with a covalent intermediate in which l-Phe is shown to be transferred from Phe-tRNA^Phe to an active serine. These findings provide insight into the molecular bases of the interactions between CDPSs and their aa-tRNAs substrates, and the catalytic mechanism used by CDPSs to achieve the non-ribosomal synthesis of cyclodipeptides.

High level soluble production of functional ribonuclease inhibitor in Escherichia coli by fusing it to soluble partners

Ribonuclease inhibitor (RI) is a 50-kDa cytosolic scavenger of pancreatic-type ribonucleases which inhibits ribonucleolytic activity. Expression of recombinant RI is extremely difficult to reach high levels in soluble form in the cytoplasm of Escherichia coli. Here, we utilized five N-terminal fusion partners to improve the soluble expression of RI. Among these five fusion partners which have been screened, maltose-binding protein (MBP), N-utilization substance A (NusA) and translation initiation factor 2 domain I (IF2) have greatly improved the soluble expression level of recombinant murine RI under the drive of T7 promoter, while glutathione S-transferase (GST) and small ubiquitin modifying protein (SUMO) were much less efficient. All these RI-fusion proteins remained to be highly active in inhibiting RNase A activity. Furthermore, all fusion tags can be efficiently removed by enterokinase digestion to generate native RI which results the highest yield to date (>30mg of native RI per liter culture). And a convenient two-step immobilized metal affinity chromatography (IMAC) method has been implemented in our study, comparing with the traditional RNase A affinity chromatography method.

Tagging recombinant proteins to enhance solubility and aid purification

Protein fusion technology has enormously facilitated the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has increased greatly in recent years and there now exists a considerable repertoire of these that can be used to solve issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have therefore become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. Here, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags are outlined.

Delineation of the Xrcc4-interacting region in the globular head domain of cernunnos/XLF

In mammals, the majority of DNA double-strand breaks are processed by the nonhomologous end-joining (NHEJ) pathway, composed of seven factors: Ku70, Ku80, DNA-PKcs, Artemis, Xrcc4 (X4), DNA-ligase IV (L4), and Cernunnos/XLF. Cernunnos is part of the ligation complex, constituted by X4 and L4. To improve our knowledge on the structure and function of Cernunnos, we performed a systematic mutagenesis study on positions selected from an analysis of the recent three-dimensional structures of this factor. Ten of 27 screened mutants were nonfunctional in several DNA repair assays. Outside amino acids critical for the expression and stability of Cernunnos, we identified three amino acids (Arg(64), Leu(65), and Leu(115)) essential for the interaction with X4 and the proper function of Cernunnos. Docking the crystal structures of the two factors further validated this probable interaction surface of Cernunnos with X4.

Asymmetric signal transduction through paralogs that comprise a genetic switch for sugar sensing in Saccharomyces cerevisiae

Efficient uptake of glucose is especially critical to Saccharomyces cerevisiae because its preference to ferment this carbon source demands high flux through glycolysis. Glucose induces expression of HXT genes encoding hexose transporters through a signal generated by the Snf3 and Rgt2 glucose sensors that leads to depletion of the transcriptional regulators Mth1 and Std1. These paralogous proteins bind to Rgt1 and enable it to repress expression of HXT genes. Here we show that Mth1 and Std1 can substitute for one another and provide nearly normal regulation of their targets. However, their roles in the glucose signal transduction cascade have diverged significantly. Mth1 is the prominent effector of Rgt1 function because it is the more abundant of the two paralogs under conditions in which both are active (in the absence of glucose). Moreover, the cellular level of Mth1 is quite sensitive to the amount of available glucose. The abundance of Std1 protein, on the other hand, remains essentially constant over a similar range of glucose concentrations. The signal generated by low levels of glucose is amplified by rapid depletion of Mth1; the velocity of this depletion is dependent on both its rate of degradation and swift repression of MTH1 transcription by the Snf1-Mig1 glucose repression pathway. Quantitation of the contributions of Mth1 and Std1 to regulation of HXT expression reveals the unique roles played by each paralog in integrating nutrient availability with metabolic capacity: Mth1 is the primary regulator; Std1 serves to buffer the response to glucose.

Specific and efficient cleavage of fusion proteins by recombinant plum pox virus NIa protease

Site-specific proteases are the most popular kind of enzymes for removing the fusion tags from fused target proteins. Nuclear inclusion protein a (NIa) proteases obtained from the family Potyviridae have become promising due to their high activities and stringencies of sequences recognition. NIa proteases from tobacco etch virus (TEV) and tomato vein mottling virus (TVMV) have been shown to process recombinant proteins successfully in vitro. In this report, recombinant PPV (plum pox virus) NIa protease was employed to process fusion proteins with artificial cleavage site in vitro. Characteristics such as catalytic ability and affecting factors (salt, temperature, protease inhibitors, detergents, and denaturing reagents) were investigated. Recombinant PPV NIa protease expressed and purified from Escherichia coli demonstrated efficient and specific processing of recombinant GFP and SARS-CoV nucleocapsid protein, with site F (N V V V H Q▾A) for PPV NIa protease artificially inserted between the fusion tags and the target proteins. Its catalytic capability is similar to those of TVMV and TEV NIa protease. Recombinant PPV NIa protease reached its maximal proteolytic activity at approximately 30 °C. Salt concentration and only one of the tested protease inhibitors had minor influences on the proteolytic activity of PPV NIa protease. Recombinant PPV NIa protease was resistant to self-lysis for at least five days.

A correlation analysis of protein characteristics associated with genome-wide high throughput expression and solubility of Streptococcus pneumoniae proteins

We have developed and evaluated a highly parallel protein expression and purification system using ORFs derived from the pathogenic bacterium Streptococcus pneumoniae as a representative test case in conjunction with the Gateway cloning technology. Establishing high throughput protein production capability is essential for genome-wide characterization of protein function. In this study, we focused on protein expression and purification outcomes generated from an expression vector which encodes an NH(2)-terminal hexa-histidine tag and a COOH-terminal S-tag. Purified recombinant proteins were validated by SDS-PAGE, followed by in-gel digestion and identification by MALDI-TOF/TOF analysis. Starting with 1360 sequence-validated destination clones we examined correlation analyses of expression and solubility of a wide variety of recombinant proteins. In total, 428 purified proteins (31%) were recovered in soluble form. We describe a semi-quantitative scoring method using an S-tag assay to improve the throughput and efficiency of expression and solubility studies for recombinant proteins. Given a relatively large dataset derived from proteins representing all functional groups in a microbial genome we correlated various protein characteristics as they relate to protein expression outcomes.

Enhancement of soluble protein expression through the use of fusion tags

The soluble expression of heterologous proteins in Escherichia coli remains a serious bottleneck in protein production. Although alteration of expression conditions can sometimes solve the problem, the best available tools to date have been fusion tags that enhance the solubility of expressed proteins. However, a systematic analysis of the utility of these solubility fusions has been difficult, and it appears that many proteins react differently to the presence of different solubility tags. The advent of high-throughput structural genomics programs and advances in cloning and expression technology afford us a new way to compare the effectiveness of solubility tags. This data should allow us to better predict the effectiveness of tags currently in use, and might also provide the information needed to identify new fusion tags.

The carboxyl-terminal nucleoplasmic region of MAN1 exhibits a DNA binding winged helix domain

MAN1 is an integral protein of the inner nuclear membrane that interacts with nuclear lamins and emerin, thus playing a role in nuclear organization. It also binds to chromatin-associated proteins and transcriptional regulators, including the R-Smads, Smad1, Smad2, and Smad3. Mutations in the human gene encoding MAN1 cause sclerosing bone dysplasias, which sometimes have associated skin abnormalities. At the molecular level, these mutations lead to loss of the MAN1-R-Smads interaction, thus perturbing transforming growth factor beta superfamily signaling pathway. As a first step to understanding the physical basis of MAN1 interaction with R-Smads, we here report the structural characterization of the carboxyl-terminal nucleoplasmic region of MAN1, which is responsible for Smad binding. This region exhibits an amino-terminal globular domain adopting a winged helix fold, as found in several Smad-associated sequence-specific DNA binding factors. Consistently, it binds to DNA through the positively charged recognition helix H3 of its winged helix motif. However, it does not show the predicted carboxyl-terminal U2AF homology domain in solution, suggesting that the folding and stability of such a domain in MAN1 depend upon binding to an unidentified partner. Modeling the complex between DNA and the winged helix domain shows that the regions involved in DNA binding are essentially distinct from those reported to be involved in Smad binding. This suggests that MAN1 binds simultaneously to R-Smads and their targeted DNA sequences.

Crystallization and halide phasing of the C-terminal domain of human KIN17

Here, the crystallization and initial phasing of the C-terminal domain of human KIN17, a 45 kDa protein mainly expressed in response to ionizing radiation and overexpressed in certain tumour cell lines, are reported. Crystals diffracting to 1.4 A resolution were obtained from 10% ethylene glycol, 27% PEG 6000, 500 mM LiCl and 100 mM sodium acetate pH 6.3 in space group P2(1)2(1)2(1), with unit-cell parameters a = 45.75, b = 46.31, c = 60.80 A and one molecule in the asymmetric unit. Since this domain has a basic pI, heavy-atom derivatives were obtained by soaking the crystals with negatively charged ions such as tungstate and iodine. The replacement of LiCl by KI in the cryosolution allowed the determination of phases from iodide ions to give an interpretable electron-density map.