Analysis of the disulfide linkage pattern in circulin A and B, HIV-inhibitory macrocyclic peptides

Enzymatic Properties of Recombinant Ligase Butelase-1 and Its Application in Cyclizing Food-Derived Angiotensin I-Converting Enzyme Inhibitory Peptides

Butelase-1 is an efficient ligase from Clitoria ternatea with wide applications in the food and biopharmaceutical fields. This research aimed to achieve high-efficiency expression of butelase-1 and explore its application in food-derived angiotensin I-converting enzyme (ACE) inhibitory peptides. The recombinant butelase-1 zymogen was prepared at a yield of 100 mg/L in Escherichia coli and successfully activated at pH 4.5, resulting in a 6973.8 U/L yield of activated butelase-1 with a specific activity of 348.69 U/mg and a catalytic efficiency of 9956 M^-1 s^-1. Activated butelase-1 exhibited considerable resistance to Tween-20, Triton X-100, and methanol. The "traceless" cyclization of ACE inhibitory peptides was realized using activated butelase-1, which resulted in higher stability and ACE inhibitory activity than those of the linear peptides. Our work proposed an efficient method for the preparation of butelase-1 and provided a promising model for its application in food fields.

Cyclotides: From Structure to Function

This Review explores the class of plant-derived macrocyclic peptides called cyclotides. We include an account of their discovery, characterization, and distribution in the plant kingdom as well as a detailed analysis of their sequences and structures, biosynthesis and chemical synthesis, biological functions, and applications. These macrocyclic peptides are around 30 amino acids in size and are characterized by their head-to-tail cyclic backbone and cystine knot motif, which render them to be exceptionally stable, with resistance to thermal or enzymatic degradation. Routes to their chemical synthesis have been developed over the past two decades, and this capability has facilitated a wide range of mutagenesis and structure-activity relationship studies. In turn, these studies have both led to an increased understanding of their mechanisms of action as well as facilitated a range of applications in agriculture and medicine, as ecofriendly crop protection agents, and as drug leads or scaffolds for pharmaceutical design. Our overall objective in this Review is to provide readers with a comprehensive overview of cyclotides that we hope will stimulate further work on this fascinating family of peptides.

A non-cyclic baboon θ-defensin derivative exhibiting antimicrobial activity against the phytopathogen Verticillium dahliae

θ-Defensins are the only natural cyclic proteins found in primates. They have strong antimicrobial activity related to their trisulfide ladders and macrocyclic conformation. A non-cyclic baboon θ-defensin (BTD) was synthesized by substituting valine with phenylalanine at position 17, at the C-terminal end of the BTD; this was termed "BTD-S." The antimicrobial activities of this synthetic peptide were investigated against Escherichia coli and two cotton phytopathogens: Verticillium dahliae and Fusarium oxysporum. The minimum inhibitory concentration (MIC) of BTD-S for E. coli was 10 μg/mL and for V. dahliae was 5 μg/mL, significantly lower than that for F. oxysporum (40.0 μg/mL). A time course analysis of fungal cultures indicated that the growth of V. dahliae was completely inhibited after 96 h of BTD-S treatment. Furthermore, hemolysis assays revealed that BTD-S was not toxic to mammalian cells as it could not induce lysis of sheep red blood cells even at ten times the MIC (50 μg/mL). Scanning electron microscopy and double-stained (calcofluor white and propidium iodide binding) fluorescence microscopy showed that exposure of spores of V. dahliae to BTD-S either disabled normal germination or disintegrated the spores. The size of cells exposed to BTD-S was significantly reduced compared with controls, and their number increased in a dose-dependent curve when measured by flow cytometry. These findings suggest that BTD-S has great potential to inhibit the growth of V. dahliae and can be utilized as an effective remedy to control economic losses caused by Verticillium wilt in the development of wilt-resistant cotton.

Mechanisms and modifications of naturally occurring host defense peptides for anti-HIV microbicide development

Despite advances in the treatment of HIV infection, heterosexual transmission of HIV remains high, and vaccines to prevent HIV acquisition have been unfruitful. Vaginal microbicides, on the other hand, have demonstrated considerable potential for HIV prevention, and a variety of compounds have been screened for their activity and safety as anti-HIV microbicides. Among these are the naturally occurring host defense peptides, small peptides from diverse lineages with intrinsic antiviral activity. Naturally occurring host defense peptides with anti-HIV activity are promising candidates for vaginal microbicide development. Their structural variance and accompanying mechanistic diversity provide a wide range of inhibitors whose antiviral activity can be exerted at nearly every stage of the HIV lifecycle. Additionally, peptide modification has been explored as a method for improving the anti-HIV activity of host defense peptides. Structure- and sequence-based alterations have achieved varying success in improving the potency and specificity of anti-HIV peptides. Overall, peptides have been discovered or engineered to inhibit HIV with therapeutic indices of > 1000, encouraging their advancement toward clinical trials. Here we review the naturally occurring anti-HIV host defense peptides, demonstrating their breadth of mechanistic diversity, and exploring approaches to enhance and optimize their activity in order to expedite their development as safe and effective anti-HIV vaginal microbicides.

Isolation, sequencing, and structure-activity relationships of cyclotides

Cyclotides are a topologically fascinating family of miniproteins discovered over the past decade that have expanded the diversity of plant-derived natural products. They are approximately 30 amino acids in size and occur in plants of the Violaceae, Rubiaceae, and Cucurbitaceae families. Despite their proteinaceous composition, cyclotides behave in much the same way as many nonpeptidic natural products in that they are resistant to degradation by enzymes or heat and can be extracted from plants using methanol. Their stability arises, in large part, due to their characteristic cyclic cystine knot (CCK) structural motif. Cystine knots are present in a variety of proteins of insect, plant, and animal origin, comprising a ring formed by two disulfide bonds and their connecting backbone segments that is threaded by a third disulfide bond. In cyclotides, the cystine knot is uniquely embedded within a head-to-tail cyclized peptide backbone, leading to the ultrastable CCK structural motif. Apart from the six absolutely conserved cysteine residues, the majority of amino acids in the six backbone loops of cyclotides are tolerant to variation. It has been predicted that the family might include up to 50,000 members; although, so far, sequences for only 140 have been reported. Cyclotides exhibit a variety of biological activities, including insecticidal, nematocidal, molluscicidal, antimicrobial, antibarnacle, anti-HIV, and antitumor activities. Due to their diverse activities and common structural core from which variable loops protrude, cyclotides can be thought of as combinatorial peptide templates capable of displaying a variety of amino acid sequences. They have thus attracted interest in drug design as well as in crop protection applications.

Elucidating the structure of cyclotides by partial acid hydrolysis and LC-MS/MS analysis

We describe here a rapid method to determine the cyclic structure and disulfide linkages of highly stable cyclotides via a combination of flash partial acid hydrolysis, LC-MS/MS, and computational tools. Briefly, a mixture of closely related cyclotides, kalata B1 and varv A purified from Viola yedoensis was partially hydrolyzed in 2 M HCl for 5 min by microwave-assisted hydrolysis or for 30 min in an autoclave oven (121 degrees C and 15 psi). The partially hydrolyzed peptide mixture was then subjected to LC-MS/MS analysis, with the disulfide linked-peptides fragmented by collision activated dissociation (CAD). A computer program written in-house (available for download at http://proteomics.sbs.ntu.edu.sg/cyclotide_SS ) was used for interpreting LC-MS/MS spectra and assigning the disulfide bonds. Time-point analysis of single-disulfide fragments revealed that nonrandom acid catalyzed fragmentation mostly occurred at the turns which are solvent-exposed and often contain side chain functionalized amino acids such as Asx/Glx and Ser/Thr. In particular, the most susceptible bond for acid hydrolysis in kalata B1 and varv A was found to be the highly conserved N25-G26 which is also the head-to-tail ligation site of the linear precursor proteins, indicating that formation of the three disulfide bonds might precede cyclic structure closure by N25-G26 ligation. This observation is consistent with the recent report that the N25-G26 bond formation is the last step in the cyclotide biosynthetic pathway. The process demonstrated here can potentially be a high throughput method that is generally applicable to determine disulfide bonds of other relatively low-abundance cyclotides.

Cyclotides as natural anti-HIV agents

Cyclotides are disulfide rich macrocyclic plant peptides that are defined by their unique topology in which a head-to-tail cyclized backbone is knotted by the interlocking arrangement of three disulfide bonds. This cyclic cystine knot motif gives the cyclotides exceptional resistance to thermal, chemical, or enzymatic degradation. Over 100 cyclotides have been reported and display a variety of biological activities, including a cytoprotective effect against HIV infected cells. It has been hypothesized that cyclotides from one subfamily, the Möbius subfamily, may be more appropriate than bracelet cyclotides as drug candidates given their lower toxicity to uninfected cells. Here, we report the anti-HIV and cytotoxic effects of three cyclotides, including two from the Möbius subfamily. We show that Möbius cyclotides have comparable inhibitory activity against HIV infection to bracelet cyclotides and that they are generally less cytotoxic to the target cells. To explore the structure activity relationships (SARs) of the 29 cyclotides tested so far for anti-HIV activity, we modeled the structures of the 21 cyclotides whose structures have not been previously solved. We show that within cyclotide subfamilies there is a correlation between hydrophobicity of certain loop regions and HIV inhibition. We also show that charged residues in these loops impact on the activity of the cyclotides, presumably by modulating membrane binding. In addition to providing new SAR data, this report is a mini-review that collates all cyclotide anti-HIV information reported so far and provides a resource for future studies on the therapeutic potential of cyclotides as natural anti-HIV agents.

NMR as a tool for elucidating the structures of circular and knotted proteins

Cyclotides are a recently discovered family of mini-proteins that have a head-to-tail cyclised backbone stabilized by a knotted arrangement of three disulfide bonds. They have a wide range of biological activities, including uterotonic, anti-bacterial, anti-HIV, and anti-tumour activity but their insecticidal activities suggest that their natural function is in plant defense. They are exceptionally resistant to chemical, enzymatic and thermal treatments because of their unique structural scaffold. This stability and resistance to proteolysis makes them a potentially valuable protein engineering tool at the interface of chemistry and biology: they have the structure of proteins but the stability and biophysical properties of organic molecules. In this review the role of NMR in defining the structures of cyclotides is described.

Conformation and mode of membrane interaction in cyclotides. Spatial structure of kalata B1 bound to a dodecylphosphocholine micelle

Cyclotides are a family of bioactive plant peptides that are characterized by a circular protein backbone and three conserved tightly packed disulfide bonds. The antimicrobial and hemolytic properties of cyclotides, along with the relative hydrophobicity of the peptides, point to the biological membrane as a target for cyclotides. To assess the membrane-induced conformation and orientation of cyclotides, the interaction of the Möbius cyclotide, kalata B1, from the African perennial plant Oldenlandia affinis, with dodecylphosphocholine micelles was studied using NMR spectroscopy. Under conditions where the cyclotide formed a well-defined complex with micelles, the spatial structure of kalata B1 was calculated from NOE and J couplings data, and the model for the peptide-micelle complex was built using 5- and 16-doxylstearate relaxation probes. The binding of divalent cations to the peptide-micelle complex was quantified by Mn2+ titration. The results show that the peptide binds to the micelle surface, with relatively high affinity, via two hydrophobic loops (loop 5, Trp19-Val21; and loop6, Leu27-Val29). The charged residues (Glu3 and Arg24), along with the cation-binding site (near Glu3) are segregated on the other side of the molecule and in contact with polar head groups of detergent. The spatial structure of kalata B1 is only slightly changed during incorporation into micelles and represents a distorted triple-stranded beta-sheet cross-linked by a cystine knot. Detailed structural analysis and comparison with other knottins revealed structural conservation of the two-disulfide motif in cyclic and acyclic peptides. The results thus obtained provide the first model for interaction of cyclotides with membranes and permit consideration of the cyclotides as membrane-active cationic antimicrobial peptides.

The cyclotide family of circular miniproteins: Nature's combinatorial peptide template

The cyclotides are a recently discovered family of miniproteins that contain a head-to-tail cyclized backbone and a knotted arrangement of disulfide bonds. They are approximately 30 amino acids in size and are present in high abundance in plants from the Violaceae, Rubiaceae, and Cucurbitaceae families, with individual plants containing a suite of up to 100 cyclotides. They have a diverse range of biological activities, including uterotonic, anti-HIV, antitumor, and antimicrobial activities, although their natural function is likely that of defending their host plants from pathogens and pests. This review focuses on the structural aspects of cyclotides, which may be thought of as a natural combinatorial peptide template in which a wide range of amino acids is displayed on a compact molecular core made up of the cyclic cystine knot structural motif. Cyclotides are exceptionally stable and are resistant to denaturation via thermal, chemical, or enzymatic treatments. The structural features that contribute to their remarkable stability are described in this review.

A continent of plant defense peptide diversity: cyclotides in Australian Hybanthus (Violaceae)

Cyclotides are plant-derived miniproteins that have the unusual features of a head-to-tail cyclized peptide backbone and a knotted arrangement of disulfide bonds. It had been postulated that they might be an especially large family of host defense agents, but this had not yet been tested by field data on cyclotide variation in wild plant populations. In this study, we sampled Australian Hybanthus (Violaceae) to gain an insight into the level of variation within populations, within species, and between species. A wealth of cyclotide diversity was discovered: at least 246 new cyclotides are present in the 11 species sampled, and 26 novel sequences were characterized. A new approach to the discovery of cyclotide sequences was developed based on the identification of a conserved sequence within a signal sequence in cyclotide precursors. The number of cyclotides in the Violaceae is now estimated to be >9000. Cyclotide physicochemical profiles were shown to be a useful taxonomic feature that reflected species and their morphological relationships. The novel sequences provided substantial insight into the tolerance of the cystine knot framework in cyclotides to amino acid substitutions and will facilitate protein engineering applications of this framework.

The role of the cyclic peptide backbone in the anti-HIV activity of the cyclotide kalata B1

The plant cyclotides, the largest known family of circular proteins, have tightly folded structures and a range of biological activities that lend themselves to potential pharmaceutical and agricultural applications. Based on sequence homology, they are classified into the bracelet and Möbius subfamilies. The bracelet subfamily has previously been shown to display anti-HIV activity. We show here that a member of the Möbius subfamily, kalata B1, also exhibits anti-HIV activity despite extensive sequence differences between the subfamilies. In addition, acyclic permutants of kalata B1 displayed no anti-HIV activity, suggesting that this activity is critically dependent on an intact circular backbone.

Disulfide mapping of the cyclotide kalata B1. Chemical proof of the cystic cystine knot motif

The cyclotides are a recently discovered family of plant proteins that have the fascinating structural feature of a continuous cyclic backbone and, putatively, a knotted arrangement of their three conserved disulfide bonds. We here show definite chemical proof of the I-IV, II-V, III-VI knotted disulfide connectivity of the prototypic cyclotide kalata B1. This has been achieved by a new approach for disulfide analysis, involving partial reduction and stepwise alkylation including introduction of charges and enzymatic cleavage sites by aminoethylation of cysteines. The approach overcomes the intrinsic difficulties for disulfide mapping of cyclotides, i.e. the cyclic amide backbone, lack of cleavage sites between cysteines, and a low or clustered content of basic amino acids, and allowed a direct determination of the disulfide bonds in kalata B1 using analysis by mass spectrometry. The established disulfide connectivity is unequivocally shown to be cystine knotted by a topological analysis. This is the first direct chemical determination of disulfides in native cyclotides and unambiguously confirms the unique cyclic cystine knot motif.

Mammalian antibiotic peptides

The increasing development of bacterial resistance to traditional antibiotics has reached alarming levels, thus creating a strong need to develop new antimicrobial agents. These new antibiotics should possess novel mechanisms of action and different cellular targets compared with existing antimicrobials. Recent discoveries and isolations of so-called animal antibiotics, mostly small cationic peptides, which represent a potent branch of natural immunity, offered the possibility to acquire new and effective antibiotics of this provenance. To this date, more than 500 antibiotic peptides have been distinguished and defined. Their antimicrobial properties present new opportunities for their use as antibiotics or for construction of their more effective derivatives, but much research is still required to pave the way to their practical use. This is a survey of substances forming an armamentarium of natural immunity of mammals.

Retrocyclin, an antiretroviral theta-defensin, is a lectin

Theta-defensins are circular octadecapeptides that contain an internal tridisulfide ladder. Because retrocyclin-1, an ancestral hominid theta-defensin, can protect human cells in vitro from infection by T- and M-tropic strains of HIV-1, we used surface plasmon resonance techniques to study its binding to glycoproteins and glycolipids implicated in HIV-1 entry. Retrocyclin-1 bound with high affinity to gp120 (K(d), 35.4 nM), CD4 (K(d), 31 nM), and galactosylceramide (K(d), 24.1 nM). Neither a circular form of retrocyclin without its tridisulfide ladder nor its beta-hairpin precursor with these disulfides intact bound gp120 or CD4 effectively. Retrocyclin also bound fetuin, an extensively glycosylated protein, with high affinity, but it did not bind nonglycosylated gp120 or BSA. However, retrocyclin did bind to a neoglycoprotein, BSA, with covalently attached sugar residues. Experiments with glycosidase-treated fetuin, gp120, and CD4 revealed that both O-linked and N-linked sugars were used as binding sites. In a panel of retrocyclin variants, binding to immobilized gp120 and CD4 were highly correlated to each other and to the peptide's ability to protect human PBMCs from infection by HIV-1. Although small, cysteine-rich antimicrobial peptides with lectin-like properties exist in plants, theta-defensins are the first such molecules to be identified in vertebrates. Retrocyclin's ability to recognize and bind carbohydrate-containing surface molecules is integrally related to its ability to protect cells from HIV-1 infection.

Enterocin CRL35 inhibits late stages of HSV-1 and HSV-2 replication in vitro

The replication of herpes simplex virus (HSV) type 1 and 2 in Vero cells is inhibited in the presence of enterocin CRL35 (ECRL), a bacteriocin produced by Enterococcus faecium CRL35. Attempts to resolve the mode of action of ECRL indicate that virus adsorption and penetration are not affected. Instead, a late step of virus multiplication is hindered since the addition of 100 microg/ml of ECRL at 8h post infection still causes a 90% inhibition of virus release. The effect of ECRL on HSV antigen expression was studied by immunofluorescence using a polyclonal serum and a monoclonal antibody against glycoprotein D (gamma protein). These studies indicated that ECRL impeded the second round of infection, apparently as a consequence of the inhibition of glycoprotein D expression. The replication of syncytial mutants of HSV-1 was significantly inhibited at a ECRL concentration of 25 microg/ml. Both the percentage of fused cells and the polykaryocyte size were affected. Studies on the effect of ECRL on viral protein synthesis showed that in the presence of ECRL, HSV late gamma proteins were not synthesized. From these findings, it is concluded that inhibition of HSV spreading by ECRL is due to the prevention of mainly late glycoprotein synthesis.

Disulfide folding pathways of cystine knot proteins. Tying the knot within the circular backbone of the cyclotides

The plant cyclotides are a fascinating family of circular proteins that contain a cyclic cystine knot motif. The knotted topology and cyclic nature of the cyclotides pose interesting questions about folding mechanisms and how the knotted arrangement of disulfide bonds is formed. In the current study we have examined the oxidative refolding and reductive unfolding of the prototypic cyclotide, kalata B1. A stable two-disulfide intermediate accumulated during oxidative refolding but not in reductive unfolding. Mass spectrometry and NMR spectroscopy were used to show that the intermediate contained a native-like structure with two native disulfide bonds topologically similar to the intermediate isolated for the related cystine knot protein EETI-II (Le-Nguyen, D., Heitz, A., Chiche, L., El Hajji, M., and Castro B. (1993) Protein Sci. 2, 165-174). However, the folding intermediate observed for kalata B1 is not the immediate precursor of the three-disulfide native peptide and does not accumulate in the reductive unfolding process, in contrast to the intermediate observed for EETI-II. These alternative pathways of linear and cyclic cystine knot proteins appear to be related to the constraints imposed by the cyclic backbone of kalata B1 and the different ring size of the cystine knot. The three-dimensional structure of a synthetic version of the two-disulfide intermediate of kalata B1 in which Ala residues replace the reduced Cys residues provides a structural insight into why the two-disulfide intermediate is a kinetic trap on the folding pathway.

Refined structure and metal binding site of the kalata B1 peptide

The cyclic polypeptide kalata B1 from the African plant Oldenlandia affinis DC consists of 29 amino acid residues with three disulfide linkages. In this study we used two-dimensional NMR spectroscopy to investigate the three-dimensional structure of the peptide and to determine the disulfide connectivities. Nuclear Overhauser effects (NOEs) between neighboring beta-protons of the cysteines detected at 750 MHz provided evidence for the disulfide connectivity pattern 5-13, 17-29, and 22-27. These disulfide linkages were confirmed by three-dimensional structures calculated from input constraints derived solely from NOEs without explicit disulfide connectivities. Kalata B1 is insoluble in aqueous solution above pH 3.5, but in a 50-50 water-methanol mixture, it was possible to use natural abundance two-dimensional (15)N-(1)H heteronuclear single quantum coherence spectroscopy to study the hydrophobic peptide from pH 2 to 10. The addition of methanol resulted in no significant structural changes. Although the peptide contains three prolyl residues, no evidence of multiple conformers was detected at any pH. The addition of Mn(2+) to kalata B1 resulted in selective broadening of resonances from Asn 23, Thr 24, and Glu 15; these results suggest that these three residues are involved in a specific metal binding site.

Acyclic permutants of naturally occurring cyclic proteins. Characterization of cystine knot and beta-sheet formation in the macrocyclic polypeptide kalata B1

Kalata B1 is a prototypic member of the unique cyclotide family of macrocyclic polypeptides in which the major structural features are a circular peptide backbone, a triple-stranded beta-sheet, and a cystine knot arrangement of three disulfide bonds. The cyclotides are the only naturally occurring family of circular proteins and have prompted us to explore the concept of acyclic permutation, i.e. opening the backbone of a cross-linked circular protein in topologically permuted ways. We have synthesized the complete suite of acyclic permutants of kalata B1 and examined the effect of acyclic permutation on structure and activity. Only two of six topologically distinct backbone loops are critical for folding into the native conformation, and these involve disruption of the embedded ring in the cystine knot. Surprisingly, it is possible to disrupt regions of the beta-sheet and still allow folding into native-like structure, provided the cystine knot is intact. Kalata B1 has mild hemolytic activity, but despite the overall structure of the native peptide being retained in all but two cases, none of the acyclic permutants displayed hemolytic activity. This loss of activity is not localized to one particular region and suggests that cyclization is critical for hemolytic activity.

New circulin macrocyclic polypeptides from Chassalia parvifolia

Four new macrocyclic polypeptides were isolated and identified from an extract of the tropical tree Chassalia parvifolia. Circulins C-F are 29-30 amino acid cyclic peptides in which the entire primary amino acid chain is covalently cyclized via peptide bonds. Their structures were deduced from a combination of FABMS analyses, N-terminal Edman degradation, endoproteinase digestion, and amino acid analyses. All the peptides share a high degree of sequence homology and contain six cysteine residues forming three intramolecular disulfide bridges. Circulins C-F inhibited the cytopathic effects of in vitro HIV-1 infection with EC(50) values of 50-275 nM.

Cycloviolins A-D, anti-HIV macrocyclic peptides from Leonia cymosa

Four novel anti-HIV macrocyclic peptides containing 28-31 amino acid residues, named cycloviolins A-D, have been isolated from the hitherto unstudied tropical plant Leonia cymosa. Their primary structure, including amino acid composition and sequence, was determined by a combination of MALDI-TOF and FAB MS and by enzymatic digestion of reduced derivatives, followed by Edman degradation and mass analyses. All of the cycloviolins contain six cysteines, which are present as three intramolecular disulfide bridges. Intriguingly, cycloviolins A-D showed high degrees of sequence homology to the known cyclopsychotride A and circulins A and B from the Rubiaceae family but much less homology to the varv peptides from Viola, a member of the same family (Violaceae).

A cyclic antimicrobial peptide produced in primate leukocytes by the ligation of two truncated alpha-defensins

Analysis of rhesus macaque leukocytes disclosed the presence of an 18-residue macrocyclic, tridisulfide antibiotic peptide in granules of neutrophils and monocytes. The peptide, termed rhesus theta defensin-1 (RTD-1), is microbicidal for bacteria and fungi at low micromolar concentrations. Antibacterial activity of the cyclic peptide was threefold greater than that of an open-chain analog, and the cyclic conformation was required for antimicrobial activity in the presence of 150 millimolar sodium chloride. Biosynthesis of RTD-1 involves the head-to-tail ligation of two alpha-defensin-related nonapeptides, requiring the formation of two new peptide bonds. Thus, host defense cells possess mechanisms for synthesis and granular packaging of macrocyclic antibiotic peptides that are components of the phagocyte antimicrobial armamentarium.

An unusual structural motif of antimicrobial peptides containing end-to-end macrocycle and cystine-knot disulfides

Four macrocyclic cystine-knot peptides of 29-31 residues, kalata, circulin A and B (CirA and CirB), and cyclopsychotride, have been isolated from coffee plants but have undetermined physiological functions. These macrocycles and 10 of their analogs prepared by chemical synthesis were tested against nine strains of microbes. Kalata and CirA were specific for the Gram-positive Staphylococcus aureus with a minimum inhibition concentration of approximately 0.2 microM. They were relatively ineffective against Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa. However, CirB and cyclopsychotride were active against both Gram-positive and Gram-negative bacteria. In particular, CirB showed potent activity against E. coli with a minimum inhibitory concentration of 0.41 microM. All four cyclic peptides were moderately active against two strains of fungi, Candida kefyr and Candida tropicalis, but were inactive against Candida albicans. These macrocycles are cytotoxic and lysed human red blood cell with a lethal dose 50% of 400 microM. Modifying the Arg residue in kalata with a keto aldehyde significantly reduced its activity against S. aureus whereas blocking the arg in CirA produced no significant effect. The two-disulfide variants and their scrambled disulfide isomers exhibited antimicrobial profiles and potency similar to their native peptides. However, in high-salt assays (100 mM NaCl), few of these macrocyclic peptides, natives or analogs, retained antimicrobial activity. These results show that the macrocyclic peptides possess specific and potent antimicrobial activity that is salt-dependent and that their initial interactions with the microbial surfaces may be electrostatic, an effect commonly found in defensin antimicrobial peptides. Furthermore, their end-to-end cyclic structure with a cystine-knot motif represents a molecular structure of antimicrobials and may provide a useful template for the design of novel peptide antibiotics.

Antiviral activity of enterocin CRL35 against herpesviruses

Enterocin CRL35 is an antibacterial polypeptide of 3.5 x 10(3) Da produced by Enterococcus faecium CRL35. A series of experiments are described that show the enterocin also had antiviral activity against thymidine-kinase positive (tk+) and deficient (tk-) strains of herpes simplex (HSV) type 1 and 2 in Vero and BHK-21 cells. This activity was observed at 100 microg/ml, 15-fold lower than the cytotoxic concentration. In both cell lines there was a 2 log inhibition of infectivity. The compound inhibited viral multiplication in a dose-dependent manner and had no virucidal effect. Enterocin CRL35 also inhibited the virion-associated host shutoff in infected Vero cells showing that intracellular viral multiplication was affected.

Seven novel macrocyclic polypeptides from Viola arvensis

Seven novel macrocyclic polypeptides, designated as varv peptides B-H, have been isolated from the aerial parts of Viola arvensis. Their primary structures have been elucidated by automated Edman degradation and mass spectrometry. They all consist of 29 or 30 amino acid residues, covalently cyclized via the amide backbone and by three internal disulfide bridges. Their amino acid sequences are as follows: varv peptide B, cyclo-(TCFGGTCNTPGCSCDPWPMCSRNGLPVCGE); varv peptide C, cyclo-(TCVGGTCNTPGCSCSWPVCTRNGVPICGE); varv peptide D, cyclo-(TCVGGSCNTPGCSCSWPVCTRNGLPICGE); varv peptide E, cyclo-(TCVGGTCNTPGCSCSWPVCTRNGLPICGE); varv peptide F, cyclo-(TCTLGTCYTAGCSCSWPVCTRNGVPICGE); varv peptide G, cyclo-(TCFGGTCNTPGCSCDPWPVCSRNGVPVCGE); and varv peptide H, cyclo-(TCFGGTCNTPGCSCETWPVCSRNGLPVCGE). The varv peptides B-H exhibited high degrees of homology with the hitherto known macrocyclic peptides varv peptide A, kalata B1, violapeptide I, circulins A and B, and cyclopsychotride A.

Solution structure by NMR of circulin A: a macrocyclic knotted peptide having anti-HIV activity

The three-dimensional solution structure of circulin A, a 30 residue polypeptide from the African plant Chassalia parvifolia, has been determined using two-dimensional 1H-NMR spectroscopy. Circulin A was originally identified based upon its inhibition of the cytopathic effects and replication of the human immunodeficiency virus. Structural restraints consisting of 369 interproton distances inferred from nuclear Overhauser effects, and 21 backbone dihedral and nine chi1 angle restraints from spin-spin coupling constants were used as input for simulated annealing calculations and energy minimisation in the program X-PLOR. The final set of 12 structures had mean pairwise rms differences over the whole molecule of 0.91 A for the backbone atom, and 1.68 A for all heavy atoms. For the well-defined region encompassing residues 2-12 and 18-27, the corresponding values were 0.71 and 1.66 A, respectively. Circulin A adopts a compact structure consisting of beta-turns and a distorted segment of triple-stranded beta-sheet. Fluorescence spectroscopy provided additional evidence for a solvent-exposed Trp residue. The molecule is stabilised by three disulfide bonds, two of which form an embedded loop completed by the backbone fragments connecting the cysteine residues. A third disulfide bond threads through the centre of this loop to form a "cystine-knot" motif. This motif is present in a range of other biologically active proteins, including omega-contoxin GVIA and Cucurbita maxima trypsin inhibitor. Circulin A belongs to a novel class of macrocyclic peptides which have been isolated from plants in the Rubiaceae family. The global fold of circulin A is similar to kalata B1, the only member of this class for which a structure has previously been determined.

The structure of synenkephalin (pro-enkephalin 1-73) is dictated by three disulfide bridges

Mass spectrometry of fragments produced by limited proteolytic digestion of pro-enkephalin was used to locate the disulfide bridges in synenkephalin (pro-enkephalin 1-73), a domain which contains sorting information for targeting the pro-neuropeptide to the granules of the regulated secretory pathway in neuroendocrine cells. Mass spectrometric analysis was optimized by using chemicals that gave low interference with the ionization and desorption processes, and computer software which simplified the identification of all possible disulfide-linked peptide fragments. Three disulfide bridges between Cys2-Cys24, Cys6-Cys28, and Cys9-Cys41 were identified. Protein conformational prediction of synenkephalin1-42 shows beta-turns which facilitate the formation of these disulfide bonds.