Synthesis of PAF, an antifungal protein from P. chrysogenum, by native chemical ligation: native disulfide pattern and fold obtained upon oxidative refolding

Confirmation of the Disulfide Connectivity and Strategies for Chemical Synthesis of the Four-Disulfide-Bond-Stabilized Aspergillus giganteus Antifungal Protein, AFP

Emerging fungal infections require new, more efficient antifungal agents and therapies. AFP, a protein from Aspergillus giganteus with four disulfide bonds, is a promising candidate because it selectively inhibits the growth of filamentous fungi. In this work, the reduced form of AFP was prepared using native chemical ligation. The native protein was synthesized via oxidative folding with uniform protection for cysteine thiols. AFP's biological activity depends heavily on the pattern of natural disulfide bonds. Enzymatic digestion and MS analysis provide proof for interlocking disulfide topology (abcdabcd) that was previously assumed. With this knowledge, a semi-orthogonal thiol protection method was designed. By following this strategy, out of a possible 105, only 6 disulfide isomers formed and 1 of them proved to be identical with the native protein. This approach allows the synthesis of analogs for examining structure-activity relationships and, thus, preparing AFP variants with higher antifungal activity.

The Epichloë festucae Antifungal Protein Efe-AfpA Protects Creeping Bentgrass (Agrostis stolonifera) from the Plant Pathogen Clarireedia jacksonii, the Causal Agent of Dollar Spot Disease

Dollar spot disease, caused by the fungal pathogen Clarireedia jacksonii, is a major problem in many turfgrass species, particularly creeping bentgrass (Agrostis stolonifera). It is well-established that strong creeping red fescue (Festuca rubra subsp. rubra) exhibits good dollar spot resistance when infected by the fungal endophyte Epichloë festucae. This endophyte-mediated disease resistance is unique to the fine fescues and has not been observed in other grass species infected with other Epichloë spp. The mechanism underlying the unique endophyte-mediated disease resistance in strong creeping red fescue has not yet been established. We pursued the possibility that it may be due to the presence of an abundant secreted antifungal protein produced by E. festucae. Here, we compare the activity of the antifungal protein expressed in Escherichia coli, Pichia pastoris, and Penicillium chrysogenum. Active protein was recovered from all systems, with the best activity being from Pe. chrysogenum. In greenhouse assays, topical application of the purified antifungal protein to creeping bentgrass and endophyte-free strong creeping red fescue protected the plants from developing severe symptoms caused by C. jacksonii. These results support the hypothesis that Efe-AfpA is a major contributor to the dollar spot resistance observed with E. festucae-infected strong creeping red fescue in the field, and that this protein could be developed as an alternative or complement to fungicides for the management of this disease on turfgrasses.

Cysteine-Rich Antifungal Proteins from Filamentous Fungi are Promising Bioactive Natural Compounds in Anti-Candida Therapy

The emerging number of life-threatening invasive fungal infections caused by drug-resistant Candida strains urges the need for the development and application of fundamentally new and safe antifungal strategies in the clinical treatment. Recent studies demonstrated that the extracellular cysteine-rich and cationic antifungal proteins (crAFPs) originating from filamentous fungi, and de novo designed synthetic peptide derivatives of these crAFPs provide a feasible basis for this approach. This mini-review focuses on the global challenges of the anti-Canidia therapy and on the crAFPs as potential drug candidates to overcome existing problems. The advantages and limitations in the use of crAFPs and peptide derivatives compared to those of conventional antifungal drugs will also be critically discussed.

Structure and Synthesis of Antifungal Disulfide β-Strand Proteins from Filamentous Fungi

The discovery and understanding of the mode of action of new antimicrobial agents is extremely urgent, since fungal infections cause 1.5 million deaths annually. Antifungal peptides and proteins represent a significant group of compounds that are able to kill pathogenic fungi. Based on phylogenetic analyses the ascomycetous, cysteine-rich antifungal proteins can be divided into three different groups: Penicillium chrysogenum antifungal protein (PAF), Neosartorya fischeri antifungal protein 2 (NFAP2) and “bubble-proteins” (BP) produced, for example, by P. brevicompactum. They all dominantly have β-strand secondary structures that are stabilized by several disulfide bonds. The PAF group (AFP antifungal protein from Aspergillus giganteus, PAF and PAFB from P. chrysogenum,Neosartorya fischeri antifungal protein (NFAP)) is the best characterized with their common β-barrel tertiary structure. These proteins and variants can efficiently be obtained either from fungi production or by recombinant expression. However, chemical synthesis may be a complementary aid for preparing unusual modifications, e.g., the incorporation of non-coded amino acids, fluorophores, or even unnatural disulfide bonds. Synthetic variants up to ca. 6–7 kDa can also be put to good use for corroborating structure determination. A short overview of the structural peculiarities of antifungal β-strand disulfide bridged proteins will be given. Here, we describe the structural propensities of some known antifungal proteins from filamentous fungi which can also be prepared with modern synthetic chemistry methods.

Calcium binding of the antifungal protein PAF: Structure, dynamics and function aspects by NMR and MD simulations

Calcium ions (Ca²⁺) play an important role in the toxicity of the cysteine-rich and cationic antifungal protein PAF from Penicillium chrysogenum: high extracellular Ca²⁺ levels reduce the toxicity of PAF in the sensitive model fungus Neurospora crassa in a concentration dependent way. However, little is known about the mechanistic details of the Ca²⁺ ion impact and the Ca²⁺ binding capabilities of PAF outside the fungal cell, which might be the reason for the activity loss. Using nuclear magnetic resonance (NMR), isothermal titration calorimetry and molecular dynamics (MD) simulations we demonstrated that PAF weakly, but specifically binds Ca²⁺ ions. MD simulations of PAF predicted one major Ca²⁺ binding site at the C-terminus involving Asp53 and Asp55, while Asp19 was considered as putative Ca²⁺ binding site. The exchange of Asp19 to serine had little impact on the Ca²⁺ binding, however caused the loss of antifungal activity, as was shown in our recent study. Now we replaced the C-terminal aspartates and expressed the serine variant PAF^D53S/D55S. The specific Ca²⁺ binding affinity of PAF^D53S/D55S decreased significantly if compared to PAF, whereas the antifungal activity was retained. To understand more details of Ca²⁺ interactions, we investigated the NMR and MD structure/dynamics of the free and Ca²⁺-bound PAF and PAF^D53S/D55S. Though we found some differences between these protein variants and the Ca²⁺ complexes, these effects cannot explain the observed Ca²⁺ influence. In conclusion, PAF binds Ca²⁺ ions selectively at the C-terminus; however, this Ca²⁺ binding does not seem to play a direct role in the previously documented modulation of the antifungal activity of PAF.

Three Antifungal Proteins From Penicillium expansum: Different Patterns of Production and Antifungal Activity

Antifungal proteins of fungal origin (AFPs) are small, secreted, cationic, and cysteine-rich proteins. Filamentous fungi encode a wide repertoire of AFPs belonging to different phylogenetic classes, which offer a great potential to develop new antifungals for the control of pathogenic fungi. The fungus Penicillium expansum is one of the few reported to encode three AFPs each belonging to a different phylogenetic class (A, B, and C). In this work, the production of the putative AFPs from P. expansum was evaluated, but only the representative of class A, PeAfpA, was identified in culture supernatants of the native fungus. The biotechnological production of PeAfpB and PeAfpC was achieved in Penicillium chrysogenum with the P. chrysogenum-based expression cassette, which had been proved to work efficiently for the production of other related AFPs in filamentous fungi. Western blot analyses confirmed that P. expansum only produces PeAfpA naturally, whereas PeAfpB and PeAfpC could not be detected. From the three AFPs from P. expansum, PeAfpA showed the highest antifungal activity against all fungi tested, including plant and human pathogens. P. expansum was also sensitive to its self-AFPs PeAfpA and PeAfpB. PeAfpB showed moderate antifungal activity against filamentous fungi, whereas no activity could be attributed to PeAfpC at the conditions tested. Importantly, none of the PeAFPs showed hemolytic activity. Finally, PeAfpA was demonstrated to efficiently protect against fungal infections caused by Botrytis cinerea in tomato leaves and Penicillium digitatum in oranges. The strong antifungal potency of PeAfpA, together with the lack of cytotoxicity, and significant in vivo protection against phytopathogenic fungi that cause postharvest decay and plant diseases, make PeAfpA a promising alternative compound for application in agriculture, but also in medicine or food preservation.

Identification and Rationalization of Kinetic Folding Intermediates for a Low-Density Lipoprotein Receptor Ligand-Binding Module

Many mutations that cause familial hypercholesterolemia localize to ligand-binding domain 5 (LA5) of the low-density lipoprotein receptor, motivating investigation of the folding and misfolding of this small, disulfide-rich, calcium-binding domain. LA5 folding is known to involve non-native disulfide isomers, yet these folding intermediates have not been structurally characterized. To provide insight into these intermediates, we used nuclear magnetic resonance (NMR) to follow LA5 folding in real time. We demonstrate that misfolded or partially folded disulfide intermediates are indistinguishable from the unfolded state when focusing on the backbone NMR signals, which provide information on the formation of only the final, native state. However, ¹³C labeling of cysteine side chains differentiated transient intermediates from the unfolded and native states and reported on disulfide bond formation in real time. The cysteine pairings in a dominant intermediate were identified using ¹³C-edited three-dimensional NMR, and coarse-grained molecular dynamics simulations were used to investigate the preference of this disulfide set over other non-native arrangements. The transient population of LA5 species with particular non-native cysteine connectitivies during folding supports the conclusion that cysteine pairing is not random and that there is a bias toward certain structural ensembles during the folding process, even prior to the binding of calcium.

Anti-Candidal Activity and Functional Mapping of Recombinant and Synthetic Neosartorya fischeri Antifungal Protein 2 (NFAP2)

The increasing number of life-threatening Candida infections caused by antifungal drug-resistant strains urges the development of new therapeutic strategies. The small, cysteine-rich, and cationic Neosartorya fischeri antifungal protein 2 (NFAP2) effectively inhibits the growth of Candida spp. Limiting factors of its future application, are the low-yield production by the native producer, unavailable information about potential clinical application, and the unsolved relationship between the structure and function. In the present study we adopted a Penicillium chrysogenum-based expression system for bulk production of recombinant NFAP2. Furthermore, solid-phase peptide synthesis and native chemical ligation were applied to produce synthetic NFAP2. The average yield of recombinant and synthetic NFAP2 was 40- and 16-times higher than in the native producer, respectively. Both proteins were correctly processed, folded, and proved to be heat-stable. They showed the same minimal inhibitory concentrations as the native NFAP2 against clinically relevant Candida spp. Minimal inhibitory concentrations were higher in RPMI 1640 mimicking the human inner fluid than in a low ionic strength medium. The recombinant NFAP2 interacted synergistically with fluconazole, the first-line Candida therapeutic agent and significantly decreased its effective in vitro concentrations in RPMI 1640. Functional mapping with synthetic peptide fragments of NFAP2 revealed that not the evolutionary conserved antimicrobial γ-core motif, but the mid-N-terminal part of the protein influences the antifungal activity that does not depend on the primary structure of this region. Preliminary nucleic magnetic resonance measurements signed that the produced recombinant NFAP2 is suitable for further structural investigations.

The Epichloë festucae antifungal protein has activity against the plant pathogen Sclerotinia homoeocarpa, the causal agent of dollar spot disease

Epichloë spp. are naturally occurring fungal endophytic symbionts of many cool-season grasses. Infection by the fungal endophytes often confers biotic and abiotic stress tolerance to their hosts. Endophyte-mediated disease resistance is well-established in the fine fescue grass Festuca rubra subsp. rubra (strong creeping red fescue) infected with E. festucae. Resistance to fungal pathogens is not an established effect of endophyte infection of other grass species, and may therefore be unique to the fine fescues. The underlying mechanism of the disease resistance is unknown. E. festucae produces a secreted antifungal protein that is highly expressed in the infected plant tissues and may therefore be involved in the disease resistance. Most Epichloë spp. do not have a gene for a similar antifungal protein. Here we report the characterization of the E. festucae antifungal protein, designated Efe-AfpA. The antifungal protein partially purified from the apoplastic proteins of endophyte-infected plant tissue and the recombinant protein expressed in the yeast Pichia pastoris was found to have activity against the important plant pathogen Sclerotinia homoeocarpa. Efe-AfpA may therefore be a component of the disease resistance seen in endophyte-infected strong creeping red fescue.

Structural determinants of Neosartorya fischeri antifungal protein (NFAP) for folding, stability and antifungal activity

The recent global challenges to prevent and treat fungal infections strongly demand for the development of new antifungal strategies. The structurally very similar cysteine-rich antifungal proteins from ascomycetes provide a feasible basis for designing new antifungal molecules. The main structural elements responsible for folding, stability and antifungal activity are not fully understood, although this is an essential prerequisite for rational protein design. In this study, we used the Neosartorya fischeri antifungal protein (NFAP) to investigate the role of the disulphide bridges, the hydrophobic core, and the N-terminal amino acids in the formation of a highly stable, folded, and antifungal active protein. NFAP and its mutants carrying cysteine deletion (NFAPΔC), hydrophobic core deletion (NFAPΔh), and N-terminal amino acids exchanges (NFAPΔN) were produced in Pichia pastoris. The recombinant NFAP showed the same features in structure, folding, stability and activity as the native protein. The data acquired with mass spectrometry, structural analyses and antifungal activity assays of NFAP and its mutants proved the importance of the disulphide bonding, the hydrophobic core and the correct N-terminus for folding, stability and full antifungal function. Our findings provide further support to the comprehensive understanding of the structure-function relationship in members of this protein group.

Mapping and Identification of Antifungal Peptides in the Putative Antifungal Protein AfpB from the Filamentous Fungus Penicillium digitatum

Antifungal proteins (AFPs) from Ascomycetes are small cysteine-rich proteins that are abundantly secreted and show antifungal activity against non-producer fungi. A gene coding for a class B AFP (AfpB) was previously identified in the genome of the plant pathogen Penicillium digitatum. However, previous attempts to detect the AfpB protein were not successful despite the high expression of the corresponding afpB gene. In this work, the structure of the putative AfpB was modeled. Based on this model, four synthetic cysteine-containing peptides, PAF109, PAF112, PAF118, and PAF119, were designed and their antimicrobial activity was tested and characterized. PAF109 that corresponds to the γ-core motif present in defensin-like antimicrobial proteins did not show antimicrobial activity. On the contrary, PAF112 and PAF118, which are cationic peptides derived from two surface-exposed loops in AfpB, showed moderate antifungal activity against P. digitatum and other filamentous fungi. It was also confirmed that cyclization through a disulfide bridge prevented peptide degradation. PAF116, which is a peptide analogous to PAF112 but derived from the Penicillium chrysogenum antifungal protein PAF, showed activity against P. digitatum similar to PAF112, but was less active than the native PAF protein. The two AfpB-derived antifungal peptides PAF112 and PAF118 showed positive synergistic interaction when combined against P. digitatum. Furthermore, the synthetic hexapeptide PAF26 previously described in our laboratory also exhibited synergistic interaction with the peptides PAF112, PAF118, and PAF116, as well as with the PAF protein. This study is an important contribution to the mapping of antifungal motifs within the AfpB and other AFPs, and opens up new strategies for the rational design and application of antifungal peptides and proteins.

D19S Mutation of the Cationic, Cysteine-Rich Protein PAF: Novel Insights into Its Structural Dynamics, Thermal Unfolding and Antifungal Function

The cysteine-rich, cationic, antifungal protein PAF is abundantly secreted into the culture supernatant of the filamentous Ascomycete Penicillium chrysogenum. The five β-strands of PAF form a compact β-barrel that is stabilized by three disulphide bonds. The folding of PAF allows the formation of four surface-exposed loops and distinct charged motifs on the protein surface that might regulate the interaction of PAF with the sensitive target fungus. The growth inhibitory activity of this highly stable protein against opportunistic fungal pathogens provides great potential in antifungal drug research. To understand its mode of action, we started to investigate the surface-exposed loops of PAF and replaced one aspartic acid at position 19 in loop 2 that is potentially involved in PAF active or binding site, with a serine (Asp19 to Ser19). We analysed the overall effects, such as unfolding, electrostatic changes, sporadic conformers and antifungal activity when substituting this specific amino acid to the fairly indifferent amino acid serine. Structural analyses revealed that the overall 3D solution structure is virtually identical with that of PAF. However, PAFD19S showed slightly increased dynamics and significant differences in the surface charge distribution. Thermal unfolding identified PAFD19S to be rather a two-state folder in contrast to the three-state folder PAF. Functional comparison of PAFD19S and PAF revealed that the exchange at residue 19 caused a dramatic loss of antifungal activity: the binding and internalization of PAFD19S by target cells was reduced and the protein failed to trigger an intracellular Ca2+ response, all of which are closely linked to the antifungal toxicity of PAF. We conclude that the negatively charged residue Asp19 in loop 2 is essential for full function of the cationic protein PAF.

NFAP2, a novel cysteine-rich anti-yeast protein from Neosartorya fischeri NRRL 181: isolation and characterization

The increasing incidence of fungal infections and damages due to drug-resistant fungi urges the development of new antifungal strategies. The cysteine-rich antifungal proteins from filamentous ascomycetes provide a feasible base for protection against molds due to their potent antifungal activity on them. In contrast to this, they show no or weak activity on yeasts, hence their applicability against this group of fungi is questionable. In the present study a 5.6 kDa anti-yeast protein (NFAP2) is isolated, identified and characterized from the ferment broth of Neosartorya fischeri NRRL 181. Based on a phylogenetic analysis, NFAP2 and its putative homologs represent a new group of ascomycetous cysteine-rich antifungal proteins. NFAP2 proved to be highly effective against tested yeasts involving clinically relevant Candida species. NFAP2 did not cause metabolic inactivity and apoptosis induction, but its plasma membrane disruption ability was observed on Saccharomyces cerevisiae. The antifungal activity was maintained after high temperature treatment presumably due to the in silico predicted stable tertiary structure. The disulfide bond-stabilized, heat-resistant folded structure of NFAP2 was experimentally proved. After further investigations of antifungal mechanism, structure and toxicity, NFAP2 could be applicable as a potent antifungal agent against yeasts.

A Penicillium chrysogenum-based expression system for the production of small, cysteine-rich antifungal proteins for structural and functional analyses

Background

Small, cysteine-rich and cationic antifungal proteins (APs) from filamentous ascomycetes, such as NFAP from Neosartorya fischeri and PAF from Penicillium chrysogenum, are promising candidates for novel drug development. A prerequisite for their application is a detailed knowledge about their structure–function relation and mode of action, which would allow protein modelling to enhance their toxicity and specificity. Technologies for structure analyses, such as electronic circular dichroism (ECD) or NMR spectroscopy, require highly purified samples and in case of NMR milligrams of uniformly ¹⁵N-/¹³C-isotope labelled protein. To meet these requirements, we developed a P. chrysogenum-based expression system that ensures sufficient amount and optimal purity of APs for structural and functional analyses.

Results

The APs PAF, PAF mutants and NFAP were expressed in a P. chrysogenum ∆paf mutant strain that served as perfect microbial expression factory. This strain lacks the paf-gene coding for the endogenous antifungal PAF and is resistant towards several APs from other ascomycetes. The expression of the recombinant proteins was under the regulation of the strong paf promoter, and the presence of a paf-specific pre-pro sequence warranted the secretion of processed proteins into the supernatant. The use of defined minimal medium allowed a single-step purification of the recombinant proteins. The expression system could be extended to express PAF in the related fungus Penicillium digitatum, which does not produce detectable amounts of APs, demonstrating the versatility of the approach. The molecular masses, folded structures and antifungal activity of the recombinant proteins were analysed by ESI–MS, ECD and NMR spectroscopy and growth inhibition assays.

Conclusion

This study demonstrates the implementation of a paf promoter driven expression cassettes for the production of cysteine-rich, cationic, APs in different Penicillium species. The system is a perfect tool for the generation of correctly folded proteins with high quality for structure–function analyses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0586-4) contains supplementary material, which is available to authorized users.

Real-time broadband proton-homodecoupled CLIP/CLAP-HSQC for automated measurement of heteronuclear one-bond coupling constants

A new method is proposed that allows broadband homonuclear decoupled CLIP/CLAP-HSQC NMR spectra to be acquired at virtually no extra cost in measurement time.

Real-time pure shift ¹⁵N HSQC of proteins: a real improvement in resolution and sensitivity

Spectral resolution in proton NMR spectroscopy is reduced by the splitting of resonances into multiplets due to the effect of homonuclear scalar couplings. Although these effects are often hidden in protein NMR spectroscopy by low digital resolution and routine apodization, behind the scenes homonuclear scalar couplings increase spectral overcrowding. The possibilities for biomolecular NMR offered by new pure shift NMR methods are illustrated here. Both resolution and sensitivity are improved, without any increase in experiment time. In these experiments, free induction decays are collected in short bursts of data acquisition, with durations short on the timescale of J-evolution, interspersed with suitable refocusing elements. The net effect is real-time (t 2) broadband homodecoupling, suppressing the multiplet structure caused by proton-proton interactions. The key feature of the refocusing elements is that they discriminate between the resonances of active (observed) and passive (coupling partner) spins. This can be achieved either by using band-selective refocusing or by the BIRD element, in both cases accompanied by a nonselective 180° proton pulse. The latter method selects the active spins based on their one-bond heteronuclear J-coupling to (15)N, while the former selects a region of the (1)H spectrum. Several novel pure shift experiments are presented, and the improvements in resolution and sensitivity they provide are evaluated for representative samples: the N-terminal domain of PGK; ubiquitin; and two mutants of the small antifungal protein PAF. These new experiments, delivering improved sensitivity and resolution, have the potential to replace the current standard HSQC experiments.

Protein kinase A signaling and calcium ions are major players in PAF mediated toxicity against Aspergillus niger

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Aspergillus niger is highly susceptible to the antifungal protein PAF.

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Ca²⁺ and cAMP/PKA signalling are interconnected in response to PAF.

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PAF toxicity requires the activation of the cAMP/PkaA signaling cascade.

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PAF evokes a sustained increase in the [Ca²⁺]_c resting level.

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Only a functional PAF protein interferes with the fungal Ca²⁺ homeostasis.

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PAF represents a promising molecule to develop new antifungal strategies.

The Penicillium chrysogenum antifungal protein PAF is toxic against potentially pathogenic Ascomycetes. We used the highly sensitive aequorin-expressing model Aspergillus niger to identify a defined change in cytoplasmic free Ca²⁺ dynamics in response to PAF. This Ca²⁺ signature depended on an intact positively charged lysine-rich PAF motif. By combining Ca²⁺ measurements in A. niger mutants with deregulated cAMP/protein kinase A (PKA) signaling, we proved the interconnection of Ca²⁺ perturbation and cAMP/PKA signaling in the mechanistic function of PAF. A deep understanding of the mode of action of PAF is an invaluable prerequisite for its future application as new antifungal drug.

"Invisible" conformers of an antifungal disulfide protein revealed by constrained cold and heat unfolding, CEST-NMR experiments, and molecular dynamics calculations

Transition between conformational states in proteins is being recognized as a possible key factor of function. In support of this, hidden dynamic NMR structures were detected in several cases up to populations of a few percent. Here, we show by two- and three-state analysis of thermal unfolding, that the population of hidden states may weight 20-40 % at 298 K in a disulfide-rich protein. In addition, sensitive (15) N-CEST NMR experiments identified a low populated (0.15 %) state that was in slow exchange with the folded PAF protein. Remarkably, other techniques failed to identify the rest of the NMR "dark matter". Comparison of the temperature dependence of chemical shifts from experiments and molecular dynamics calculations suggests that hidden conformers of PAF differ in the loop and terminal regions and are most similar in the evolutionary conserved core. Our observations point to the existence of a complex conformational landscape with multiple conformational states in dynamic equilibrium, with diverse exchange rates presumably responsible for the completely hidden nature of a considerable fraction.

Antifungal plant defensins: mechanisms of action and production

Plant defensins are small, cysteine-rich peptides that possess biological activity towards a broad range of organisms. Their activity is primarily directed against fungi, but bactericidal and insecticidal actions have also been reported. The mode of action of various antifungal plant defensins has been studied extensively during the last decades and several of their fungal targets have been identified to date. This review summarizes the mechanism of action of well-characterized antifungal plant defensins, including RsAFP2, MsDef1, MtDef4, NaD1 and Psd1, and points out the variety by which antifungal plant defensins affect microbial cell viability. Furthermore, this review summarizes production routes for plant defensins, either via heterologous expression or chemical synthesis. As plant defensins are generally considered non-toxic for plant and mammalian cells, they are regarded as attractive candidates for further development into novel antimicrobial agents.

Accurate determination of one-bond heteronuclear coupling constants with "pure shift" broadband proton-decoupled CLIP/CLAP-HSQC experiments

We report broadband proton-decoupled CLIP/CLAP-HSQC experiments for the accurate determination of one-bond heteronuclear couplings and, by extension, for the reliable measurement of small residual dipolar coupling constants. The combination of an isotope-selective BIRD((d)) filter module with a non-selective (1)H inversion pulse is employed to refocus proton-proton coupling evolution prior to the acquisition of brief chunks of free induction decay that are subsequently assembled to reconstruct the fully-decoupled signal evolution. As a result, the cross-peaks obtained are split only by the heteronuclear one-bond coupling along the F2 dimension, allowing coupling constants to be extracted by measuring simple frequency differences between singlet maxima. The proton decoupling scheme presented has also been utilized in standard HSQC experiments, resulting in a fully-decoupled pure shift correlation map with significantly improved resolution.

NMR investigation of disulfide containing peptides and proteins

Peptides and proteins with disulfide bonds are abundant in all kingdoms and play essential role in many biological events. Because small disulfide-rich peptides (proteins) are usually difficult to crystallize, nuclear magnetic resonance (NMR) is by far one of the most powerful techniques for the determination of their solution structure. Besides the “static” three-dimensional structure, NMR has unique opportunities to acquire additional information about molecular dynamics and folding at atomic resolution. Nowadays it is becoming increasingly evident, that “excited”, “disordered” or “fuzzy” protein states may exhibit biological function and disulfide proteins are also promising targets for such studies. In this short two-three years overview those disulfide peptides and proteins were cited from the literature that were studied by NMR. Though we may have missed some, their structural diversity and complexity as well as their wide repertoire of biological functions is impressive. We emphasised especially antimicrobial peptides and peptide based toxins in addition to some biologically important other structures. Besides the general NMR methods we reviewed some contemporary techniques suitable for disclosing the peculiar properties of disulfide bonds. Interesting dynamics and folding studies of disulfide proteins were also mentioned. It is important to disclose the essential structure, dynamics, function aspects of disulfide proteins since this aids the design of new compounds with improved activity and reduced toxicity. Undoubtedly, NMR has the potential to accelerate the development of new disulfide peptides/proteins with pharmacological activity.

Production of a defensin-like antifungal protein NFAP from Neosartorya fischeri in Pichia pastoris and its antifungal activity against filamentous fungal isolates from human infections

Neosartorya fischeri NRRL 181 isolate secretes a defensin-like antifungal protein (NFAP) which has a remarkable antifungal effect against ascomycetous filamentous fungi. This protein is a promising antifungal agent of biotechnological value; however in spite of the available knowledge of the nature of its 5'-upstream transcriptional regulation elements, the bulk production of NFAP has not been resolved yet. In this study we carried out its heterologous expression in the yeast Pichia pastoris and investigated the growth inhibition effect exerted by the heterologous NFAP (hNFAP) on filamentous fungal isolates from human infections compared with what was caused by the native NFAP. P. pastoris KM71H transformant strain harboring the pPICZαA plasmid with the mature NFAP encoding gene produced the protein. The final yield of the hNFAP was sixfold compared to the NFAP produced by N. fischeri NRRL 181. Based on the signal dispersion of the amide region, it was proven that the hNFAP exists in folded state. The purified hNFAP effectively inhibited the growth of fungal isolates belonging to the Aspergillus and to the Fusarium genus, but all investigated zygomycetous strain proved to be insusceptible. There was no significant difference between the growth inhibition effect exerted by the native and the heterologous NFAP. These data indicated that P. pastoris KM71H can produce the NFAP in an antifungally active folded state. Our results provide a base for further research, e.g., investigation the connection between the protein structure and the antifungal activity using site directed mutagenesis.