Monoclonal antibodies to the multienzyme enniatin synthetase. Production and use in structural studies

Nonribosomal cyclic peptides: specific markers of fungal infections

Some cyclic peptides and depsipeptides are synthesized in microorganisms by large multienzymes called nonribosomal peptide synthetases. The structures of peptide products originating in this way are complex and diverse and are microorganism-specific. This work proposes the use of fungal cyclic peptides and depsipeptides as extremely specific markers of fungal infections. Since a reliable molecular tool for diagnosing fungal infections at an early stage is still missing, we present mass spectrometry as a new, modern, broadband (with respect to fungal strain) and specific tool for clinical mycologists. More than 40 different fungal species can be rapidly characterized according to specific families of cyclic peptides, and in some cases, a particular fungal strain can be identified on the basis of its cyclopeptide profile. This paper is also aimed at initiating discussion on the biological role of these secondary metabolites, especially of those synthesized by medically important strains. Proven cytotoxic, anti-inflammatory or immunosuppressive activities of some cyclic peptides indicate that these molecules may contribute to the synergistic array of fungal virulence factors and support microbial invasion during fungal infection. In addition to an overview on recent mass spectrometric protocols for cyclic peptide sequencing, the structures of new peptides from Paecilomyces and Pseudallescheria are presented.

A major cell wall lipopeptide of Mycobacterium avium subspecies paratuberculosis

Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne disease in cattle and other ruminants, is proposed to be at least one of the causes of Crohn disease in humans. MAP and Mycobacterium avium subspecies avium, a closely related opportunistic environmental bacterium, share 95% of their genes and exhibit homologies of more than 99% between these genes. The identification of molecules specific for MAP is essential for understanding its pathogenicity and for development of useful diagnostic tools. The application of gas chromatography, mass spectrometry, and nuclear magnetic resonance led to the structural identification of a major cell wall lipopeptide of MAP, termed Para-LP-01, defined as C20 fatty acyl-D-Phe-N-Me-L-Val-L-Ile-L-Phe-L-Ala methyl ester. Variations of this lipopeptide with different fatty acyl moieties (C16 fatty acyl through C17, C18, C19, C21 to C22) were also identified. Besides the specificity of this lipopeptide for MAP, the presence of an N-Me-L-valine represents the first reported N-methylated amino acid within an immunogenic lipopeptide of mycobacteria. Sera from animals with Johne disease, but not sera from uninfected cattle, reacted with this lipopeptide, indicating potential biological importance.

Mutational analysis of the N-methyltransferase domain of the multifunctional enzyme enniatin synthetase

N-Methylcyclopeptides like cyclosporins and enniatins are synthesized by multifunctional enzymes representing hybrid systems of peptide synthetases and S-adenosyl-l-methionine (AdoMet)-dependent N-methyltransferases. The latter constitute a new family of N-methyltransferases sharing high homology within procaryotes and eucaryotes. Here we describe the mutational analysis of the N-methyltransferase domain of enniatin synthetase from Fusarium scirpi to gain insight into the assembly of the AdoMet-binding site. The role of four conserved motifs (I, (2085)VLEIGTGSGMIL; II/Y, (2105)SYVGLDPS; IV, (2152)DLVVFNSVVQYFTPPEYL; and V, (2194)ATNGHFLAARA) in cofactor binding as measured by photolabeling was studied. Deletion of the first 21 N-terminal amino acid residues of the N-methyltransferase domain did not affect AdoMet binding. Further shortening close to motif I resulted in loss of binding activity. Truncation of 38 amino acids from the C terminus and also internal deletions containing motif V led to complete loss of AdoMet-binding activity. Point mutations converting the conserved Tyr(223) (corresponding to position 2106 in enniatin synthetase) in motif II/Y (close to motif I) into Val, Ala, and Ser, respectively, strongly diminished AdoMet binding, whereas conversion of this residue to Phe restored AdoMet-binding activity to approximately 70%, indicating that Tyr(223) is important for AdoMet binding and that the aromatic Tyr(223) may be crucial for AdoMet binding in N-methylpeptide synthetases.

How do peptide synthetases generate structural diversity?

Many low-molecular-weight peptides of microbial origin are synthesized nonribosomally on large multifunctional proteins, termed peptide synthetases. These enzymes contain repeated building blocks in which several defined domains catalyze specific reactions of peptide synthesis. The order of these domains within the enzyme determines the sequence and structure of the peptide product.

Arrangement of catalytic sites in the multifunctional enzyme enniatin synthetase

Enniatin synthetase is an N-methyl peptide synthetase comprising 3131 amino acids. Catalytic sites of the 347-kDa multifunctional enzyme were mapped by N-terminal sequencing of substrate affinity-labelled enzyme fragments formed by proteolysis, and functional studies of purified enniatin synthetase fragments. An N-terminal 200-kDa fragment containing the cofactor 4'-phosphopantetheine was able to activate D-hydroxyisovaleric acid (D-HOiVl) as a thioester. The N-termini of two [14C]HOiVl-labelled enzyme fragments could be assigned to amino acid position 429 within the N-terminal conserved enniatin synthetase portion named EA. This portion of about 600 amino acids shares high similarity to microbial peptide synthetase regions. A 68-kDa L-[14C]Val-labelled enniatin synthetase fragment was localized at amino acid position 2294 within the second C-terminal conserved protein portion EB. Additionally enniatin synthetase was labelled with isovaleryl-L-[14C]Val, an analogue of the D-hydroxyisovaleryl-L-Val intermediate in enniatin biosynthesis. The N-terminus of a 30-kDa isovaleryl-L-[14C]Val-labelled enniatin synthetase fragment was mapped in a C-terminal segment of the protein portion EA. The same N-terminal sequence was obtained from a 60-kDa enniatin synthetase fragment modified with [3H]beta Ala, a constituent of the cofactor 4'-phosphopantetheine. This indicates the presence of the cofactor in this protein fragment. Localization of the methyltransferase function of enniatin synthetase in an amino acid portion integrated into region EB was achieved by N-terminal sequencing of a photolabelled S-[methyl-14C]adenosyl methionine 45-kDa fragment and the identification of a photolabelled peptide Asn-Leu-Asn-Pro-Gly-Leu-Asn-Ser-Tyr.

Peptides

If we include beta-lactam antibiotics on the grounds that they have the same biosynthetic origin, peptides remain commercially the most important group of pharmaceuticals. However, our increasing knowledge of the genetic and enzymic background to biosynthesis, and of the regulation of metabolite production, will eventually bring a more unified approach to bioactive compounds. Mixing of structural types will become important, and we will be able to use our knowledge of biosynthetic genes and their regulatory networks. We will also benefit from an appreciation of the modular organization of catalytic functions, substrate transfer mechanisms and signalling between interacting enzymes. Since all of this is, in fact, the basis for enzymic synthesis of complex natural products in vivo, the exploitation of living cells requires mastery of a formidable network of cellular controls and compartments. For the present we are able to see fascinating connections emerging between genes in a variety of reaction sequences, not only in biosynthetic but also in degradative pathways. Peptide synthetases show surprising similarities to acylcoenzyme A synthetases, which are key enzymes in forming polyketides as well as in generating the CoA-derivatives that serve as substrates in degradative pathways. 4'-Phosphopantetheine, the functional half of CoA, plays a key role as the intrinsic transfer cofactor in various multienzyme systems. The comparatively small catalogue of reactions modifying natural products, notably epimerization, methylation, hydroxylation, decarboxylation (of peptides) and reduction/dehydration (of polyketides) can be found within or amongst biosynthetic proteins, generally as modules and organized in a specified order. The biochemist is coming close to the synthetic chemist's recipes, and may soon be recruiting proteins to carry them out.

Molecular characterization of the enniatin synthetase gene encoding a multifunctional enzyme catalysing N-methyldepsipeptide formation in Fusarium scirpi

The gene encoding the multifunctional enzyme enniatin synthetase from Fusarium scirpi (esyn1) was isolated and characterized by transcriptional mapping and expression studies in Escherichia coli. This is the first example of a gene encoding an N-methyl peptide synthetase. The nucleotide sequence revealed an open reading frame of 9393 bp encoding a protein of 3131 amino acids (M(r) 346,900). Two domains designated EA and EB within the protein were identified which share similarity to each other and to microbial peptide synthetase domains. In contrast to the N-terminal domain EA, the carboxyl terminal domain EB is interrupted by a 434-amino-acid portion which shows local similarity to a motif apparently conserved within adenine and cytosine RNA and DNA methyltransferases and therefore seems to harbour the N-methyl-transferase function of the multienzyme.

Nonribosomal biosynthesis of peptide antibiotics

Peptide antibiotics are known to contain non-protein amino acids, D-amino acids, hydroxy acids, and other unusual constituents. In addition they may be modified by N-methylation and cyclization reactions. Their biosynthetic origin has been connected in many cases to an enzymatic system referred to as the 'thiotemplate multienzymic mechanism'. This mechanism includes the activation of the constituent residues as adenylates on the enzymic template, the acylation of specific template thiol groups, epimerization or N-methylation at this thioester stage, and polymerization in the sequence directed by the multienzymic structure with the aid of 4'-phosphopantetheine as a cofactor, including possible cyclization or terminal modification reactions. The reaction sequences leading to gramicidin S, tyrocidine, cyclosporine, bacitracin, polymyxin, actinomycin, enniatin, beauvericin, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine and linear gramicidin are discussed. The structures of the multienzymes, their genetic organization, the biological functions of these peptides and results on related systems are discussed.

Constitutive Expression of Enniatin Synthetase during Fermentative Growth of Fusarium scirpi

The production of enniatins by Fusarium scirpi during fermentative growth in submerged cultures was measured. The fungus produced the antibiotic during mycelial growth, but not during the stationary phase of cultivation. By contrast, enniatin synthetase, the enzyme responsible for enniatin synthesis, was present during growth, during the stationary phase, and even in spores. Similarly, the enniatin synthetase mRNA was present at every stage of the cultivation of the fungus. Therefore, this multifunctional peptide synthetase is a constitutive enzyme, the expression of which is not regulated by any specific mechanism. The findings stand in contrast to the common assumption that production of secondary metabolites underlies regulatory control, leading to separation of the trophophase and the idiophase.