Pentapeptide Scanning Mutagenesis Research Articles

Characterization of permissive and non-permissive peptide insertion sites in chloramphenicol acetyltransferase

The growing prevalence of antibiotic resistance in numerous pathogenic bacteria is a major public health concern and urgently requires the development of new therapeutic approaches. Multidrug resistant species that remain sensitive to chloramphenicol (CAM) treatment have engendered renewed interest in using this drug as a modern day antimicrobial agent. High-level resistance to CAM commonly is mediated by chloramphenicol acetyltransferase (CAT) which catalyzes the acetylation of CAM and renders the drug inactive. Of the three main types (CATI, CATII and CATIII), CATI is of broad clinical significance. Despite this importance, understanding of the catalytic mechanism of CATI largely is extrapolated from studies of CATIII. Here, pentapeptide scanning mutagenesis was used to generate a library of random insertions in CATI to gain a better understanding of structure-function relationships in the enzyme. Pentapeptide insertions in secondary structure elements which contain residues that form part of the CATI active site abolished CAM resistance in Escherichia coli. Insertions in secondary structures that have key roles in protein folding and CAM binding led to a reduction in resistance. In contrast, insertions in loop regions between the major secondary structure features exerted modest, if any, effects on CAM resistance. The analysis pinpoints regions of CATI that may serve as targets for the design of novel inhibitors that prevent the spread of CAM-resistant pathogens thereby enabling the drug to be re-deployed as a broad range antimicrobial agent. Moreover, regions of CATI that are tolerant of insertions may be suitable for the construction of bifunctional enzymes in which peptides, mini-proteins or amino acid tags are introduced at the permissive sites.

The Histidine Kinase PdhS Controls Cell Cycle Progression of the Pathogenic Alphaproteobacterium Brucella abortus

Bacterial differentiation is often associated with the asymmetric localization of regulatory proteins, such as histidine kinases. PdhS is an essential and polarly localized histidine kinase in the pathogenic alphaproteobacterium Brucella abortus. After cell division, PdhS is asymmetrically segregated between the two sibling cells, highlighting a differentiation event. However, the function(s) of PdhS in the B. abortus cell cycle remains unknown. We used an original approach, the pentapeptide scanning mutagenesis method, to generate a thermosensitive allele of pdhS. We report that a B. abortus strain carrying this pdhS allele displays growth arrest and an altered DivK-yellow fluorescent protein (YFP) polar localization at the restrictive temperature. Moreover, the production of a nonphosphorylatable PdhS protein or truncated PdhS proteins leads to dominant-negative effects by generating morphological defects consistent with the inhibition of cell division. In addition, we have used a domain mapping approach combined with yeast two-hybrid and fluorescence microscopy methods to better characterize the unusual PdhS sensory domain. We have identified a fragment of the PdhS sensory domain required for protein-protein interaction (amino acids [aa] 210 to 434), a fragment sufficient for polar localization (aa 1 to 434), and a fragment (aa 527 to 661) whose production in B. abortus correlates with the generation of cell shape alterations. The data support a model in which PdhS acts as an essential regulator of cell cycle progression in B. abortus and contribute to a better understanding of the differentiation program inherited by the two sibling cells.

Escherichia coliArgR mutants defective incer/Xer recombination, but not in DNA binding

The Escherichia coli arginine repressor (ArgR) is an L-arginine-dependent DNA-binding protein that controls the expression of the arginine biosynthetic genes and is required as an accessory factor for Xer site-specific recombination at cer and related recombination sites in plasmids. We used the technique of pentapeptide scanning mutagenesis to isolate a series of ArgR mutants that were considerably reduced in cer recombination, but were still able to repress an argA::lacZ fusion. DNA sequence analysis showed that all of the mutants mapped to the same nucleotide, resulting in a five amino acid insertion between residues 149 and 150 of ArgR, corresponding to the end of the alpha6 helix. A truncated ArgR containing a stop codon at residue 150 displayed the same phenotype as the protein with the five amino acid insertion, and both mutants displayed sequence-specific DNA-binding activity that was L-arginine dependent. These results show that the C-terminus of ArgR is more important in cer/Xer site-specific recombination than in DNA binding.

Functional mapping of PVX RNA-dependent RNA-replicase using pentapeptide scanning mutagenesis—Identification of regions essential for replication and subgenomic RNA amplification

The replicase protein of Potato virus X (PVX), type species of the genus Potexvirus, was selected to identify regions essential for replication and subgenomic RNA synthesis. Replicase amino acid (aa) sequence alignment of 16 Potexvirus species resulted in the detection of overall sequence homology of 34.4–65.4%. Two regions of consensus with a high proportion of conserved aa (1–411 and 617–1437 according to PVX) were separated by a hyper-variable linker region. Pentapeptide scanning (PS) mutagenesis in a PVX full-length clone expressing green fluorescent protein (GFP) was carried out. For 69 selected PS-mutants where insertions were spread randomly over the replicase ORF the position of the insertion was determined. The replication activity was evaluated by GFP expression from subgenomic viral RNA of PVX replicase mutants. Only one functional PS-mutant was detected in the N-terminal 430 aa, containing the conserved methyltransferase domain of the protein. In the linker region from aa 430–595, nine mutations were discovered which did not induce significant effects on the replicase replication ability. The part of the protein including helicase and polymerase domains was highly intolerant for the PS insertion as demonstrated by 24 independent more or less uniformly spread mutants.

Identification of regions critically affecting kinetics and allosteric regulation of the Escherichia coli ADP-glucose pyrophosphorylase by modeling and pentapeptide-scanning mutagenesis.

ADP-glucose pyrophosphorylase (ADP-Glc PPase) is the enzyme responsible for the regulation of bacterial glycogen synthesis. To perform a structure-function relationship study of the Escherichia coli ADP-Glc PPase enzyme, we studied the effects of pentapeptide insertions at different positions in the enzyme and analyzed the results with a homology model. We randomly inserted 15 bp in a plasmid with the ADP-Glc PPase gene. We obtained 140 modified plasmids with single insertions of which 21 were in the coding region of the enzyme. Fourteen of them generated insertions of five amino acids, whereas the other seven created a stop codon and produced truncations. Correlation of ADP-Glc PPase activity to these modifications validated the enzyme model. Six of the insertions and one truncation produced enzymes with sufficient activity for the E. coli cells to synthesize glycogen and stain in the presence of iodine vapor. These were in regions away from the substrate site, whereas the mutants that did not stain had alterations in critical areas of the protein. The enzyme with a pentapeptide insertion between Leu(102) and Pro(103) was catalytically competent but insensitive to activation. We postulate this region as critical for the allosteric regulation of the enzyme, participating in the communication between the catalytic and regulatory domains.

The Unstructured N-terminal Tail of ParG Modulates Assembly of a Quaternary Nucleoprotein Complex in Transcription Repression

ParG is the prototype of a group of small (<10 kDa) proteins involved in accurate plasmid segregation. The protein is a dimeric DNA binding factor, which consists of symmetric paired C-terminal domains that interleave into a ribbon-helix-helix fold that is crucial for the interaction with DNA, and unstructured N-terminal domains of previously unknown function. Here the ParG protein is shown to be a transcriptional repressor of the parFG genes. The protein assembles on its operator site initially as a tetramer (dimer of dimers) and, at elevated protein concentrations, as a pair of tetramers. Progressive deletion of the mobile N-terminal tails concomitantly decreased transcriptional repression by ParG and perturbed the DNA binding kinetics of the protein. The flexible tails are not necessary for ParG dimerization but instead modulate the organization of a higher order nucleoprotein complex that is crucial for proper transcriptional repression. This is achieved by transient associations between the flexible and folded domains in complex with the target DNA. Numerous ParG homologs encoded by plasmids of Gram-negative bacteria similarly are predicted to possess N-terminal disordered tails, suggesting that this is a common feature of partition operon autoregulation. The results provide new insights into the role of natively unfolded domains in protein function, the molecular mechanisms of transcription regulation, and the control of plasmid segregation.

Analysis of Human Pex19p's Domain Structure by Pentapeptide Scanning Mutagenesis

Pex19p, a primarily cytosolic protein, is essential for the biogenesis of numerous peroxisomal membrane proteins (PMPs); however, its precise function is unclear. Pex19p might function as a PMP-specific chaperone, a cycling PMP-receptor protein, a PMP membrane insertion factor, or an association/dissociation factor of membrane-associated protein complexes. Alternatively, Pex19p might act as a multifunctional peroxin and participate in a number of these activities. Here, we have employed transposon mutagenesis to generate a library of human pex19 alleles coding for Pex19p variants containing random in-frame pentapeptide insertions. A total of 87 different variants were characterized to identify functionally important regions. These studies revealed that Pex19p has a tripartite domain structure consisting of: (i) an amino-terminal domain that binds to Pex3p and is essential for docking at the peroxisome membrane; (ii) a central domain that competes with Pex5p and Pex13p for binding to Pex14p and may play a role in the assembly of PTS-receptor docking complexes; and (iii) a carboxy-terminal domain that interacts with multiple PMPs including Pex3p, Pex11pbeta, Pex12p, Pex13p, Pex16p, and Pex26p. Whether the latter interactions constitute the chaperone or transport functions (or both), remains to be determined. Finally, our observation that Pex19p contains two distinct binding sites for Pex3p suggests that the peroxin may bind PMPs in multiple places and for multiple purposes.

Mutagenesis of Escherichia coli uridine phosphorylase by random pentapeptide insertions

Escherichia coli uridine phosphorylase (UP) is encoded by the udp gene and catalyzes the reversible phosphorolysis of uridine to uracil and ribose-1-phosphate. Only few key residues involved in UP catalysis, identified by site-directed mutagenesis and selective chemical modification studies, were reported. A recent paper reporting the crystal structure at 2.0 Å resolution indicated that UP shares a high structural homology with E. coli purine nucleoside phosphorylase. This latter enzyme is better known and a number of residues in its active site have been identified. In this work, we used pentapeptide scanning mutagenesis to cause random insertions of a 5 amino acid cassette in the UP polypeptide chain. Several insertional mutants located in different regions of UP maintained or increased the enzymatic activity and provided new insights into protein structure–function relationships. Moreover, this mutagenesis approach appears to be useful for the rapid preparation of mutants that present altered enzymatic activities.

Structure-function correlations in the XerD site-specific recombinase revealed by pentapeptide scanning mutagenesis

Xer-mediated site-specific recombination contributes to the stability of circular chromosomes in bacteria by resolving plasmid multimers and chromosome dimers to monomers prior to cell division. Two related site-specific recombinases, XerC and XerD, each catalyse one pair of strand exchange during Xer recombination. In order to relate the recently determined structure of XerD to its function, the XerD protein was subjected to pentapeptide scanning mutagenesis, which leads to a variable five amino acid cassette being introduced randomly into the target protein. This has allowed identification of regions of XerD involved in specific DNA binding, in communicating with the partner recombinase, XerC, and in catalysis and its control. The C-terminal domain of XerD, comprising two-thirds of the protein, contains the catalytic active site and comprises ten α helices (αE to αN) and a β hairpin. A flexible linker connects this domain to the N-terminal domain that comprises four α helices (αA to αD). Pentapeptide insertions into αB, αD, αG, or αJ interfered with DNA binding. Helices αG and αJ comprise a pseudo helix-turn-helix DNA binding motif that may provide specificity of recombinase binding. An insertion in αL, adjacent to an active site arginine residue, led to loss of cooperative interactions between XerC and XerD and abolished recombination activity. Other insertions close to active site residues also abolished recombination activity. Proteins with an insertion in the β hairpin turn bound DNA, interacted cooperatively with XerC and had a phenotype that is consistent with the protein being defective in XerD catalysis. This β hairpin appears to be highly conserved in related proteins. Insertions at a number of dispersed locations did not impair XerD catalytic activity or DNA binding, but failed to allow XerC catalysis in vivo, indicating that several sites of interaction between XerD and XerC may be important for activation of XerC catalysis by XerD.

Insertion Mutagenesis as a Tool in the Modification of Protein Function: EXTENDED SUBSTRATE SPECIFICITY CONFERRED BY PENTAPEPTIDE INSERTIONS IN THE Ω-LOOP OF TEM-1 β-LACTAMASE

The TEM-1 beta-lactamase enzyme efficiently hydrolyzes beta-lactam antibiotics such as ampicillin but cleaves third generation cephalosporin antibiotics poorly. Variant beta-lactamases that conferred elevated levels of resistance to the cephalosporin ceftazidime were identified in a set of beta-lactamase derivatives previously generated by pentapeptide scanning mutagenesis in which a variable 5-amino acid cassette was introduced randomly in the target protein. This mutagenesis procedure was also modified to allow the direct selection of variant beta-lactamases with pentapeptide insertions that conferred extended substrate specificities. All insertions associated with enhanced resistance to ceftazidime were targetted to the 19-amino acid Omega-loop region, which forms part of the catalytic pocket of the beta-lactamase enzyme. However, pentapeptide insertions in the C- and N-terminal halves of this region had different effects on the ability of the enzyme to hydrolyze ampicillin in vivo. Larger insertions that increased the length of the Omega-loop by up to 2-fold also retained catalytic activity toward ampicillin and/or ceftazidime in vivo. In accord with previous substitution mutation studies, these results emphasize the extreme flexibility of the Omega-loop with regards the primary structure requirements for ceftazidime hydrolysis by beta-lactamase. The potential of pentapeptide scanning mutagenesis in mimicking evolution events that result from the insertion and excision of transposons in nature is discussed.

Pentapeptide scanning mutagenesis: random insertion of a variable five amino acid cassette in a target protein.

A new insertion method for probing protein functional organization was developed. The method relies on the random insertion of transposon Tn 4430 and subsequent in vitro deletion of the bulk of the transposon after which a 15 bp insertion remains within the target gene. This results in pentapeptide insertions randomly distributed in the target protein. Characterization of 23 pentapeptide insertions in TEM-1beta-lactamase demonstrated the utility of the method. The phenotypes associated with the mutated beta-lactamase proteins equated both with the sorts of local peptide structures in which the pentapeptide insertions occurred and their position in the three-dimensional structure of the enzyme.