T-rich DNA single strands bind to a preformed site on the bacterial cold shock protein Bs-CspB

Structural and biochemical insights into the bacteriophage PlyGRCS endolysin targeting methicillin-resistant Staphylococcus aureus (MRSA) and serendipitous discovery of its interaction with a cold shock protein C (CspC)

Methicillin-resistant Staphylococcus aureus (MRSA) causes life-threatening human infections. Bacteriophage-encoded endolysins degrade the cell walls of Gram-positive bacteria by selectively hydrolyzing the peptidoglycan layer and thus are promising candidates to combat bacterial infections. PlyGRCS, the S. aureus-specific bacteriophage endolysin, contains a catalytic CHAP domain and a cell-wall binding SH3_5 domain connected by a linker. Here, we show the crystal structure of full-length PlyGRCS refined to 2.1 Å resolution. In addition, a serendipitous finding revealed that PlyGRCS binds to cold-shock protein C (CspC) by interacting with its CHAP and SH3_5 domains. CspC is an RNA chaperone that plays regulatory roles by conferring bacterial adaptability to various stress conditions. PlyGRCS has substantial lytic activity against S. aureus and showed only minimal change in its lytic activity in the presence of CspC. Whereas the PlyGRCS-CspC complex greatly reduced CspC-nucleic acid binding, the aforesaid complex may downregulate the CspC function during bacterial infection. Overall, the crystal structure and biochemical results of PlyGRCS provide a molecular basis for the bacteriolytic activity of PlyGRCS against S. aureus.

Upstream of N-Ras C-terminal cold shock domains mediate poly(A) specificity in a novel RNA recognition mode and bind poly(A) binding protein

RNA binding proteins (RBPs) often engage multiple RNA binding domains (RBDs) to increase target specificity and affinity. However, the complexity of target recognition of multiple RBDs remains largely unexplored. Here we use Upstream of N-Ras (Unr), a multidomain RBP, to demonstrate how multiple RBDs orchestrate target specificity. A crystal structure of the three C-terminal RNA binding cold-shock domains (CSD) of Unr bound to a poly(A) sequence exemplifies how recognition goes beyond the classical ππ-stacking in CSDs. Further structural studies reveal several interaction surfaces between the N-terminal and C-terminal part of Unr with the poly(A)-binding protein (pAbp). All interactions are validated by mutational analyses and the high-resolution structures presented here will guide further studies to understand how both proteins act together in cellular processes.

Two energy barriers and a transient intermediate state determine the unfolding and folding dynamics of cold shock protein

Cold shock protein (Csp) is a typical two-state folding model protein which has been widely studied by biochemistry and single molecule techniques. Recently two-state property of Csp was confirmed by atomic force microscopy (AFM) through direct pulling measurement, while several long-lifetime intermediate states were found by force-clamp AFM. We systematically studied force-dependent folding and unfolding dynamics of Csp using magnetic tweezers with intrinsic constant force capability. Here we report that Csp mostly folds and unfolds with a single step over force range from 5 pN to 50 pN, and the unfolding rates show different force sensitivities at forces below and above ~8 pN, which determines a free energy landscape with two barriers and a transient intermediate state between them along one transition pathway. Our results provide a new insight on protein folding mechanism of two-state proteins.

Fluorine NMR Spectroscopy Enables to Quantify the Affinity Between DNA and Proteins in Cell Lysate

The determination of the binding affinity quantifying the interaction between proteins and nucleic acids is of crucial interest in biological and chemical research. Here, we have made use of site-specific fluorine labeling of the cold shock protein from Bacillus subtilis, BsCspB, enabling to directly monitor the interaction with single stranded DNA molecules in cell lysate. High-resolution 19 F NMR spectroscopy has been applied to exclusively report on resonance signals arising from the protein under study. We have found that this experimental approach advances the reliable determination of the binding affinity between single stranded DNA molecules and its target protein in this complex biological environment by intertwining analyses based on NMR chemical shifts, signal heights, line shapes and simulations. We propose that the developed experimental platform offers a potent approach for the identification of binding affinities characterizing intermolecular interactions in native surroundings covering the nano-to-micromolar range that can be even expanded to in cell applications in future studies.

Impact of crowded environments on binding between protein and single-stranded DNA

The concept of Molecular Crowding depicts the high density of diverse molecules present in the cellular interior. Here, we determine the impact of low molecular weight and larger molecules on binding capacity of single-stranded DNA (ssDNA) to the cold shock protein B (CspB). Whereas structural features of ssDNA-bound CspB are fully conserved in crowded environments as probed by high-resolution NMR spectroscopy, intrinsic fluorescence quenching experiments reveal subtle changes in equilibrium affinity. Kinetic stopped-flow data showed that DNA-to-protein association is significantly retarded independent of choice of the molecule that is added to the solution, but dissociation depends in a nontrivial way on its size and chemical characteristics. Thus, for this DNA-protein interaction, excluded volume effect does not play the dominant role but instead observed effects are dictated by the chemical properties of the crowder. We propose that surrounding molecules are capable of specific modification of the protein's hydration shell via soft interactions that, in turn, tune protein-ligand binding dynamics and affinity.

SpyTag/SpyCatcher tether as a fingerprint and force marker in single-molecule force spectroscopy experiments

Single molecule force spectroscopy has emerged as a powerful tool to study protein folding dynamics, ligand-receptor interactions, and various mechanobiological processes. High force precision does not necessarily lead to high force accuracy, as the uncertainties in calibration can bring serious systematic errors. In the case of magnetic tweezers, accurate determination of the applied forces for short biomolecular tethers, by measuring thermal fluctuations of inhomogeneous magnetic beads, remains difficult. Here we address this challenge by showing that the SpyTag/SpyCatcher complex is not only a convenient and genetically encodable covalent linker but also an ideal molecular fingerprint and force marker in single molecule force spectroscopy experiments. By stretching the N-termini of both SpyCatcher and SpyTag, the complex unfolds locally up to the isopeptide bond position in an unzipping geometry, resulting in equilibrium transitions at ∼30 pN with step sizes of ∼3.4 nm. This mechanical feature can be used as the fingerprint to identify single-molecular events. Moreover, the transitions occur with a fast exchange rate and in a narrow force range. Therefore, the real applied forces can be determined accurately based on the force-dependent transitions. The equilibrium forces are insensitive to buffer conditions and temperature, making the calibration applicable to many complicated experimental systems. We provide an example to calibrate protein unfolding forces using this force marker and expect that this method can greatly simplify force calibration in single-molecule force spectroscopy experiments and improve the force accuracy.

Cold-Shock Domains-Abundance, Structure, Properties, and Nucleic-Acid Binding

Simple Summary

Proteins are composed of compact domains, often of known three-dimensional structure, and natively unstructured polypeptide regions. The abundant cold-shock domain is among the set of canonical nucleic acid-binding domains and conserved from bacteria to man. Proteins containing cold-shock domains serve a large variety of biological functions, which are mostly linked to DNA or RNA binding. These functions include the regulation of transcription, RNA splicing, translation, stability and sequestration. Cold-shock domains have a simple architecture with a conserved surface ideally suited to bind single-stranded nucleic acids. Because the binding is mostly by non-specific molecular interactions which do not involve the sugar-phosphate backbone, cold-shock domains are not strictly sequence-specific and do not discriminate reliably between DNA and RNA. Many, but not all functions of cold shock-domain proteins in health and disease can be understood based of the physical and structural properties of their cold-shock domains.

Abstract

The cold-shock domain has a deceptively simple architecture but supports a complex biology. It is conserved from bacteria to man and has representatives in all kingdoms of life. Bacterial cold-shock proteins consist of a single cold-shock domain and some, but not all are induced by cold shock. Cold-shock domains in human proteins are often associated with natively unfolded protein segments and more rarely with other folded domains. Cold-shock proteins and domains share a five-stranded all-antiparallel β-barrel structure and a conserved surface that binds single-stranded nucleic acids, predominantly by stacking interactions between nucleobases and aromatic protein sidechains. This conserved binding mode explains the cold-shock domains’ ability to associate with both DNA and RNA strands and their limited sequence selectivity. The promiscuous DNA and RNA binding provides a rationale for the ability of cold-shock domain-containing proteins to function in transcription regulation and DNA-damage repair as well as in regulating splicing, translation, mRNA stability and RNA sequestration.

RNA and DNA Binding Epitopes of the Cold Shock Protein TmCsp from the Hyperthermophile Thermotoga maritima

Prokaryotic cold shock proteins (CSPs) are considered to play an important role in the transcriptional and translational regulation of gene expression, possibly by acting as transcription anti-terminators and “RNA chaperones”. They bind with high affinity to single-stranded nucleic acids. Here we report the binding epitope of TmCsp from Thermotoga maritima for both single-stranded DNA and RNA, using heteronuclear 2D NMR spectroscopy. At “physiological” growth temperatures of TmCsp (≥ 343 K), all oligonucleotides studied have dissociation constants between 1.6 ((dT)₇) and 25.2 ((dA)₇) μM as determined by tryptophan fluorescence quenching. Reduction of the temperature to 303 K leads to a pronounced increase of affinity for thymidylate (dT)₇ and uridylate (rU)₇ heptamers with dissociation constants of 4.0 and 10.8 nM, respectively, whereas the weak binding of TmCsp to cytidylate, adenylate, and guanylate heptamers (dC)₇, (dA)₇, and (dT)₇ is almost unaffected by temperature. The change of affinities of TmCsp for (dT)₇ and (rU)₇ by approximately 3 orders of magnitude shows that it represents a cold chock sensor that switches on the cold shock reaction of the cell. A temperature dependent conformational switch of the protein is required for this action. The binding epitope on TmCsp for the ssDNA and RNA heptamers is very similar and comprises β-strands 1 and 2, the loop β1–β2 as well as the loops connecting β3 with β4 and β4 with β5. Besides the loop regions, surprisingly, mainly the RNA-binding motif RNP1 is involved in ssDNA and RNA binding, while only two amino acids, H28 and W29, of the postulated RNA-binding motif RNP2 interact with the uridylate and thymidylate homonucleotides, although a high affinity in the nanomolar range is achieved. This is in contrast to the binding properties of other CSPs or cold shock domains, where RNP1 as well as RNP2 are involved in binding. TmCsp takes up a unique position since it is the only one which possesses a tryptophan residue instead of a usually highly conserved phenylalanine or tyrosine residue at the end of RNP2. NMR titrations suggest that neither (dT)₇ nor (rU)₇ represent the full binding motif and that non-optimal intercalation of W29 into these oligonucleotides blocks the access of the RNP2 site to the DNA or RNA. NMR-experiments with (dA)₇ suggest an interaction of W29 with the adenine ring. Full binding seems to require at least one single purine base well-positioned within a thymine- or uracil-rich stretch of nucleic acids.

One evolutionarily selected amino acid variation is sufficient to provide functional specificity in the cold shock protein paralogs of Staphylococcus aureus

Bacterial genomes encode several families of protein paralogs. Discrimination between functional divergence and redundancy among paralogs is challenging due to their sequence conservation. Here, we investigated whether the amino acid differences present in the cold shock protein (CSP) paralogs of Staphylococcus aureus were responsible for functional specificity. Since deletion of cspA reduces the synthesis of staphyloxanthin (STX), we used it as an in vivo reporter of CSP functionality. Complementation of a ΔcspA strain with the different S. aureus CSP variants showed that only CspA could specifically restore STX production by controlling the activity of the stress‐associated sigma B factor (σ^B). To determine the amino acid residues responsible for CspA specificity, we created several chimeric CSPs that interchanged the amino acid differences between CspA and CspC, which shared the highest identity. We demonstrated that CspA Pro58 was responsible for the specific control of σ^B activity and its associated phenotypes. Interestingly, CspC gained the biological function of CspA when the E58P substitution was introduced. This study highlights how just one evolutionarily selected amino acid change may be sufficient to modify the specific functionality of CSP paralogs.

Crystal structure of a Y-box binding protein 1 (YB-1)-RNA complex reveals key features and residues interacting with RNA

The Y-box binding protein 1 (YB-1) is a member of the cold shock domain (CSD) protein family and is recognized as an oncogenic factor in several solid tumors. By binding to RNA, YB-1 participates in several steps of posttranscriptional regulation of gene expression, including mRNA splicing, stability, and translation; microRNA processing; and stress granule assembly. However, the mechanisms in YB-1-mediated regulation of RNAs are unclear. Previously, we used both systematic evolution of ligands by exponential enrichment (SELEX) and individual-nucleotide resolution UV cross-linking and immunoprecipitation coupled RNA-Seq (iCLIP-Seq) analyses, which defined the RNA-binding consensus sequence of YB-1 as CA(U/C)C. We also reported that through binding to its core motif CAUC in primary transcripts, YB-1 regulates the alternative splicing of a CD44 variable exon and the biogenesis of miR-29b-2 during both Drosha and Dicer steps. To elucidate the molecular basis of the YB-1-RNA interactions, we report high-resolution crystal structures of the YB-1 CSD in complex with different RNA oligos at 1.7 Å resolution. The structure revealed that CSD interacts with RNA mainly through π-π stacking interactions assembled by four highly conserved aromatic residues. Interestingly, YB-1 CSD forms a homodimer in solution, and we observed that two residues, Tyr-99 and Asp-105, at the dimer interface are important for YB-1 CSD dimerization. Substituting these two residues with Ala reduced CSD's RNA-binding activity and abrogated the splicing activation of YB-1 targets. The YB-1 CSD-RNA structures presented here at atomic resolution provide mechanistic insights into gene expression regulated by CSD-containing proteins.

Structure, stability and specificity of the binding of ssDNA and ssRNA with proteins

Recognition of single-stranded DNA (ssDNA) or single-stranded RNA (ssRNA) is important for many fundamental cellular functions. A variety of single-stranded DNA-binding proteins (ssDBPs) and single-stranded RNA-binding proteins (ssRBPs) have evolved that bind ssDNA and ssRNA, respectively, with varying degree of affinities and specificities to form complexes. Structural studies of these complexes provide key insights into their recognition mechanism. However, computational modeling of the specific recognition process and to predict the structure of the complex is challenging, primarily due to the heterogeneity of their binding energy landscape and the greater flexibility of ssDNA or ssRNA compared with double-stranded nucleic acids. Consequently, considerably fewer computational studies have explored interactions between proteins and single-stranded nucleic acids compared with protein interactions with double-stranded nucleic acids. Here, we report a newly developed energy-based coarse-grained model to predict the structure of ssDNA–ssDBP and ssRNA–ssRBP complexes and to assess their sequence-specific interactions and stabilities. We tuned two factors that can modulate specific recognition: base–aromatic stacking strength and the flexibility of the single-stranded nucleic acid. The model was successfully applied to predict the binding conformations of 12 distinct ssDBP and ssRBP structures with their cognate ssDNA and ssRNA partners having various sequences. Estimated binding energies agreed well with the corresponding experimental binding affinities. Bound conformations from the simulation showed a funnel-shaped binding energy distribution where the native-like conformations corresponded to the energy minima. The various ssDNA–protein and ssRNA–protein complexes differed in the balance of electrostatic and aromatic energies. The lower affinity of the ssRNA–ssRBP complexes compared with the ssDNA–ssDBP complexes stems from lower flexibility of ssRNA compared to ssDNA, which results in higher rate constants for the dissociation of the complex (k_off) for complexes involving the former.

Quantifying bimolecular self-assembly is pivotal to understanding cellular function. In recent years, a large progress has been made in understanding the structure and biophysics of protein-protein interactions. Particularly, various computational tools are available for predicting these structures and to estimate their stability and the driving forces of their formation. The understating of the interactions between proteins and nucleic acids, however, is still limited, presumably due to the involvement of non-specific interactions as well as the high conformational plasticity that may demand an induced-fit mechanism. In particular, the interactions between proteins and single-stranded nucleic acids (i.e., single-stranded DNA and RNA) is very challenging due to their high flexibility. Furthermore, the interface between proteins and single-stranded nucleic acids is often chemically more heterogeneous than the interface between proteins and double-stranded DNA. In this study, we developed a coarse-grained computational model to predict the structure of complexes between proteins and single-stranded nucleic acids. The model was applied to estimate binding affinities and the estimated binding energies agreed well with the corresponding experimental binding affinities. The kinetics of association as well as the specificity of the complexes between proteins and ssDNA are different than those with ssRNA, mostly due to differences in their conformational flexibility.

Y-box proteins combine versatile cold shock domains and arginine-rich motifs (ARMs) for pleiotropic functions in RNA biology

Y-box proteins are single-strand DNA- and RNA-binding proteins distinguished by a conserved cold shock domain (CSD) and a variable C-terminal domain organized into alternating short modules rich in basic or acidic amino acids. A huge literature depicts Y-box proteins as highly abundant, staggeringly versatile proteins that interact with all mRNAs and function in most forms of mRNA-specific regulation. The mechanisms by which Y-box proteins recognize mRNAs are unclear, because their CSDs bind a jumble of diverse elements, and the basic modules in the C-terminal domain are considered to bind nonspecifically to phosphates in the RNA backbone. A survey of vertebrate Y-box proteins clarifies the confusing names for Y-box proteins, their domains, and RNA-binding motifs, and identifies several novel conserved sequences: first, the CSD is flanked by linkers that extend its binding surface or regulate co-operative binding of the CSD and N-terminal and C-terminal domains to proteins and RNA. Second, the basic modules in the C-terminal domain are bona fide arginine-rich motifs (ARMs), because arginine is the predominant amino acid and comprises 99% of basic residues. Third, conserved differences in AA (amino acid) sequences between isoforms probably affect RNA-binding specificity. C-terminal ARMs connect with many studies, demonstrating that ARMs avidly bind sites containing specific RNA structures. ARMs crystallize insights into the under-appreciated contributions of the C-terminal domain to site-specific binding by Y-box proteins and difficulties in identifying site-specific binding by the C-terminal domain. Validated structural biology techniques are available to elucidate the mechanisms by which YBXprot (Y-box element-binding protein) CSDs and ARMs identify targets.

Structure and interaction of Corynebacterium pseudotuberculosis cold shock protein A with Y-box single-stranded DNA fragment

Cold shock proteins (Csps) function to preserve cell viability at low temperatures by binding to nucleic acids and consequently control gene expression. The mesophilic bacterium Corynebacterium pseudotuberculosis is the causative agent of caseous lymphadenitis in animals, and infection in livestock is a considerable economic burden worldwide. In this report, the structure of cold shock protein A from Cp (Cp-CspA) and biochemical analysis of its temperature-dependent interaction with a Y-box ssDNA motif is presented. The Cp-CspA structure contains five β-strands making up a β-barrel fold with 11 hydrophobic core residues and two salt bridges that confers it with a melting temperature of ~ 54 °C that is similar to mesophilic Bs-CspB. Chemical shift perturbations analysis revealed that residues in the nucleic acid-binding motifs (RNP 1 and 2) and loop 3 are involved in binding to the Y-box fragment either by direct interaction or by conformational rearrangements remote from the binding region. Fluorescence quenching experiments of Cp-CspA showed that the dissociation constants for Y-box ssDNA binding is nanomolar and the binding affinity decreased as the temperature increased, indicating that the interaction is enthalpically driven and the hydrogen bonds and van der Waals forces are important contributions for complex stabilization. The Y31 of Cp-CspA is a particular occurrence among Csps from mesophilic bacteria that provide a possible explanation for the higher binding affinity to ssDNA than that observed for Bs-CspB. Anisotropy measurements indicated that the reduction in molecular mobility of Cp-CspA upon Y-box binding is characterized by a cooperative process.

DATABASE

Resonance assignment and structural data are available in the Biological Magnetic Resonance Data Bank and Protein Data Bank under accession number 26802 and 5O6F, respectively.

Electronic Structures of LNA Phosphorothioate Oligonucleotides

Important oligonucleotides in anti-sense research have been investigated in silico and experimentally. This involves quantum mechanical (QM) calculations and chromatography experiments on locked nucleic acid (LNA) phosphorothioate (PS) oligonucleotides. iso-potential electrostatic surfaces are essential in this study and have been calculated from the wave functions derived from the QM calculations that provide binding information and other properties of these molecules. The QM calculations give details of the electronic structures in terms of e.g., energy and bonding, which make them distinguish or differentiate between the individual PS diastereoisomers determined by the position of sulfur atoms. Rules are derived from the electronic calculations of these molecules and include the effects of the phosphorothioate chirality and formation of electrostatic potential surfaces. Physical and electrochemical descriptors of the PS oligonucleotides are compared to the experiments in which chiral states on these molecules can be distinguished. The calculations demonstrate that electronic structure, electrostatic potential, and topology are highly sensitive to single PS configuration changes and can give a lead to understanding the activity of the molecules.

Evidence for a cytoplasmic pool of ribosome-free mRNAs encoding inner membrane proteins in Escherichia coli

Translation-independent mRNA localization represents an emerging concept in cell biology. In Escherichia coli, mRNAs encoding integral membrane proteins (MPRs) are targeted to the membrane where they are translated by membrane associated ribosomes and the produced proteins are inserted into the membrane co-translationally. In order to better understand aspects of the biogenesis and localization of MPRs, we investigated their subcellular distribution using cell fractionation, RNA-seq and qPCR. The results show that MPRs are overrepresented in the membrane fraction, as expected, and depletion of the signal recognition particle-receptor, FtsY reduced the amounts of all mRNAs on the membrane. Surprisingly, however, MPRs were also found relatively abundant in the soluble ribosome-free fraction and their amount in this fraction is increased upon overexpression of CspE, which was recently shown to interact with MPRs. CspE also conferred a positive effect on the membrane-expression of integral membrane proteins. We discuss the possibility that the effects of CspE overexpression may link the intriguing subcellular localization of MPRs to the cytosolic ribosome-free fraction with their translation into membrane proteins and that the ribosome-free pool of MPRs may represent a stage during their targeting to the membrane, which precedes translation.

RNA binding and chaperone activity of the E. coli cold-shock protein CspA

Ensuring the correct folding of RNA molecules in the cell is of major importance for a large variety of biological functions. Therefore, chaperone proteins that assist RNA in adopting their functionally active states are abundant in all living organisms. An important feature of RNA chaperone proteins is that they do not require an external energy source to perform their activity, and that they interact transiently and non-specifically with their RNA targets. So far, little is known about the mechanistic details of the RNA chaperone activity of these proteins. Prominent examples of RNA chaperones are bacterial cold shock proteins (Csp) that have been reported to bind single-stranded RNA and DNA. Here, we have used advanced NMR spectroscopy techniques to investigate at atomic resolution the RNA-melting activity of CspA, the major cold shock protein of Escherichia coli, upon binding to different RNA hairpins. Real-time NMR provides detailed information on the folding kinetics and folding pathways. Finally, comparison of wild-type CspA with single-point mutants and small peptides yields insights into the complementary roles of aromatic and positively charged amino-acid side chains for the RNA chaperone activity of the protein.

Unusual dimerization of a BcCsp mutant leads to reduced conformational dynamics

Cold shock proteins (Csp) constitute a family of ubiquitous small proteins that act as RNA-chaperones to avoid cold-induced termination of translation. All members contain two subdomains composed of 2 and 3 β-strands, respectively, which are connected by a hinge loop and fold into a β-barrel. Bacillus caldolyticus Csp (BcCsp) is one of the most studied members of the family in terms of its folding, function, and structure. This protein has been described as a monomer in solution, although a recent crystal structure showed dimerization via domain swapping (DS). In contrast, other cold shock proteins of the same fold are known to dimerize in a nonswapped arrangement. Hypothesizing that reducing the size of the hinge loop may promote swapping as in several other DS proteins with different folds we deleted two residues from these region (BcCsp∆36-37), leading to a protein in monomer-dimer equilibrium with similar folding stability to that of the wild-type. Strikingly, the crystal structure of BcCsp∆36-37 revealed a nonswapped dimer with its interface located at the nucleic acid-binding surface, showing that the deletion led to structural consequences far from the perturbation site. Concomitantly, circular dichroism experiments on BcCsp∆36-37 demonstrated that binding of the oligonucleotide hexathymidine disrupts the dimer. Additionally, HDXMS shows a protective effect on the protein structure upon dimerization, where the resulting interactions between ligand-binding surfaces in the dimer reduced the extent of exchange throughout the whole protein. Our work provides evidence of the complex interplay between conformational dynamics, deletions, and oligomerization within the Csp protein family.

DATABASES

Structural data are available in the Protein Data Bank under accession number 5JX4.

Terahertz Frequency Spectroscopy to Determine Cold Shock Protein Stability upon Solvation and Evaporation - A Molecular Dynamics Study

Infrared (IR) and Terahertz (THz) spectroscopy simulations were carried out using CHARMM35b2 to determine protein stability. The stabilities of three bacterial cold shock proteins (Csps) originating from mesophiles, thermophiles and hyper- thermophiles respectively were investigated in this study. The three different Csps were investigated by Normal-Mode analysis and Molecular Dynamics simulation of THz spectra using the Hessian matrix for solvated systems, interpreted in the harmonic approximation at optimum near-melting temperatures of each homologue, by incorporating differences in the hydrous and anhydrous states of the Csps. The results show slight variations in the large scale protein motion. However, the IR spectra of Csps observed at the low frequency saddle surface region, clearly distinguishes the thermophilic and mesophilic proteins based on their stability. Further studies on protein stability employing low-frequency collective modes have the potential to reveal functionally important conformational changes that are biologically significant.

The Cold Shock Domain of YB-1 Segregates RNA from DNA by Non-Bonded Interactions

The human YB-1 protein plays multiple cellular roles, of which many are dictated by its binding to RNA and DNA through its Cold Shock Domain (CSD). Using molecular dynamics simulation approaches validated by experimental assays, the YB1 CSD was found to interact with nucleic acids in a sequence-dependent manner and with a higher affinity for RNA than DNA. The binding properties of the YB1 CSD were close to those observed for the related bacterial Cold Shock Proteins (CSP), albeit some differences in sequence specificity. The results provide insights in the molecular mechanisms whereby YB-1 interacts with nucleic acids.

Solution structure of the RNA-binding cold-shock domain of the Chlamydomonas reinhardtii NAB1 protein and insights into RNA recognition

Light-harvesting complex (LHC) proteins are among the most abundant proteins on Earth and play critical roles in photosynthesis, both in light capture and in photoprotective mechanisms. The Chlamydomonas reinhardtii nucleic acid-binding protein 1 (NAB1) is a negative regulator of LHC protein translation. Its N-terminal cold-shock domain (CSD) binds to a 13-nt element [CSD consensus sequence (CSDCS)] found in the mRNA of specific LHC proteins associated with Photosystem II (PSII), an interaction which regulates LHC expression and, consequently, PSII-associated antenna size, structure and function. In the present study, we elucidated the solution structure of the NAB1 CSD as determined by heteronuclear NMR. The CSD adopts a characteristic five-stranded anti parallel β-barrel fold. Upon addition of CSDCS RNA, a large number of NMR chemical shift perturbations were observed, corresponding primarily to surface-exposed residues within the highly conserved β2- and β3-strands in the canonical RNA-binding region, but also to residues on β-strand 5 extending the positive surface patch and the overall RNA-binding site. Additional chemical shift perturbations that accompanied RNA binding involved buried residues, suggesting that transcript recognition is accompanied by conformational change. Our results indicate that NAB1 associates with RNA transcripts through a mechanism involving its CSD that is conserved with mechanisms of sequence-specific nucleic acid recognition employed by ancestrally related bacterial cold-shock proteins (CSPs).

Purification, characterization and safety assessment of the introduced cold shock protein B in DroughtGard maize

DroughtGard maize was developed through constitutive expression of cold shock protein B (CSPB) from Bacillus subtilis to improve performance of maize (Zea mays) under water-limited conditions. B. subtilis commonly occurs in fermented foods and CSPB has a history of safe use. Safety studies were performed to further evaluate safety of CSPB introduced into maize. CSPB was compared to proteins found in current allergen and protein toxin databases and there are no sequence similarities between CSPB and known allergens or toxins. In order to validate the use of Escherichia coli-derived CSPB in other safety studies, physicochemical and functional characterization confirmed that the CSPB produced by DroughtGard possesses comparable molecular weight, immunoreactivity, and functional activity to CSPB produced from E. coli and that neither is glycosylated. CSPB was completely digested with sequential exposure to pepsin and pancreatin for 2 min and 30 s, respectively, suggesting that CSPB will be degraded in the mammalian digestive tract and would not be expected to be allergenic. Mice orally dosed with CSPB at 2160 mg/kg, followed by analysis of body weight gains, food consumption and clinical observations, showed no discernible adverse effects. This comprehensive safety assessment indicated that the CSPB protein from DroughtGard is safe for food and feed consumption.

Structure and flexibility of the thermophilic cold-shock protein of Thermus aquaticus

The thermophilic bacterium Thermus aquaticus is a well-known source of Taq polymerase. Here, we studied the structure and dynamics of the T. aquaticus cold-shock protein (Ta-Csp) to better understand its thermostability using NMR spectroscopy. We found that Ta-Csp has a five-stranded β-barrel structure with five salt bridges which are important for more rigid structure and a higher melting temperature (76 °C) of Ta-Csp compared to mesophilic and psychrophilic Csps. Microsecond to millisecond time scale exchange processes occur only at the β1-β2 surface region of the nucleic acid binding site with an average conformational exchange rate constant of 674 s(-1). The results imply that thermophilic Ta-Csp has a more rigid structure and may not need high structural flexibility to accommodate nucleic acids upon cold shock compared to its mesophile and psychrophile counterparts.

Quantum mechanical studies of DNA and LNA

Quantum mechanical (QM) methodology has been employed to study the structure activity relations of DNA and locked nucleic acid (LNA). The QM calculations provide the basis for construction of molecular structure and electrostatic surface potentials from molecular orbitals. The topologies of the electrostatic potentials were compared among model oligonucleotides, and it was observed that small structural modifications induce global changes in the molecular structure and surface potentials. Since ligand structure and electrostatic potential complementarity with a receptor is a determinant for the bonding pattern between molecules, minor chemical modifications may have profound changes in the interaction profiles of oligonucleotides, possibly leading to changes in pharmacological properties. The QM modeling data can be used to understand earlier observations of antisense oligonucleotide properties, that is, the observation that small structural changes in oligonucleotide composition may lead to dramatic shifts in phenotypes. These observations should be taken into account in future oligonucleotide drug discovery, and by focusing more on non RNA target interactions it should be possible to utilize the exhibited property diversity of oligonucleotides to produce improved antisense drugs.

Single-stranded DNA-binding proteins: multiple domains for multiple functions

The recognition of single-stranded DNA (ssDNA) is integral to myriad cellular functions. In eukaryotes, ssDNA is present stably at the ends of chromosomes and at some promoter elements. Furthermore, it is formed transiently by several cellular processes including telomere synthesis, transcription, and DNA replication, recombination, and repair. To coordinate these diverse activities, a variety of proteins have evolved to bind ssDNA in a manner specific to their function. Here, we review the recognition of ssDNA through the analysis of high-resolution structures of proteins in complex with ssDNA. This functionally diverse set of proteins arises from a limited set of structural motifs that can be modified and arranged to achieve distinct activities, including a range of ligand specificities. We also investigate the ways in which these domains interact in the context of large multidomain proteins/complexes. These comparisons reveal the structural features that define the range of functions exhibited by these proteins.

Mechanisms of Lin28-mediated miRNA and mRNA regulation--a structural and functional perspective

Lin28 is an essential RNA-binding protein that is ubiquitously expressed in embryonic stem cells. Its physiological function has been linked to the regulation of differentiation, development, and oncogenesis as well as glucose metabolism. Lin28 mediates these pleiotropic functions by inhibiting let-7 miRNA biogenesis and by modulating the translation of target mRNAs. Both activities strongly depend on Lin28’s RNA-binding domains (RBDs), an N-terminal cold-shock domain (CSD) and a C-terminal Zn-knuckle domain (ZKD). Recent biochemical and structural studies revealed the mechanisms of how Lin28 controls let-7 biogenesis. Lin28 binds to the terminal loop of pri- and pre-let-7 miRNA and represses their processing by Drosha and Dicer. Several biochemical and structural studies showed that the specificity of this interaction is mainly mediated by the ZKD with a conserved GGAGA or GGAGA-like motif. Further RNA crosslinking and immunoprecipitation coupled to high-throughput sequencing (CLIP-seq) studies confirmed this binding motif and uncovered a large number of new mRNA binding sites. Here we review exciting recent progress in our understanding of how Lin28 binds structurally diverse RNAs and fulfills its pleiotropic functions.

CRT1 is a nuclear-translocated MORC endonuclease that participates in multiple levels of plant immunity

Arabidopsis thaliana CRT1 (compromised for recognition of Turnip Crinkle Virus) was previously shown to be required for effector-triggered immunity. Sequence analyses previously revealed that CRT1 contains the ATPase and S5 domains characteristic of Microchidia (MORC) proteins; these proteins are associated with DNA modification and repair. Here we show that CRT1 and its closest homologue, CRH1, are also required for pathogen-associated molecular pattern (PAMP)-triggered immunity, basal resistance, non-host resistance and systemic acquired resistance. Consistent with its role in PAMP-triggered immunity, CRT1 interacted with the PAMP recognition receptor FLS2. Subcellular fractionation and transmission electron microscopy detected a subpopulation of CRT1 in the nucleus, whose levels increased following PAMP treatment or infection with an avirulent pathogen. These results, combined with the demonstration that CRT1 binds DNA, exhibits endonuclease activity, and affects tolerance to the DNA-damaging agent mitomycin C, argue that this prototypic eukaryotic member of the MORC superfamily has important nuclear functions during immune response activation.

Identification of LIN28B-bound mRNAs reveals features of target recognition and regulation

The conserved human LIN28 RNA-binding proteins function in development, maintenance of pluripotency and oncogenesis. We used PAR-CLIP and a newly developed variant of this method, iDo-PAR-CLIP, to identify LIN28B targets as well as sites bound by the individual RNA-binding domains of LIN28B in the human transcriptome at nucleotide resolution. The position of target binding sites reflected the known structural relative orientation of individual LIN28B-binding domains, validating iDo-PAR-CLIP. Our data suggest that LIN28B directly interacts with most expressed mRNAs and members of the let-7 microRNA family. The Lin28-binding motif detected in pre-let-7 was enriched in mRNA sequences bound by LIN28B. Upon LIN28B knockdown, cell proliferation and the cell cycle were strongly impaired. Quantitative shotgun proteomics of LIN28B depleted cells revealed significant reduction of protein synthesis from its RNA targets. Computational analyses provided evidence that the strength of protein synthesis reduction correlated with the location of LIN28B binding sites within target transcripts.

Identification of mRNAs bound and regulated by human LIN28 proteins and molecular requirements for RNA recognition

Human LIN28A and LIN28B are RNA-binding proteins (RBPs) conserved in animals with important roles during development and stem cell reprogramming. We used Photoactivatable-Ribonucleoside-Enhanced Crosslinking and Immunoprecipitation (PAR-CLIP) in HEK293 cells and identified a largely overlapping set of ∼3000 mRNAs at ∼9500 sites located in the 3' UTR and CDS. In vitro and in vivo, LIN28 preferentially bound single-stranded RNA containing a uridine-rich element and one or more flanking guanosines and appeared to be able to disrupt base-pairing to access these elements when embedded in predicted secondary structure. In HEK293 cells, LIN28 protein binding mildly stabilized target mRNAs and increased protein abundance. The top targets were its own mRNAs and those of other RBPs and cell cycle regulators. Alteration of LIN28 protein levels also negatively regulated the abundance of some but not all let-7 miRNA family members, indicating sequence-specific binding of let-7 precursors to LIN28 proteins and competition with cytoplasmic miRNA biogenesis factors.

Structural and dynamic features of cold-shock proteins of Listeria monocytogenes, a psychrophilic bacterium

Cold-shock proteins (Csps), proteins expressed when the ambient temperature drops below the growth-supporting temperature, bind to single-stranded nucleic acids and act as RNA chaperones to regulate translation. Listeria monocytogenes is a psychrophilic food-borne pathogen that is problematic for the food industry. Structures of Csps from psychrophilic bacteria have not yet been studied. Despite dramatic differences in the thermostability of Csps of various thermophilic microorganisms, these proteins share a high degree of primary sequence homology and a high degree of three-dimensional structural similarity. Here, we investigated the structural and dynamic features as well as the thermostability of L. monocytogenes CspA (Lm-CspA). Lm-CspA has a five-stranded β-barrel structure with hydrophobic core packing and two salt bridges. When heptathymidine (dT(7)) binds, values for the heteronuclear nuclear Overhauser effect and order parameters of residues in surface loop regions near nucleic acid binding sites increase dramatically. Moreover, Carr-Purcell-Meiboom-Gill experiments showed that slow motions observed for the nucleic acid binding residues K7, W8, F15, F27, and R56 disappeared in Lm-CspA-dT(7). Lm-CspA is less thermostable than mesophilic and thermophilic Csps, with a lower melting temperature (40 °C). The structural flexibility that accompanies longer surface loops and less hydrophobic core packing and a number of salt bridges and unfavorable electrostatic repulsion are likely key factors in the low thermostability of Lm-CspA. This implies that the large conformational flexibility of psychrophilic Lm-CspA, which more easily accommodates nucleic acids at low temperature, is required for RNA chaperone function under cold-shock conditions and for the cold adaptation of L. monocytogenes.

Solution structure of the cold-shock-like protein from Rickettsia rickettsii

The solution structure of the cold-shock-like protein from R. rickettsii, the causative agent of Rocky Mountain spotted fever, is reported.

Rocky Mountain spotted fever is caused by Rickettsia rickettsii infection. R. rickettsii can be transmitted to mammals, including humans, through the bite of an infected hard-bodied tick of the family Ixodidae. Since the R. rickettsii genome contains only one cold-shock-like protein and given the essential nature of cold-shock proteins in other bacteria, the structure of the cold-shock-like protein from R. rickettsii was investigated. With the exception of a short α-helix found between β-strands 3 and 4, the solution structure of the R. rickettsii cold-shock-like protein has the typical Greek-key five-stranded β-barrel structure found in most cold-shock domains. Additionally, the R. rickettsii cold-shock-like protein, with a ΔG of unfolding of 18.4 kJ mol⁻¹, has a similar stability when compared with other bacterial cold-shock proteins.

Prediction of ligand binding site by insilico approach in cold resistant protein isolated from cold resistant mutant of Pseudomonas fluorescens

Cold shock proteins perform vital functions, such as mRNA masking, coupling of transcription to translation and developmental timing and regulation, which aids in survival of microbes in cold stress. Pseudomonas fluorescens is an ecologically important bacterium which helps in plant growth promotion. Since the cold tolerant mutant of the bacterium is able to grow at the temperature ranges from 30 to 4°C, it is of interest to study the process of its survival in the extreme temperatures. Therefore, this study is focused on the three dimensional structure and molecular modeling of cold resistant protein (CRP) from P. fluorescens to predict its molecular mechanism. Investigating the structure of CRP confirmed the presence of a conserved domain characteristic of the cold shock domain (CSD) family and a single nucleotide binding domain. When 3D structure of CRP was compared with the existing cold shock proteins, major deviations were found in the loop regions connecting the β2-β3, β3-β4 and β4-β5 sheets. Docking studies showed that CRP forms a significant clamp like structure at the substrate binding cleft which stabilizes the ligand. Therefore, it can be concluded that CRP has a strong affinity for the poly thymidine (poly T) stretch and can be considered a candidate for transcription regulation.

The Lin28 cold-shock domain remodels pre-let-7 microRNA

The RNA-binding protein Lin28 regulates the processing of a developmentally important group of microRNAs, the let-7 family. Lin28 blocks the biogenesis of let-7 in embryonic stem cells and thereby prevents differentiation. It was shown that both RNA-binding domains (RBDs) of this protein, the cold-shock domain (CSD) and the zinc-knuckle domain (ZKD) are indispensable for pri- or pre-let-7 binding and blocking its maturation. Here, we systematically examined the nucleic acid-binding preferences of the Lin28 RBDs and determined the crystal structure of the Lin28 CSD in the absence and presence of nucleic acids. Both RNA-binding domains bind to single-stranded nucleic acids with the ZKD mediating specific binding to a conserved GGAG motif and the CSD showing only limited sequence specificity. However, only the isolated Lin28 CSD, but not the ZKD, can bind with a reasonable affinity to pre-let-7 and thus is able to remodel the terminal loop of pre-let-7 including the Dicer cleavage site. Further mutagenesis studies reveal that the Lin28 CSD induces a conformational change in the terminal loop of pre-let-7 and thereby facilitates a subsequent specific binding of the Lin28 ZKD to the conserved GGAG motif.

A non-cold-inducible cold shock protein homolog mainly contributes to translational control under optimal growth conditions

Cold shock proteins (Csps) include both cold-induced and non-cold-induced proteins, contrary to their name. Cold-induced Csps are well studied; they function in cold acclimation by controlling transcription and translation. Some Csps have been reported to contribute to other cellular processes. However, the functions of non-cold-induced Csps under optimal growth conditions remain unknown. To elucidate these functions, we used transcriptome and proteome analyses as comprehensive approaches and have compared the outputs of wild-type and non-cold-induced Csp-deletion mutant cells. As a model organism, we selected Thermus thermophilus HB8 because it has only two csp genes (ttcsp1 and ttcsp2); ttCsp1 is the only non-cold-induced Csp. Surprisingly, the amount of transcripts and proteins upon deletion of the ttcsp1 gene was quite different. DNA microarray analysis revealed that the deletion of ttcsp1 did not affect the amount of transcripts, although the ttcsp1 gene was constantly expressed in the wild-type cell. Nonetheless, proteomic analysis revealed that the expression levels of many proteins were significantly altered when ttcsp1 was deleted. These results suggest that ttCsp1 functions in translation independent of transcription. Furthermore, ttCsp1 is involved in both the stimulation and inhibition of translation of specific proteins. Here, we have determined the crystal structure of ttCsp1 at 1.65 Å. This is the first report to present the structure of a non-cold-inducible cold shock protein. We also report the nucleotide binding affinity of ttCsp1. Finally, we discuss the functions of non-cold-induced Csps and propose how they modulate the levels of specific proteins to suit the prevailing environmental conditions.

RNA single strands bind to a conserved surface of the major cold shock protein in crystals and solution

Bacterial cold shock proteins (CSPs) regulate the cellular response to temperature downshift. Their general principle of function involves RNA chaperoning and transcriptional antitermination. Here we present two crystal structures of cold shock protein B from Bacillus subtilis (Bs-CspB) in complex with either a hexanucleotide (5'-UUUUUU-3') or heptanucleotide (5'-GUCUUUA-3') single-stranded RNA (ssRNA). Hydrogen bonds and stacking interactions between RNA bases and aromatic sidechains characterize individual binding subsites. Additional binding subsites which are not occupied by the ligand in the crystal structure were revealed by NMR spectroscopy in solution on Bs-CspB·RNA complexes. Binding studies demonstrate that Bs-CspB associates with ssDNA as well as ssRNA with moderate sequence specificity. Varying affinities of oligonucleotides are reflected mainly in changes of the dissociation rates. The generally lower binding affinity of ssRNA compared to its ssDNA analog is attributed solely to the substitution of thymine by uracil bases in RNA.

Molecular basis for interaction of let-7 microRNAs with Lin28

MicroRNAs (miRNAs) are small noncoding RNA molecules that regulate gene expression. Among these, members of the let-7 miRNA family control many cell-fate determination genes to influence pluripotency, differentiation, and transformation. Lin28 is a specific, posttranscriptional inhibitor of let-7 biogenesis. We report crystal structures of mouse Lin28 in complex with sequences from let-7d, let-7-f1, and let-7 g precursors. The two folded domains of Lin28 recognize two distinct regions of the RNA and are sufficient for inhibition of let-7 in vivo. We also show by NMR spectroscopy that the linker connecting the two folded domains is flexible, accommodating Lin28 binding to diverse let-7 family members. Protein-RNA complex formation imposes specific conformations on both components that could affect downstream recognition by other processing factors. Our data provide a molecular explanation for Lin28 specificity and a model for how it regulates let-7.

Redesign of a WW domain peptide for selective recognition of single-stranded DNA

A β-sheet miniprotein based on the FBP11 WW1 domain sequence has been redesigned for the molecular recognition of ssDNA. A previous report showed that a β-hairpin peptide dimer, (WKWK)(2), binds ssDNA with low micromolar affinity but with little selectivity over duplex DNA. This report extends those studies to a three-stranded β-sheet miniprotein designed to mimic the OB-fold. The new peptide binds ssDNA with low micromolar affinity and shows about 10-fold selectivity for ssDNA over duplex DNA. The redesigned peptide no longer binds its native ligand, the polyproline helix, confirming that the peptide has been redesigned for the function of binding ssDNA. Structural studies provide evidence that this peptide consists of a well-structured β-hairpin made of strands 2 and 3 with a less structured first strand that provides affinity for ssDNA but does not improve the stability of the full peptide. These studies provide insight into protein-DNA interactions as well as a novel example of protein redesign.

Structure-functional analyses of CRHSP-24 plasticity and dynamics in oxidative stress response

The cold shock domain (CSD) is an evolutionarily conserved nucleic acid binding domain that exhibits binding activity to RNA, ssDNA, and dsDNA. Mammalian CRHSP-24 contains CSD, but its structure-functional relationship has remained elusive. Here we report the crystal structure of human CRHSP-24 and characterization of the molecular trafficking of CRHSP-24 between stress granules and processing bodies in response to oxidative stress. The structure of CRHSP-24 determined by single-wavelength anomalous dispersion exhibits an α-helix and a compact β-barrel formed by five curved anti-parallel β strands. Ligand binding activity of the CSD is orchestrated by residues Ser(41) to Leu(43). Interestingly, a phosphomimetic S41D mutant abolishes the ssDNA binding in vitro and causes CRHSP-24 liberated from stress granules in vivo without apparent alternation of its localization to the processing bodies. This new class of phosphorylation-regulated interaction between the CSD and nucleic acids is unique in stress granule plasticity. Importantly, the association of CRHSP-24 with stress granules is blocked by PP4/PP2A inhibitor calyculin A as PP2A catalyzes the dephosphorylation of Ser(41) of CRHSP-24. Therefore, we speculate that CRHSP-24 participates in oxidative stress response via a dynamic and temporal association between stress granules and processing bodies.

Influence of the stability of a fused protein and its distance to the amyloidogenic segment on fibril formation

Conversion of native proteins into amyloid fibrils is irreversible and therefore it is difficult to study the interdependence of conformational stability and fibrillation by thermodynamic analyses. Here we approached this problem by fusing amyloidogenic poly-alanine segments derived from the N-terminal domain of the nuclear poly (A) binding protein PABPN1 with a well studied, reversibly unfolding protein, CspB from Bacillus subtilis. Earlier studies had indicated that CspB could maintain its folded structure in fibrils, when it was separated from the amyloidogenic segment by a long linker. When CspB is directly fused with the amyloidogenic segment, it unfolds because its N-terminal chain region becomes integrated into the fibrillar core, as shown by protease mapping experiments. Spacers of either 3 or 16 residues between CspB and the amyloidogenic segment were not sufficient to prevent this loss of CspB structure. Since the low thermodynamic stability of CspB (ΔG(D) = 12.4 kJ/mol) might be responsible for unfolding and integration of CspB into fibrils, fusions with a CspB mutant with enhanced thermodynamic stability (ΔG(D) = 26.9 kJ/mol) were studied. This strongly stabilized CspB remained folded and prevented fibril formation in all fusions. Our data show that the conformational stability of a linked, independently structured protein domain can control fibril formation.

Origins of specificity in protein-DNA recognition

Specific interactions between proteins and DNA are fundamental to many biological processes. In this review, we provide a revised view of protein-DNA interactions that emphasizes the importance of the three-dimensional structures of both macromolecules. We divide protein-DNA interactions into two categories: those when the protein recognizes the unique chemical signatures of the DNA bases (base readout) and those when the protein recognizes a sequence-dependent DNA shape (shape readout). We further divide base readout into those interactions that occur in the major groove from those that occur in the minor groove. Analogously, the readout of the DNA shape is subdivided into global shape recognition (for example, when the DNA helix exhibits an overall bend) and local shape recognition (for example, when a base pair step is kinked or a region of the minor groove is narrow). Based on the >1500 structures of protein-DNA complexes now available in the Protein Data Bank, we argue that individual DNA-binding proteins combine multiple readout mechanisms to achieve DNA-binding specificity. Specificity that distinguishes between families frequently involves base readout in the major groove, whereas shape readout is often exploited for higher resolution specificity, to distinguish between members within the same DNA-binding protein family.

The NMR solution structures of the five constituent cold-shock domains (CSD) of the human UNR (upstream of N-ras) protein

Upon cold shock, the amounts of most proteins dramatically decrease from normal levels, but those of cold shock proteins (CSPs) and proteins containing cold-shock domains (CSDs) greatly increase. Although their biological function is still not completely clear, cold-shock proteins might control translation via RNA chaperoning. Many cold-shock proteins contain the motifs (Y/F)GFI and (V/F)(V/F)H, which are known as ribonucleoprotein (RNP)-1 and RNP-2 motifs implicated in RNA/DNA binding. We determined the solution NMR structures of all five constituent CSDs of the human UNR (upstream of N-ras) protein. The spatial arrangements of the sidechains in the RNP-1 and RNP-2 motifs are mostly conserved; however, the conformations of the following residues in the first CSD are different: F43 and H45 (the first phenylalanine residue and the histidine residue in the putative binding site RNP-2) and Y30 (the first residue in the putative binding site RNP-1). F43 and H45 affect each other, and H45 is further influenced by C46. The altered binding site of the first CSD, and its putatively enhanced intrinsic stability, may provide an explanation for the observation that the first CSD has 20-fold higher RNA-binding activity than the fifth CSD. It also lends support to the hypothesis that the UNR protein arose by repeated duplication of a protein that originally contained just one CSD, and that the proto-UNR protein acquired cysteine C46 by mutation during evolution.

Crystallization and X-ray structure of cold-shock protein E from Salmonella typhimurium

In prokaryotic organisms, cold shock triggers the production of a small highly conserved family of cold-shock proteins (CSPs). CSPs have been well studied structurally and functionally in Escherichia coli and Bacillus subtilis, but Salmonella typhimurium CSPs remain relatively uncharacterized. In S. typhimurium, six homologous CSPs have been identified: StCspA-E and StCspH. The crystal structure of cold-shock protein E from S. typhimurium (StCspE) has been determined at 1.1 A resolution and has an R factor of 0.203 after refinement. The three-dimensional structure is similar to those of previously determined CSPs and is composed of five antiparallel beta-strands forming a classic OB fold/five-stranded beta-barrel. This first structure of a CSP from S. typhimurium provides new insight into the cold-shock response of this bacterium.

A computational approach for the rational design of stable proteins and enzymes: optimization of surface charge-charge interactions

The design of stable proteins and enzymes is not only of particular biotechnological importance, but also addresses some important fundamental questions. While there are a number of different options available for designing or engineering stable proteins, the field of computational design provides fast and universal methods for stabilizing proteins of interest. One of the successful computational design strategies focuses on stabilizing proteins through the optimization of charge-charge interactions on the protein surface. By optimizing surface interactions, it is possible to alleviate some of the challenges that accompany efforts to redesign the protein core. The rational design of surface charge-charge interactions also allows one to optimize only the interactions that are distant from binding sites or active sites, making it possible to increase stability without adversely affecting activity. The optimization of surface charge-charge interactions is discussed in detail along with the experimental evidence to demonstrate that this is a robust and universal approach to designing proteins with enhanced stability.

Crystal structure of the Agrobacterium virulence complex VirE1-VirE2 reveals a flexible protein that can accommodate different partners

Agrobacterium tumefaciens infects its plant hosts by a mechanism of horizontal gene transfer. This capability has led to its widespread use in artificial genetic transformation. In addition to DNA, the bacterium delivers an abundant ssDNA binding protein, VirE2, whose roles in the host include protection from cytoplasmic nucleases and adaptation for nuclear import. In Agrobacterium, VirE2 is bound to its acidic chaperone VirE1. When expressed in vitro in the absence of VirE1, VirE2 is prone to oligomerization and forms disordered filamentous aggregates. These filaments adopt an ordered solenoidal form in the presence of ssDNA, which was characterized previously by electron microscopy and three-dimensional image processing. VirE2 coexpressed in vitro with VirE1 forms a soluble heterodimer. VirE1 thus prevents VirE2 oligomerization and competes with its binding to ssDNA. We present here a crystal structure of VirE2 in complex with VirE1, showing that VirE2 is composed of two independent domains presenting a novel fold, joined by a flexible linker. Electrostatic interactions with VirE1 cement the two domains of VirE2 into a locked form. Comparison with the electron microscopy structure indicates that the VirE2 domains adopt different relative orientations. We suggest that the flexible linker between the domains enables VirE2 to accommodate its different binding partners.

A folded and functional protein domain in an amyloid-like fibril

The effect of the polypeptide environment on polyalanine-induced fibril formation was investigated with amyloidogenic fragments from PAPBN1, a nuclear protein controlling polyadenylation. Mutation-caused extensions of the natural 10 alanine sequence up to maximally 17 alanines result in fibril formation of PABPN1 and the development of the disease oculopharyngeal muscular dystrophy (OPMD). We explored the influence of fibril formation on the structure and function of a one-domain protein linked to the fibril-forming part of PABPN1. The well-characterized, stably folded, one-domain protein, cold-shock protein CspB from Bacillus subtilis, was fused either to the C terminus of the entire N-terminal domain of PABPN1 or directly to peptides consisting of 10 or 17 alanine residues. The fusion protein between the N-terminal domain of PABPN1 and CspB formed fibrils in which the structure and activity of CspB were retained. In the fibrils formed by fusions in which the polyalanine sequence was directly linked to CspB, CspB was unfolded. These results indicate that the folded conformation and the function of a protein domain can be maintained in amyloid-like fibrils, and that the distance between this domain and the fibril plays an important role.

Structure of the cold-shock domain protein from Neisseria meningitidis reveals a strand-exchanged dimer

The structure of the cold-shock domain protein from Neisseria meningitidis has been solved to 2.6 A resolution and shown to comprise a dimer formed by the exchange of two beta-strands between protein monomers. The overall fold of the monomer closely resembles those of other bacterial cold-shock proteins. The neisserial protein behaved as a monomer in solution and was shown to bind to a hexathymidine oligonucleotide with a stoichiometry of 1:1 and a K(d) of 1.25 microM.

RNA misfolding and the action of chaperones

RNA folds to a myriad of three-dimensional structures and performs an equally diverse set of functions. The ability of RNA to fold and function in vivo is all the more remarkable because, in vitro, RNA has been shown to have a strong propensity to adopt misfolded, non-functional conformations. A principal factor underlying the dominance of RNA misfolding is that local RNA structure can be quite stable even in the absence of enforcing global tertiary structure. This property allows non-native structure to persist, and it also allows native structure to form and stabilize non-native contacts or non-native topology. In recent years it has become clear that one of the central reasons for the apparent disconnect between the capabilities of RNA in vivo and its in vitro folding properties is the presence of RNA chaperones, which facilitate conformational transitions of RNA and therefore mitigate the deleterious effects of RNA misfolding. Over the past two decades, it has been demonstrated that several classes of non-specific RNA binding proteins possess profound RNA chaperone activity in vitro and when overexpressed in vivo, and at least some of these proteins appear to function as chaperones in vivo. More recently, it has been shown that certain DExD/H-box proteins function as general chaperones to facilitate folding of group I and group II introns. These proteins are RNA-dependent ATPases and have RNA helicase activity, and are proposed to function by using energy from ATP binding and hydrolysis to disrupt RNA structure and/or to displace proteins from RNA-protein complexes. This review outlines experimental studies that have led to our current understanding of the range of misfolded RNA structures, the physical origins of RNA misfolding, and the functions and mechanisms of putative RNA chaperone proteins.