Introduction Of Point Mutations Research Articles

Differential effects of volatile anesthetics on leukocyte integrin macrophage-1 antigen

Macrophage-1 antigen (Mac-1, αMβ2) is a leukocyte adhesion molecule that plays a significant role in leukocyte crawling and phagocytosis, and is homologous to its sister protein leukocyte function-associated antigen-1 (LFA-1, αLβ2). The authors have previously demonstrated that volatile anesthetics isoflurane and sevoflurane bound to and inhibited LFA-1. Here, the hypothesis tested was that isoflurane and sevoflurane would inhibit Mac-1. A binding assay of Mac-1 to its ligand inter-cellular adhesion molecule-1 (ICAM-1) using V-bottom plates was established. The effect of isoflurane and sevoflurane on Mac-1 was examined using this ICAM-1 binding assay and by probing exposure of activation-sensitive epitopes. The docking program Glide was used to predict anesthetic binding site(s) on Mac-1. The functional role of this predicted binding site was then assessed by introducing point mutations in this region. Lastly, the effect of anesthetic on activating mutants was evaluated. The results indicated that isoflurane inhibited binding of Mac-1 to ICAM-1, but sevoflurane did not. Isoflurane also attenuated the exposure of the activation-sensitive epitopes. The docking simulation predicted the isoflurane binding site to be at the cavity underneath the α7 helix of the ligand binding domain (the αM I domain). Point mutants at this predicted binding site contained both activating and deactivating mutants, suggesting its functional significance. The binding of activating mutants αM-Y267A β2-WT and αM-L312A β2-WT to ICAM-1 was not affected by isoflurane, but binding of another activating mutant αM-WT β2-L132A was inhibited supporting the binding of isoflurane to this cavity. The conclusion reached from these findings was that isoflurane inhibited Mac-1 binding to ICAM-1 by binding to the cavity underneath the α7 helix of the αM I domain, but sevoflurane did not. Thus, because these common clinical volatile anesthetics demonstrated different effects on Mac-1, this implied their effects on the immune system might differ.

An engineered old yellow enzyme that enables efficient synthesis of (4R,6R)-Actinol in a one-pot reduction system.

(4R,6R)-Actinol can be stereo-selectively synthesized from ketoisophorone by a two-step conversion using a mixture of two enzymes: Candida macedoniensis old yellow enzyme (CmOYE) and Corynebacterium aquaticum (6R)-levodione reductase. However, (4S)-phorenol, an intermediate, accumulates because of the limited substrate range of CmOYE. To address this issue, we solved crystal structures of CmOYE in the presence and absence of a substrate analogue p-HBA, and introduced point mutations into the substrate-recognition loop. The most effective mutant (P295G) showed two- and 12-fold higher catalytic activities toward ketoisophorone and (4S)-phorenol, respectively, than the wild-type, and improved the yield of the two-step conversion from 67.2 to 90.1%. Our results demonstrate that the substrate range of an enzyme can be changed by introducing mutation(s) into a substrate-recognition loop. This method can be applied to the development of other favorable OYEs with different substrate preferences.

Stability of structured Kaposi's sarcoma-associated herpesvirus ORF57 protein is regulated by protein phosphorylation and homodimerization.

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 plays an essential role in KSHV lytic infection by promoting viral gene expression at the posttranscriptional level. Using bioinformatic and biochemical approaches, we determined that ORF57 contains two structurally and functionally distinct domains: a disordered nonstructural N-terminal domain (amino acids [aa] 1 to 152) and a structured α-helix-rich C-terminal domain (aa 153 to 455). The N-terminal domain mediates ORF57 interaction with several RNA-protein complexes essential for ORF57 to function. The N-terminal phosphorylation by cellular casein kinase II (CKII) at S21, T32, and S43, and other cellular kinases at S95 and S97 residues in proximity of the caspase-7 cleavage site, 30-DETD-33, inhibits caspase-7 digestion of ORF57. The structured C-terminal domain mediates homodimerization of ORF57, and the critical region for this function was mapped carefully to α-helices 7 to 9. Introduction of point mutations into α-helix 7 at ORF57 aa 280 to 299, a region highly conserved among ORF57 homologues from other herpesviruses, inhibited ORF57 homodimerization and led to proteasome-mediated degradation of ORF57 protein. Thus, homodimerization of ORF57 via its C terminus prevents ORF57 from degrading and allows two structure-free N termini of the dimerized ORF57 to work coordinately for host factor interactions, leading to productive KSHV lytic infection and pathogenesis. KSHV is a human oncogenic virus linked to the development of several malignancies. KSHV-mediated oncogenesis requires both latent and lytic infection. The KSHV ORF57 protein is essential for KSHV lytic replication, as it regulates the expression of viral lytic genes at the posttranscriptional level. This report provides evidence that the structural conformation of the ORF57 protein plays a critical role in regulation of ORF57 stability. Phosphorylation by CKII on the identified serine/threonine residues at the N-terminal unstructured domain of ORF57 prevents its digestion by caspase-7. The C-terminal domain of ORF57, which is rich in α-helices, contributes to homodimerization of ORF57 to prevent proteasome-mediated protein degradation. Elucidation of the ORF57 structure not only enables us to better understand ORF57 stability and functions but also provides an important tool for us to modulate ORF57's activity with the aim to inhibit KSHV lytic replication.

I-SceI-mediated scarless gene modification via allelic exchange in Clostridium

Although gene disruption in Clostridium spp. with the TargeTron technology is much more effective than single-crossover integration, it cannot achieve gene modification via allelic exchange. Here, we developed a targeted, nonpolar, scarless gene modification system based on the I-SceI endonuclease. First, a replicative plasmid containing homology arms on either side of the target sequence and I-SceI recognition sites was integrated into the Clostridium chromosome, resulting in single-crossover integrants containing a mutant allele. Second, the cells were transformed with plasmids containing the synthetic gene (sceC) encoding the I-SceI enzyme, resulting in double-stranded breaks at the I-SceI recognition sites, which stimulated homologous recombination and yielded double-crossover mutants. Application of the method was demonstrated by deleting two genes (adc and glcG) from C. acetobutylicum ATCC 824 and one gene (adc) from C. beijerinckii NCIMB 8052, and by introducing point mutations into xylR of C. beijerinckii NCIMB 8052. The double-crossover mutants displayed similar fermentation phenotypes to those constructed with the TargeTron technology.

Structural states of the flexible catalytic loop of M. tuberculosis tyrosyl-tRNA synthetase in different enzyme-substrate complexes.

Tyrosyl-tRNA synthetase from Mycobacterium tuberculosis (MtTyrRS) is an enzyme that belongs to class I of aminoacyl-tRNA synthetases, which catalyze the attachment of L-tyrosine to its cognate tRNATyr in the preribosomal step of protein synthesis. MtTyrRS is incapable of cross-recognition and aminoacylation of human cytoplasmic tRNATyr, so this enzyme may be a promising target for development of novel selective inhibitors as putative antituberculosis drugs. As a class I aminoacyl-tRNA synthetase, MtTyrRS contains the HIGH-like and KFGKS catalytic motifs that catalyze amino acid activation with ATP. In this study, the conformational mobility of MtTyrRS catalytic KFGKS loop was analyzed by 100-ns all-atoms molecular dynamics simulations of the free enzyme and its complexes with different substrates: tyrosine, ATP, and the tyrosyl-adenylate intermediate. It was shown that in the closed state of the active site, the KFGKS loop, readily adopts different stable conformations depending on the type of bound substrate. Molecular dynamics simulations revealed that the closed state of the loop is stabilized by dynamic formation of two antiparallel β-sheets at flanking ends which hold the KFGKS fragment inside the active center. Prevention of β-sheet formation by introducing point mutations in the loop sequence results in a rapid (<20 ns) transition of the loop from its functional "closed" M-like structure to an inactive "open" O-like structure, i.e. rapid diffusion of the catalytic loop outside the active site. The flexibility and rapid dynamics of the wild-type aaRS catalytic loop structure are crucial for formation of protein-substrate interactions and subsequently for overall enzyme functional activity.

The contest for precursors: channelling L-isoleucine synthesis in Corynebacterium glutamicum without byproduct formation.

L-Isoleucine is an essential amino acid, which is required as a pharma product and feed additive. Its synthesis shares initial steps with that of L-lysine and L-threonine, and four enzymes of L-isoleucine synthesis have an enlarged substrate specificity involved also in L-valine and L-leucine synthesis. As a consequence, constructing a strain specifically overproducing L-isoleucine without byproduct formation is a challenge. Here, we analyze for consequences of plasmid-encoded genes in Corynebacterium glutamicum MH20-22B on L-isoleucine formation, but still obtain substantial accumulation of byproducts. In a different approach, we introduce point mutations into the genome of MH20-22B to remove the feedback control of homoserine dehydrogenase, hom, and threonine dehydratase, ilvA, and we assay sets of genomic promoter mutations to increase hom and ilvA expression as well as to reduce dapA expression, the latter gene encoding the dihydrodipicolinate synthase. The promoter mutations are mirrored in the resulting differential protein levels determined by a targeted LC-MS/MS approach for the three key enzymes. The best combination of genomic mutations was found in strain K2P55, where 53 mM L-isoleucine could be obtained. Whereas in fed-batch fermentations with the plasmid-based strain, 94 mM L-isoleucine with L-lysine as byproduct was formed; with the plasmid-less strain K2P55, 109 mM L-isoleucine accumulated with no substantial byproduct formation. The specific molar yield with the latter strain was 0.188 mol L-isoleucine (mol glucose)(-1) which characterizes it as one of the best L-isoleucine producers available and which does not contain plasmids.

A versatile and highly efficient method for scarless genome editing in Escherichia coli and Salmonella enterica.

BackgroundRecently developed methods for genome editing in bacteria take advantage of the introduction of double-strand breaks by I-SceI in a mutation cassette to select for cells in which homologous recombination has healed the break and introduced a desired mutation. This elegantly designed method did not work well in our hands for most genes.ResultsWe corrected a mutation in the gene encoding I-SceI that compromised the function of a previously used Red helper plasmid. Further, we found that transcription extending into the mutation cassette interferes with cleavage by I-SceI. Addition of two transcription terminators upstream of the cleavage site dramatically increases the efficiency of genome editing. We also developed an improved method for modification of essential genes. Inclusion of a segment of the essential gene consisting of synonymous codons restores an open reading frame when the mutation cassette is integrated into the genome and decreases the frequency of recombination events that fail to incorporate the desired mutation. The optimized protocol takes only 5 days and has been 100% successful for over 100 genomic modifications in our hands.ConclusionsThe method we describe here is reliable and versatile, enabling various types of genome editing in Escherichia coli and Salmonella enterica by straightforward modifications of the mutation cassette. We provide detailed descriptions of the methods as well as designs for insertions, deletions, and introduction of point mutations.

Mutational definition of binding requirements of an hnRNP-like protein in Arabidopsis using fluorescence correlation spectroscopy

Arabidopsis thaliana glycine-rich RNA binding protein 7 (AtGRP7) is part of a negative feedback loop through which it regulates alternative splicing and steady-state abundance of its pre-mRNA. Here we use fluorescence correlation spectroscopy to investigate the requirements for AtGRP7 binding to its intron using fluorescently-labelled synthetic oligonucleotides. By systematically introducing point mutations we identify three nucleotides that lead to an increased Kd value when mutated and thus are critical for AtGRP7 binding. Simultaneous mutation of all three residues abrogates binding. The paralogue AtGRP8 binds to an overlapping motif but with a different sequence preference, in line with overlapping but not identical functions of this protein pair. Truncation of the glycine-rich domain reduces the binding affinity of AtGRP7, showing for the first time that the glycine-rich stretch of a plant hnRNP-like protein contributes to binding. Mutation of the conserved R49 that is crucial for AtGRP7 function in pathogen defence and splicing abolishes binding.

Crystal structure of the M14R mutant of oryctin in complex with trypsin

Oryctin is a 66-amino-acid protein purified from the larval haemolymph of the coconut rhinoceros beetle Oryctes rhinoceros, which shows no sequence similarity to any other protein known. We determined the solution NMR structure of oryctin, and found that oryctin had a similar backbone fold to the turkey ovomucoid domain 3, OMTKY3, a Kazal-type serine protease inhibitor [1]. Based on the structural similarity, we tested the serine protease inhibitory activity of oryctin, and found that oryctin does inhibit some serine proteases, such as α-chymotrypsin, endopeptidase K, subtilisin Carlsberg, and leukocyte elastase [1]. However, oryctin cannot inhibit trypsin at all. In this study, we have introduced point mutations to the putative inhibition loop of oryctin to obtain oryctin mutants that can inhibit trypsin. Then, we have solved the crystal structure of such an oryctin mutant, M14R-oryctin with a Ki value of 3.410.8 nM, in complex with trypsin to reveal how it binds to and inhibits trypsin. As predicted, the putative inhibition loop lay on the substrate binding cleft of trypsin. Particularly, the side chain of R14 fit into the S1 pocket of trypsin by forming hydrogen/ionic bonds with D191, S192 and G216 at the bottom of the S1 pocket and G195, D196, S197 and S212 at its entrance. In addition, R65 located in the C-terminal α-helix of M14R-oryctin formed hydrogen bonds with S40 and F44 of trypsin. The latter interaction, which is unique to oryctin, enhances its binding affinity to trypsin.

Identification of conserved residues in hepatitis C virus envelope glycoprotein E2 that modulate virus dependence on CD81 and SRB1 entry factors.

In spite of the high variability of its sequence, hepatitis C virus (HCV) envelope glycoprotein E2 contains several conserved regions. In this study, we explored the structural and functional features of the highly conserved E2 segment from amino acid (aa) 502 to 520, which had been proposed as a fusion peptide and shown to strongly overlap a potential conserved neutralizing epitope. For this purpose, we used reverse genetics to introduce point mutations within this region, and we characterized the phenotypes of these mutants in the light of the recently published structure of E2. The functional analyses showed that their phenotypes are in agreement with the positions of the corresponding residues in the E2 crystal structure. In contrast, our data ruled out the involvement of this region in membrane fusion, and they indicate that alternative conformations would be necessary to expose the potential neutralizing epitope present in this segment. Of particular interest, we identified three specific mutations (Y507L, V514A, and V515A) located within this neutralizing epitope which only mildly reduced infectivity and showed no assembly defect. These mutations modulated HCV dependence on the viral receptor SRB1, and/or they also modulated virion sensitivity to neutralizing antibodies. Importantly, their characterization also showed that amino acids Y507, V514, and V515 contribute to E2 interaction with HCV receptor CD81. In conclusion, our data show that the highly conserved E2 segment from aa 502 to 520 plays a key role in cell entry by influencing the association of the viral particle with coreceptors and neutralizing antibodies. Hepatitis C virus (HCV) envelope proteins E1 and E2 exhibit sequence variability. However, some segments of the envelope proteins are highly conserved, suggesting that these sequences play a key role at some steps of the HCV life cycle. In this work, we characterized the function and structure of a highly conserved E2 region that is targeted by neutralizing antibodies and had been proposed as a fusion peptide. Our data ruled out the involvement of this region in membrane fusion but allowed for the identification of new residues modulating the interaction of the virus with entry factors and its sensitivity to neutralizing antibodies. Moreover, structural data suggest that alternative conformations could exist for E2, which would explain the presence of a partially masked neutralizing epitope in this segment in the currently available E2 structure. Overall, our findings highlight the importance of conserved regions in the sequences of HCV envelope proteins.

Efficient genome engineering of Toxoplasma gondii using CRISPR/Cas9.

Toxoplasma gondii is a parasite of humans and animals, and a model for other apicomplexans including Plasmodium spp., the causative agents of malaria. Despite many advances, manipulating the T. gondii genome remains labor intensive, and is often restricted to lab-adapted strains or lines carrying mutations that enable selection. Here, we use the RNA-guided Cas9 nuclease to efficiently generate knockouts without selection, and to introduce point mutations and epitope tags into the T. gondii genome. These methods will streamline the functional analysis of parasite genes and enable high-throughput engineering of their genomes.

A highly efficient in vitro site-directed mutagenesis protocol for introducing multiple-site mutations into target genes

背景与目的在基因序列中引入点突变是研究基因结构和功能及其相关性的重要手段。目前已有多种对基因序列进行突变的方法，然而，这些方法大多对单一位置的基因突变有效，而对在基因序列中引入多位点突变，还有待方法的进一步改进。为适应这一需要，本研究提供了一种高效的可在基因序列的多个位点引入突变的方法。方法该方法依赖于一种Type Ⅱs类的限制性内切酶，例如Esp 3I等。本研究所提供的方法中，针对每一个突变位点，合成一对含有突变点和所选择的Type Ⅱs类的限制性内切酶位点的引物，当需要引入多位点突变时，则利用邻近的两个突变位点的引物做PCR反应，多个位点的突变，就可以得到多个扩增片段，将这些片段用和引物上的限制性内切酶位点对应的酶进行反应，然后用连接反应将片段连接形成突变基因。结果本研究所提供的方法非常简便，主要实验步骤可以在一天之内完成。我们已经利用这种方法，在绿色荧光蛋白（enhanced green fluorecence protein, EGFP）和nm23基因中，引入3个或4个突变，突变效率几乎为100%。结论本研究所提供的方法可以成为研究基因功能的有用工具。

Mechanisms of transcriptional activation of the mouse claudin-5 promoter by estrogen receptor alpha and beta

Claudin-5 is an integral membrane protein and a critical component of endothelial tight junctions that control paracellular permeability. Claudin-5 is expressed at high levels in the brain vascular endothelium. Estrogens have multiple effects on vascular physiology and function. The biological actions of estrogens are mediated by two different estrogen receptor (ER) subtypes, ER alpha and ER beta. Estrogens have beneficial effects in several vascular disorders. Recently we have cloned and characterized a murine claudin-5 promoter and demonstrated 17beta-estradiol (E2)-mediated regulation of claudin-5 in brain and heart microvascular endothelium on promoter, mRNA and protein level. Sequence analysis revealed a putative estrogen response element (ERE) and a putative Sp1 transcription factor binding site in the claudin-5 promoter. The aim of the present study was to further characterize the estrogen-responsive elements of claudin-5 promoter. First, we introduced point mutations in ERE or Sp1 site in −500/+111 or in Sp1 site of −268/+111 claudin-5 promoter construct, respectively. Basal and E2-mediated transcriptional activation of mutated constructs was abrogated in the luciferase reporter gene assay. Next, we examined whether estrogen receptor subtypes bind to the claudin-5 promoter region. For this purpose we performed chromatin immunoprecipitation assays using anti-estrogen receptor antibodies and cellular lysates of E2-treated endothelial cells followed by quantitative PCR analysis. We show enrichment of claudin-5 promoter fragments containing the ERE- and Sp1-binding site in immunoprecipitates after E2 treatment. Finally, in a gel mobility shift assay, we demonstrated DNA–protein interaction of both ER subtypes at ERE. In summary, this study provides evidence that both a non-consensus ERE and a Sp1 site in the claudin-5 promoter are functional and necessary for the basal and E2-mediated activation of the promoter.

Linker-Extended Native Cyanovirin-N Facilitates PEGylation and Potently Inhibits HIV-1 by Targeting the Glycan Ligand

Cyanovirin-N (CVN) potently inhibits human immunodeficiency virus type 1 (HIV-1) infection, but both cytotoxicity and immunogenicity have hindered the translation of this protein into a viable therapeutic. A molecular docking analysis suggested that up to 12 residues were involved in the interaction of the reverse parallel CVN dimer with the oligosaccharide targets, among which Leu-1 was the most prominent hot spot residue. This finding provided a possible explanation for the lack of anti-HIV-1 activity observed with N-terminal PEGylated CVN. Therefore, linker-CVN (LCVN) was designed as a CVN derivative with a flexible and hydrophilic linker (Gly4Ser)3 at the N-terminus. The N-terminal α-amine of LCVN was PEGylated to create 10 K PEG-aldehyde (ALD)-LCVN. LCVN and 10 K PEG-ALD-LCVN retained the specificity and affinity of CVN for high mannose N-glycans. Moreover, LCVN exhibited significant anti-HIV-1 activity with attenuated cytotoxicity in the HaCaT keratinocyte cell line and MT-4 T lymphocyte cell lines. 10 K PEG-ALD-LCVN also efficiently inactivated HIV-1 with remarkably decreased cytotoxicity and pronounced cell-to-cell fusion inhibitory activity in vitro. The linker-extended CVN and the mono-PEGylated derivative were determined to be promising candidates for the development of an anti-HIV-1 agent. This derivatization approach provided a model for the PEGylation of biologic candidates without introducing point mutations.

Affinity improvement of a therapeutic antibody to methamphetamine and amphetamine through structure-based antibody engineering

Methamphetamine (METH) abuse is a worldwide threat, without any FDA approved medications. Anti-METH IgGs and single chain fragments (scFvs) have shown efficacy in preclinical studies. Here we report affinity enhancement of an anti-METH scFv for METH and its active metabolite amphetamine (AMP), through the introduction of point mutations, rationally designed to optimize the shape and hydrophobicity of the antibody binding pocket. The binding affinity was measured using saturation binding technique. The mutant scFv-S93T showed 3.1 fold enhancement in affinity for METH and 26 fold for AMP. The scFv-I37M and scFv-Y34M mutants showed enhancement of 94, and 8 fold for AMP, respectively. Structural analysis of scFv-S93T:METH revealed that the substitution of Ser residue by Thr caused the expulsion of a water molecule from the cavity, creating a more hydrophobic environment for the binding that dramatically increases the affinities for METH and AMP.

Variation in the Binding Pocket of an Inhibitor of the Bacterial Division Protein FtsZ Across Genotypes, Nucleotide States, and Species

The recent increase in antibiotic resistance in disease-causing bacteria calls for new approaches to drug target selection and drug development. Targeting proteins involved in the bacterial cell division process provides a novel mechanism of action that bypasses problems associated with the increasingly ineffective variants of older antibiotics. One potentially powerful target for inhibition is the bacterial cytoskeletal protein FtsZ, a GTPase homolog of eukaryotic tubulin. This highly conserved and essential protein acts as a scaffold to recruit other cell division machinery and can exert mechanical forces that may be responsible for constriction of the division site. Recent work on an allosteric inhibitor (PC190723) has shown promising in vitro and in vivo antimicrobial activities on Staphylococcus aureus, however the mechanism of drug action and its effect on FtsZ of other bacterial species is unclear. Here, we examine the structural environment of the FtsZ-PC190723 binding pocket using Molecular Dynamics simulations and PocketFEATURE, a statistical method that performs pairwise comparisons of small molecule binding sites based on 3D structure information. By comparing all currently existing FtsZ crystal structures to the crystalized FtsZ-PC190723 complex, we observe that species and nucleotide binding state have significant impact on the structural properties of the binding site, with a GDP-bound Staph-FtsZ displaying the highest similarity to the inhibitor complex. However, during extended Molecular Dynamics simulations, this similarity is negated upon introduction of point mutations known to nullify the inhibitor's antimicrobial affect. Together, these results give insight into the mechanism and specificity of PC190723 interaction with various bacterial FtsZ.

Modulation of Inactivation Kinetics of the Bacterial Sodium Channel Nachbac Suggests a Complex Mode of Inhibition by Isoflurane

The bacterial sodium channel NaChBac is a prokaryotic ancestor of eukaryotic voltage-gated sodium channels (Nav), which are critical for action potential generation and propagation in nervous and cardiac tissue. Like all mammalian Nav isoforms examined thus far, NaChBac is inhibited by clinically relevant concentrations of the inhaled volatile anesthetic isoflurane, and inhibition is accompanied by enhancement of slow or C-type inactivation (Ouyang et al., 2007). However, a detailed mechanistic explanation of the interplay between C-type inactivation and isoflurane inhibition is lacking. To shed light on the relationship between inactivation and inhibition, we introduced point mutations known to alter inactivation in NaChBac (G219A, G219P, G229A, and S195E), expressed the channels in HEK293FT cells, and applied isoflurane during whole-cell patch clamp recording. Preliminary data support the idea that isoflurane acts by multiple mechanisms invovling multiple sites on NaChBac. Channel mutations that enhance inactivation show greater current reduction by isoflurane, suggesting that isoflurane binds more favorably to inactivated channels. However, isoflurane exhibits both tonic and use-dependent block of the essentially non-inactivating NaChBac G219P mutant, indicating that binding also occurs in the closed/resting and open/conducting states. A detailed biophysical and pharmacological profile of these NaChBac inactivation variants together with a recent molecular dynamics simulation study showing isoflurane interacting with NaChBac at three distinct binding sites (Carnavale et al, 2013) supports multiple mechanisms of state-dependent inhibtion. Such functional data help clarify the complex pharmacological effects of volatile anesthetics on Nav channels, and contribute to better understanding of C-type inactivation in these proteins. Supported by NIH grant GM 58055 (RS and HH) and German Research Foundation fellowship 5767/1-1 (TM).

Protecting the Genome: Mechanisms Targeting Somatic Hypermutation

Somatic hypermutation (SHM) introduces point mutations into immunoglobulin (Ig) genes to support the generation of high affinity antibodies through affinity maturation. SHM is quite dangerous, having been shown to affect many non-Ig genes in germinal center and malignant B cells. Nonetheless, the reaction exhibits a strong preference for Ig loci over non-Ig loci. The targeting mechanism responsible for this preference has remained elusive despite many years of study. We and others have used the chicken Ig light chain (IgL) locus and the chicken Dt40 B cell line as a model system for the study of SHM targeting. This system has several advantages, including efficient homologous recombination and a compact IgL locus, making the search for targeting sequences easier than in the large, complex mammalian Ig loci. We and others have reported that sequences downstream of the chicken IgL constant region are cis-acting SHM targeting sequences, which we refer to as Diversification Activator (DIVAC). Dissection of the chicken DIVAC region reveals that targeting activity is dispersed among numerous sequence elements that function in an additive, synergistic, and/or redundant manner. We find that the E-box motif (CANNTG) is critical for DIVAC function, but has no detectable DIVAC activity unless embedded in the appropriate sequence context. We have recently developed a more sensitive mutation reporter assay in Dt40 cells that allows us to assess the targeting activity of short DNA sequence elements. Two "core" portions of the chicken DIVAC region, both evolutionarily conserved among avian species, have particularly strong SHM targeting activity: the IgL enhancer, and a downstream region rich in transcription factor binding sites characteristic of Ig enhancers. Remarkably, some human and mouse Ig locus enhancers have even stronger DIVAC function in our Dt40 assay than the chicken core sequences. In the mouse Igλ locus, where the canonical enhancers have relatively weak SHM targeting activity, strongly active "shadow enhancers" work synergistically with the canonical enhancers to create powerful DIVAC elements. DIVAC fragments enhance SHM in a position- and orientation-independent manner, and deletion or mutation of certain E-box, NFκB, Mef2, PU.1, or Ets family binding sites – all known to be important for the transcriptional role of Ig enhancers – impair or abolish DIVAC activity. Importantly, the increase in SHM mediated by DIVAC is not mediated by an increase in transcription. We conclude that Ig enhancers and shadow enhancers are the central components of the cis-acting DNA sequences that target SHM to Ig loci, that targeting relies on cooperation between multiple well-known transcription factor binding motifs, and that the targeting mechanism is likely to be conserved from birds to mammals. We are currently investigating whether these insights can be used to understand the widespread mistargeting of SHM that occurs in both normal and malignant B cells. Since multiple myeloma (MM) cells can be induced to express activation-induced cytidine deaminase (AID), we are also studying the targeting of AID in the MM genome and how AID might contribute to the progression of the disease. Disclosures:No relevant conflicts of interest to declare.

Protease-activated Receptor 1 (PAR1) and PAR4 Heterodimers Are Required for PAR1-enhanced Cleavage of PAR4 by α-Thrombin

Thrombin is a potent platelet agonist that activates platelets and other cells of the cardiovascular system by cleaving its G-protein-coupled receptors, protease-activated receptor 1 (PAR1), PAR4, or both. We now show that cleaving PAR1 and PAR4 with α-thrombin induces heterodimer formation. PAR1-PAR4 heterodimers were not detected when unstimulated; however, when the cells were stimulated with 10 nm α-thrombin, we were able to detect a strong interaction between PAR1 and PAR4 by bioluminescence resonance energy transfer. In contrast, activating the receptors without cleavage using PAR1 and PAR4 agonist peptides (TFLLRN and AYPGKF, respectively) did not enhance heterodimer formation. Preventing PAR1 or PAR4 cleavage with point mutations or hirugen also prevented the induction of heterodimers. To further characterize the PAR1-PAR4 interactions, we mapped the heterodimer interface by introducing point mutations in transmembrane helix 4 of PAR1 or PAR4 that prevented heterodimer formation. Finally, we show that mutations in PAR1 or PAR4 at the heterodimer interface prevented PAR1-assisted cleavage of PAR4. These data demonstrate that PAR1 and PAR4 require allosteric changes induced via receptor cleavage by α-thrombin to mediate heterodimer formation, and we have determined the PAR1-PAR4 heterodimer interface. Our findings show that PAR1 and PAR4 have dynamic interactions on the cell surface that should be taken into account when developing and characterizing PAR antagonists.

Point Mutations in the Paramyxovirus F Protein That Enhance Fusion Activity Shift the Mechanism of Complement-Mediated Virus Neutralization

Parainfluenza virus 5 (PIV5) activates and is neutralized by the alternative pathway (AP) in normal human serum (NHS) but not by heat-inactivated (HI) serum. We have tested the relationship between the fusion activity within the PIV5 F protein, the activation of complement pathways, and subsequent complement-mediated virus neutralization. Recombinant PIV5 viruses with enhanced fusion activity were generated by introducing point mutations in the F fusogenic peptide (G3A) or at a distal site near the F transmembrane domain (S443P). In contrast to wild-type (WT) PIV5, the mutant G3A and S443P viruses were neutralized by both NHS and HI serum. Unlike WT PIV5, hyperfusogenic G3A and S443P viruses were potent C4 activators, C4 was deposited on NHS-treated mutant virions, and the mutants were neutralized by factor B-depleted serum but not by C4-depleted serum. Antibodies purified from HI human serum were sufficient to neutralize both G3A and S443P viruses in vitro but were ineffective against WT PIV5. Electron microscopy data showed greater deposition of purified human antibodies on G3A and S443P virions than on WT PIV5 particles. These data indicate that single amino acid changes that enhance the fusion activity of the PIV5 F protein shift the mechanism of complement activation in the context of viral particles or on the surface of virus-infected cells, due to enhanced binding of antibodies. We present general models for the relationship between enhanced fusion activity in the paramyxovirus F protein and increased susceptibility to antibody-mediated neutralization.