Mutational analysis of tyrosine-191 in the catalysis of Cephalosporium acremonium isopenicillin N synthase

Isopenicillin N synthase (IPNS) is a key enzyme responsible for the catalytic conversion of delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-valine (ACV) to isopenicillin N in the beta-lactam antibiotic biosynthetic pathway. The Aspergillus nidulans IPNS crystal structure implicated amino acid residues tyrosine-189, arginine-279, and serine-281 in the substrate-binding of the valine carboxylate portion of ACV via hydrogen bonds. In previous reports, we provided mutational evidence for the critical involvement of the corresponding arginine-281 and serine-283, which constitute a conserved R-X-S motif, for the catalysis of Cephalosporium acremonium IPNS (cIPNS). In this study, we report the site-directed mutagenesis of the corresponding tyrosine-191 in cIPNS to four amino acids from different amino acid groups, namely, phenylalanine, serine, histidine, and aspartate. The mutants Y191F, Y191H, and Y191R respectively yielded specific activities at levels of 3, 8.6, and 18.8% relative to the wild-type when enzyme bioassays were performed using purified protein fractions. These results were surprising, as previous mutational analyses involving arginine-281 and serine-283 resulted in non-measurable specific activities, thus suggesting that tyrosine-191 is important but not critical for the activity of cIPNS due to its involvement in ACV binding. Hence, it is likely that tyrosine-191 is the least critical of the three residues involved in binding the ACV valine carboxylate moiety.

Antigen-presenting cells recruited by Brugia malayi induce Th2 differentiation of naïve CD4(+) T cells

A key feature of nematode infection is a bias towards a type 2 immune response. To investigate the role that antigen-presenting cells (APC) may play in promoting this bias, we used adherent peritoneal exudate cells (PEC) recruited in response to the filarial nematode Brugia malayi, to stimulate naïve T cells from pigeon cytochrome c (PCC)-specific TCR transgenic (PCC-tg) mice. Although the proliferation of PCC-tg T cells was inhibited by parasite- induced PEC during primary stimulation, they proliferated normally upon secondary stimulation and were not rendered anergic. However, PCC-tg T cells primed by suppressive APC differentiated into IL-4-producing Th2 cells upon secondary stimulation instead of IFN-gamma-producing Th1 cells, as has been previously described. Studies with carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled cells indicated that Th2 differentiation was associated with the inhibition of (or failure to stimulate) IFN-gamma production during primary stimulation. Interestingly, blocking antibodies against TGF-beta (but not IL-10) restored the differentiation of IFN-gamma-producing Th1 cells. Identical results with CFSE-labeled cells were obtained using purified IL-4-dependent F4/80(+) macrophages. These data indicate that T cells exposed to parasite-induced alternatively activated macrophages are driven towards Th2 differentiation. This may be an important factor in the Th2 bias that accompanies nematode infection.

PCR cloning, heterologous expression, and characterization of isopenicillin N synthase from Streptomyces lipmanii NRRL 3584

A key step which involves the cyclization of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine to the bicyclic ring structure of isopenicillin N in the penicillin and cephalosporin biosynthetic pathway, is catalyzed by isopenicillin N synthase (IPNS). In this study, an IPNS gene from Streptomyces lipmanii NRRL 3584 (slIPNS) was cloned via PCR-based homology cloning, sequenced and expressed in Escherichia coli. Soluble slIPNS was overexpressed up to 21% of total soluble protein, and verified to be functionally active when in an IPNS enzymatic assay. Sequence comparison of the slIPNS gene obtained (excluding the consensus primer sequences) with another cloned IPNS from S. lipmanii 16884.3, revealed one three-nucleotide deletion and three closely-spaced single nucleotide deletions. Furthermore, this paper also reports the first instance of the usage of PCR as an alternative and rapid strategy for IPNS cloning using consensus primers.

PCR cloning, heterologous expression, and characterization of isopenicillin N synthase fromStreptomyces lipmaniiNRRL 3584

A key step which involves the cyclization of δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine to the bicyclic ring structure of isopenicillin N in the penicillin and cephalosporin biosynthetic pathway, is catalyzed by isopenicillin N synthase (IPNS). In this study, an IPNS gene from Streptomyces lipmanii NRRL 3584 (slIPNS) was cloned via PCR-based homology cloning, sequenced and expressed in Escherichia coli. Soluble slIPNS was overexpressed up to 21% of total soluble protein, and verified to be functionally active when in an IPNS enzymatic assay. Sequence comparison of the slIPNS gene obtained (excluding the consensus primer sequences) with another cloned IPNS from S. lipmanii 16884.3, revealed one three-nucleotide deletion and three closely-spaced single nucleotide deletions. Futhermore, this paper also reports the first instance of the usage of PCR as an alternative and rapid strategy for IPNS cloning using consensus primers. Key words: isopenicillin N synthase, β-lactam antibiotics, secondary metabolism, consensus primers.

Site-directed mutagenesis of arginine-89 supports the role of its guanidino side-chain in substrate binding by Cephalosporium acremonium isopenicillin N synthase

Isopenicillin N synthase (IPNS) catalyses a key step in the penicillin and cephalosporin biosynthetic pathway which involves the oxidative cyclisation of the acyclic peptide delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV) to isopenicillin N. Based on crystallographic evidence from the Aspergillus nidulans IPNS crystal structure complexed with the substrate ACV (Roach et al. (1997) Nature 387, 827-830), we were able to provide mutational evidence for the critical involvement of the conserved R-X-S motif in ACV binding in IPNS. The crystal structure further implicated arginine-87 in the binding of the aminoadipyl portion of ACV. Thus, in this study, the site-directed mutagenesis of the corresponding arginine-89 in Cephalosporium acremonium IPNS (cIPNS) was performed to ascertain its role in cIPNS. Alteration of arginine-89 to five amino acids from different amino acid groups, namely lysine, serine, alanine, aspartate and leucine, was performed and no activity was detected in all the mutants obtained when enzyme bioassays were performed. Furthermore, the solubility of the mutants was considerably lower than the wild-type cIPNS after expression at 37 degrees C, but could be recovered when the expression temperature was lowered to 25 degrees C. This suggests that arginine-89 could be critical for the activity of cIPNS due to its involvement in ACV binding and the solubility of wild-type enzyme.

Cytotoxic T lymphocytes can induce a condemned state and synchronous post-mitotic apoptosis of daughter target cells

We have used time-lapse video microscopy to study cytotoxic T lymphocyte (CTL)-mediated apoptosis of LDb fibroblast target cells at different phases of the cell cycle. When aphidicolin-synchronized target cells were exposed to the CTL clone F5, apoptosis occurred with similar morphology during G1, S/G2 and M phase, showing that apoptosis and mitosis are not mutually exclusive cellular events. Interestingly, following normal mitosis of target cells that had been previously contacted by CTL, pairs of daughter cells would occasionally undergo apoptosis within minutes of each other. Such synchronous post-mitotic apoptosis was also observed when using mitotically unsynchronized target cells, and also when using d11S T cell hybridomas as alternative Fas- (CD95-) based effector cells, even if these effectors were physically washed away after an initial period of co-incubation with the target cells. Our observations show that cytotoxic cells can induce a condemned state in pre-mitotic target cells, which can be inherited by both daughter cells, leading to their synchronous apoptosis after mitosis.

Glutamine-230 influences enzyme solubility but not catalysis in Streptomyces clavuligerus isopenicillin N synthase

The conversion of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine to isopenicillin N is dependent upon the catalytic action of isopenicillin N synthase (IPNS), an important enzyme in the penicillin and cephalosporin biosynthetic pathway. Recent catalytic investigations on the conserved glutamine-230 in the bacterial Streptomyces jumonjinensis IPNS and the corresponding glutamine-234 in the fungal Cephalosporium acremonium IPNS showed contrasting results whereby the former was suggested to be essential for IPNS activity whereas the latter was found not to be so. In order to unravel these conflicting results, we report the site-directed mutagenesis investigation on the corresponding glutamine-230 in a third IPNS isozyme, which is the bacterial Streptomyces clavuligerus IPNS (scIPNS). IPNS enzymatic assays showed that catalytic activity of the mutant Q230L scIPNS was reduced but not eliminated. Moreover, the solubility of the mutant enzyme was also markedly reduced. Hence, we can conclude that glutamine-230 in scIPNS is not essential for catalysis and correspondingly in all IPNS.

Analysis of glutamines in catalysis in Cephalosporium acremonium isopenicillin N synthase by site-directed mutagenesis

Isopenicillin N synthase (IPNS), an important enzyme in the beta-lactam antibiotic biosynthetic pathway, is responsible for the catalytic conversion of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine to isopenicillin N. Three catalytic ligands essential for IPNS activity have already been determined. Based on an Aspergillus nidulans IPNS crystal structure, the probable involvement of a fourth amino acid as a catalytic ligand was previously revealed. To continue the search for the fourth catalytic ligand, we report investigations on whether or not glutamines play a role in the catalytic action of Cephalosporium acremonium IPNS (cIPNS). Three glutamine residues were targeted for modification based on the previous revelation of one (Q337) via crystal structure coordinates, the conservation of one (Q234) in isozyme alignment and the proximity of one (Q227) to the catalytic centre. Analysis of the biotransformed mutant enzymes showed retention of activity, thereby rejecting the involvement of a possible glutamine as a catalytic ligand in cIPNS catalysis.

Mutational evidence for the role of serine-283 in Cephalosporium acremonium isopenicillin N synthase

Creation of isopenicillin N from delta-(L-alpha-aminodipyl)-L-cysteinyl-D-valine (ACV) in the penicillin and cephalosporin biosynthetic pathway is catalysed by isopenicillin N synthase (IPNS), a non-heme iron-containing dioxygenase. A tripeptide R-X-S motif which consists of arginine-281 and serine-283 (Cephalosporium acremonium IPNS numbering) was found to be conserved in IPNS and other related proteins. These two amino acids mentioned were proposed to have a role in ACV substrate binding by the recent Aspergillus nidulans IPNS crystal structure. Using site-directed mutagenesis arginine-281 in C. acremonium IPNS (cIPNS) was earlier found to be essential for catalysis by our group. Similarly, serine-283 in cIPNS was also altered by site-directed mutagenesis to determine its role in cIPNS. No measurable activity was detected from the resultant mutant using enzyme bioassays. It is most likely that the eliminatin of the mutant's substrate-binding capability similar to that of arginine-281 lead to the abolishment of the catalytic reaction. This highlights the importance of the R-X-S motif in the functionality of cIPNS.

Catalytic activity in Cephalosporium acremonium isopenicillin N synthase does not involve glutamine-234

The catalytic activity of isopenicillin N synthase (IPNS), a crucial enzyme which converts delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine to isopenicillin N in the beta-lactam antibiotic biosynthetic pathway, is known to be dependent upon the ligation of two histidines and an aspartate to the iron active centre. Recent studies have ruled out the suggested requirement of the penultimate glutamine, Q330 and Q328 in Aspergillus nidulans and Streptomyces jumonjinensis IPNS respectively, for catalysis. As a counter proposal, glutamine-230 from S. jumonjinensis IPNS was presented to be crucial for activity. However, we report differing results from the site-directed mutagenesis of the corresponding glutamine-234 in Cephalosporium acremonium IPNS. Based on IPNS enzymatic assays, we conclude that glutamine-234 is not essential for catalysis in cIPNS. Furthermore, we advocate the use of soluble proteins over solubilized proteins especially for studies which involve enzymatic catalysis.

Functional analysis of a conserved aspartate D218 in Cephalosporium acremonium isopenicillin N synthase

Isopenicillin N synthase (IPNS) is instrumental in the catalytic conversion of a tripeptide precursor delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine to a bioactive intermediate isopenicillin N in the beta-lactam antibiotic biosynthetic pathway. It has recently been shown that this reaction is dependent on a conserved aspartate, D214, in a bacterial Streptomyces jumonjinensis IPNS. Thus, this study was carried out to provide the experimental evidence for the involvement of a similarly conserved aspartate residue, D218, in a fungal Cephalosporium acremonium IPNS (cIPNS). Initially, alteration of the aspartate residue to generate the mutant D218L cIPNS protein was achieved by site-directed mutagenesis. Subsequent enzyme assays indicated that the catalytic property of the mutant protein was lost, attesting to the need for the corresponding conserved aspartate to maintain IPNS functionality. It is also evident from the observed results that site-directed mutagenesis of this particular aspartate residue in cIPNS can affect its solubility. It is therefore important to take these potential changes into consideration when site-directed mutant proteins are analysed for catalytic function.