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Random mutagenesis of the gene encoding a viral ligand for multiple cell entry receptors to obtain viral mutants altered for receptor usage.

ABSTRACT: Herpes simplex virus type 1 (HSV-1) can enter cells expressing any one of multiple entry receptors, including the herpesvirus entry mediator (HVEM), nectin-1, and sites in heparan sulfate generated by specific 3-O-sulfotransferases. The viral ligand for these receptors is glycoprotein D (gD). To define structural requirements for functional interactions of gD with its receptors and to obtain viral mutants altered for receptor usage, we generated a library of HSV-1 mutants with random mutations in the gD gene. Viral isolates selected on a monkey cell line (Vero) were screened for the loss of ability to infect cells expressing each of the HSV-1 receptors. The 10 HSV-1 mutants obtained had 12 mutations in gD, affecting 11 amino acids. All mutations reduced or abrogated viral entry through HVEM and 3-O-sulfated heparan sulfate, indicating that similar features of gD are critical for functional interactions with both these receptors. None of the mutations reduced viral entry through nectin-1, whereas a subset of the mutations conferred ability to use nectin-2 as an entry receptor. These and other results show that features of gD, including conformation of the N terminus, critical for functional interactions with HVEM/3-O-sulfated heparan sulfate, differ from those critical for interactions with nectin-1.

SUBMITTER: Yoon M

PROVIDER: S-EPMC535387 | biostudies-literature | 2004 Dec

REPOSITORIES: biostudies-literature

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Random mutagenesis of the gene encoding a viral ligand for multiple cell entry receptors to obtain viral mutants altered for receptor usage.

Yoon Miri M Spear Patricia G PG

Proceedings of the National Academy of Sciences of the United States of America 20041122 49

Herpes simplex virus type 1 (HSV-1) can enter cells expressing any one of multiple entry receptors, including the herpesvirus entry mediator (HVEM), nectin-1, and sites in heparan sulfate generated by specific 3-O-sulfotransferases. The viral ligand for these receptors is glycoprotein D (gD). To define structural requirements for functional interactions of gD with its receptors and to obtain viral mutants altered for receptor usage, we generated a library of HSV-1 mutants with random mutations i ...[more]

PMID: 15557552

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Project description:BackgroundDuring transcription, the nontranscribed DNA strand becomes single-stranded DNA (ssDNA), which can form secondary structures. Unpaired bases in the ssDNA are less protected from mutagens and hence experience more mutations than do paired bases. These mutations are called transcription-associated mutations. Transcription-associated mutagenesis is increased under stress and depends on the DNA sequence. Therefore, selection might significantly influence protein-coding sequences in terms of the transcription-associated mutability per transcription event under stress to improve the survival of Escherichia coli.Methodology/principal findingsThe mutability index (MI) was developed by Wright et al. to estimate the relative transcription-associated mutability of bases per transcription event. Using the most stable fold of each ssDNA that have an average length n, MI was defined as (the number of folds in which the base is unpaired)/nx(highest -DeltaG of all n folds in which the base is unpaired), where DeltaG is the free energy. The MI values show a significant correlation with mutation data under stress but not with spontaneous mutations in E. coli. Protein sequence diversity is preferred under stress but not under favorable conditions. Therefore, we evaluated the selection pressure on MI in terms of the protein sequence diversity for all the protein-coding sequences in E. coli. The distributions of the MI values were lower at bases that could be substituted with each of the other three bases without affecting the amino acid sequence than at bases that could not be so substituted. Start codons had lower distributions of MI values than did nonstart codons.Conclusions/significanceOur results suggest that the majority of protein-coding sequences have evolved to promote protein sequence diversity and to reduce gene knockout under stress. Consequently, transcription-associated mutagenesis increases protein sequence diversity more effectively than does random mutagenesis under stress. Nonrandom transcription-associated mutagenesis under stress should improve the survival of E. coli.

Project description:BackgroundProtein glycosylation is of fundamental importance in many biological systems. The discovery of N-glycosylation in bacteria and the functional expression of the N-oligosaccharyltransferase PglB of Campylobacter jejuni in Escherichia coli enabled the production of engineered glycoproteins and the study of the underlying molecular mechanisms. A particularly promising application for protein glycosylation in recombinant bacteria is the production of potent conjugate vaccines where polysaccharide antigens of pathogenic bacteria are covalently bound to immunogenic carrier proteins.ResultsIn this study capsular polysaccharides of the clinically relevant pathogen Staphylococcus aureus serotype 5 (CP5) were expressed in Escherichia coli and linked in vivo to a detoxified version of Pseudomonas aeruginosa exotoxin (EPA). We investigated which amino acids of the periplasmic domain of PglB are crucial for the glycosylation reaction using a newly established 96-well screening system enabling the relative quantification of glycoproteins by enzyme-linked immunosorbent assay. A random mutant library was generated by error-prone PCR and screened for inactivating amino acid substitutions. In addition to 15 inactive variants with amino acid changes within the previously known, strictly conserved WWDYG motif of N-oligosaccharyltransferases, 8 inactivating mutations mapped to a flexible loop in close vicinity of the amide nitrogen atom of the acceptor asparagine as revealed in the crystal structure of the homologous enzyme C. lari PglB. The importance of the conserved loop residue H479 for glycosylation was confirmed by site directed mutagenesis, while a change to alanine of the adjacent, non-conserved L480 had no effect. In addition, we investigated functional requirements in the so-called MIV motif of bacterial N-oligosaccharyltransferases. Amino acid residues I571 and V575, which had been postulated to interact with the acceptor peptide, were subjected to cassette saturation mutagenesis. With the exception of I571C only hydrophobic residues were found in active variants. Variant I571V performed equally well as the wild type, cysteine at the same position reduced glycoprotein yield slightly, while a change to phenylalanine reduced activity by a factor of three.ConclusionsThis study provides novel structure-function relationships for the periplasmic domain of the Campylobacter jejuni N-oligosaccharyltransferase PglB and describes procedures for generating and screening oligosaccharyltransferase mutant libraries in an engineered E. coli system.

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Random mutagenesis of the gene encoding a viral ligand for multiple cell entry receptors to obtain viral mutants altered for receptor usage.

Publications

Random mutagenesis of the gene encoding a viral ligand for multiple cell entry receptors to obtain viral mutants altered for receptor usage.

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