Insight into structure-function relationship in phenol-soluble modulins using an alanine screen of the phenol-soluble modulin (PSM) α3 peptide

Structure of biofilm-forming functional amyloid PSMα1 from Staphylococcus aureus

Biofilm-protected pathogenic Staphylococcus aureus causes chronic infections that are difficult to treat. An essential building block of these biofilms are functional amyloid fibrils that assemble from phenol-soluble modulins (PSMs). PSMα1 cross-seeds other PSMs into cross-β amyloid folds and is therefore a key element in initiating biofilm formation. However, the paucity of high-resolution structures hinders efforts to prevent amyloid assembly and biofilm formation. Here, we present a 3.5 Å resolution density map of the major PSMα1 fibril form revealing a left-handed cross-β fibril composed of two C2-symmetric U-shaped protofilaments whose subunits are unusually tilted out-of-plane. Monomeric α-helical PSMα1 is extremely cytotoxic to cells, despite the moderate toxicity of the cross-β fibril. We suggest mechanistic insights into the PSM functional amyloid formation and conformation transformation on the path from monomer-to-fibril formation. Details of PSMα1 assembly and fibril polymorphism suggest how S. aureus utilizes functional amyloids to form biofilms and establish a framework for developing therapeutics against infection and antimicrobial resistance.

N-Formylation modifies membrane damage associated with PSMα3 interfacial fibrillation

The virulence of Staphylococcus aureus, a multi-drug resistant pathogen, notably depends on the expression of the phenol soluble modulins α3 (PSMα3) peptides, able to self-assemble into amyloid-like cross-α fibrils. Despite remarkable advances evidencing the crucial, yet insufficient, role of fibrils in PSMα3 cytotoxic activities towards host cells, the relationship between its molecular structures, assembly propensities, and modes of action remains an open intriguing problem. In this study, combining atomic force microscopy (AFM) imaging and infrared spectroscopy, we first demonstrated in vitro that the charge provided by the N-terminal capping of PSMα3 alters its interactions with model membranes of controlled lipid composition without compromising its fibrillation kinetics or morphology. N-formylation eventually dictates PSMα3-membrane binding via electrostatic interactions with the lipid head groups. Furthermore, PSMα3 insertion within the lipid bilayer is favoured by hydrophobic interactions with the lipid acyl chains only in the fluid phase of membranes and not in the gel-like ordered domains. Strikingly, our real-time AFM imaging emphasizes how intermediate protofibrillar entities, formed along PSMα3 self-assembly and promoted at the membrane interface, likely disrupt membrane integrity via peptide accumulation and subsequent membrane thinning in a peptide concentration and lipid-dependent manner. Overall, our multiscale and multimodal approach sheds new light on the key roles of N-formylation and intermediate self-assembling entities, rather than mature fibrils, in dictating deleterious interactions of PSMα3 with membrane lipids, likely underscoring its ultimate cellular toxicity in vivo, and in turn S. aureus pathogenesis.

Cytotoxic Staphylococcus aureus PSMα3 inhibits the aggregation of human insulin in vitro

Phenol-soluble modulins (PSMs) are extracellular short amphipathic peptides secreted by the bacteria Staphylococcus aureus (S. aureus). They play an essential role in the bacterial lifecycle, biofilm formation, and stabilisation. From the PSM family, PSMα3 has been of special interest recently due to its cytotoxicity and highly stable α-helical conformation, which also remains in its amyloid fibrils. In particular, PSMα3 fibrils were shown to be composed of self-associating "sheets" of α-helices oriented perpendicular to the fibril axis, mimicking the architecture of canonical cross-β fibrils. Therefore, they were called cross-α-fibrils. PSMα3 was synthesised and verified for identity with wild-type sequences (S. aureus). Then, using several experimental techniques, we evaluated its propensity for in vitro aggregation. According to our findings, synthetic PSMα3 (which lacks the N-terminal formyl groups found in bacteria) does not form amyloid fibrils and maintains α-helical conformation in a soluble monomeric form for several days of incubation. We also evaluated the influence of PSMα3 on human insulin fibrillation in vitro, using a variety of experimental approaches in combination with computational molecular studies. First, it was shown that PSMα3 drastically inhibits the fibrillation of human insulin. The anti-fibrillation effect of PSMα3 was concentration-dependent and required a concentration ratio of PSMα3: insulin equal to or above 1 : 100. Molecular modelling revealed that PSMα3 most likely inhibits the production of insulin primary nuclei by competing for residues involved in its dimerization.

Differential Effects of Lipid Bilayers on αPSM Peptide Functional Amyloid Formation

Phenol-soluble modulins (PSMs) are key virulence factors of S. aureus, and they comprise the structural scaffold of biofilm as they self-assemble into functional amyloids. They have been shown to interact with cell membranes as they display toxicity towards human cells through cell lysis, with αPSM3 being the most cytotoxic. In addition to causing cell lysis in mammalian cells, PSMs have also been shown to interact with bacterial cell membranes through antimicrobial effects. Here, we present a study on the effects of lipid bilayers on the aggregation mechanism of αPSM using chemical kinetics to study the effects of lipid vesicles on the aggregation kinetics and using circular dichroism (CD) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) to investigate the corresponding secondary structure of the aggregates. We found that the effects of lipid bilayers on αPSM aggregation were not homogeneous between lipid type and αPSM peptides, although none of the lipids caused changes in the dominating aggregation mechanism. In the case of αPSM3, all types of lipids slowed down aggregation to a varying degree, with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) having the most pronounced effect. For αPSM1, lipids had opposite effects, where DOPC decelerated aggregation and lipopolysaccharide (LPS) accelerated the aggregation, while 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DOPG) had no effect. For αPSM4, both DOPG and LPS accelerated the aggregation, but only at high concentration, while DOPC showed no effect. None of the lipids was capable of inducing aggregation of αPSM2. Our data reveal a complex interaction pattern between PSMs peptides and lipid bilayers that causes changes in the aggregation kinetics by affecting different kinetic parameters along with only subtle changes in morphology.

Staphylococcus aureus functional amyloids catalyze degradation of β-lactam antibiotics

Antibiotic resistance of bacteria is considered one of the most alarming developments in modern medicine. While varied pathways for bacteria acquiring antibiotic resistance have been identified, there still are open questions concerning the mechanisms underlying resistance. Here, we show that alpha phenol-soluble modulins (PSMαs), functional bacterial amyloids secreted by Staphylococcus aureus, catalyze hydrolysis of β-lactams, a prominent class of antibiotic compounds. Specifically, we show that PSMα2 and, particularly, PSMα3 catalyze hydrolysis of the amide-like bond of the four membered β-lactam ring of nitrocefin, an antibiotic β-lactam surrogate. Examination of the catalytic activities of several PSMα3 variants allowed mapping of the active sites on the amyloid fibrils' surface, specifically underscoring the key roles of the cross-α fibril organization, and the combined electrostatic and nucleophilic functions of the lysine arrays. Molecular dynamics simulations further illuminate the structural features of β-lactam association upon the fibril surface. Complementary experimental data underscore the generality of the functional amyloid-mediated catalytic phenomenon, demonstrating hydrolysis of clinically employed β-lactams by PSMα3 fibrils, and illustrating antibiotic degradation in actual S. aureus biofilms and live bacteria environments. Overall, this study unveils functional amyloids as catalytic agents inducing degradation of β-lactam antibiotics, underlying possible antibiotic resistance mechanisms associated with bacterial biofilms.

Phenol-soluble modulins form amyloids in contact with multiple surface chemistries

Functional amyloids are commonly produced by many microorganisms and their biological functions are numerous. Staphylococcus aureus can secrete a group of peptides named phenol-soluble modulins (PSMs) in their biofilm extracellular matrix. PSMs have been found inside biofilms both in their soluble form and assembled into amyloid structures. Yet, the actual biological function of these amyloids has been highly debated. Here, we assessed the ability of PSMs to form amyloids in contact with different abiotic surfaces to unravel a potential unknown bioadhesive and/or biofilm stabilization function. We combined surface plasmon resonance imaging, fluorescence aggregation kinetics, and FTIR spectroscopy in order to evaluate the PSM adsorption as well as amyloid formation properties in the presence of various surface chemistries. Overall, PSMs adsorb even on low-binding surfaces, making them highly adaptable adsorbants in the context of bioadhesion. Moreover, the PSM aggregation potential to form amyloid aggregates is not impacted by the presence of the surface chemistries tested. This versatility regarding adsorption and amyloid formation may imply a possible role of PSMs in biofilm adhesion and/or structure integrity.

The Molecular Mechanism of PSMα3 Aggregation: A New View

The emergence of a novel cross-α fibrillar structure, unlike the commonly observed sequence-independent cross-β one, of a 22-residue bacterial virulent amphipathic α-helical peptide of the phenol soluble modulin (PSM) family, PSMα3, with many deleterious effects on human life, has infused uncertainty to the paradigm of the intrinsically polymorphic, multivariate, multiphasic, and cross-sequence-cross-disease entangled protein aggregation landscape and hence on the identity of the therapeutic target. We, here, deconvolute the factors contributing to the genesis and hence the transition of lower to higher order aggregates of PSMα3 in its natural state and three noncanonical designed variants using conventional and enhanced sampling approach-based atomistic simulations. PSMα3 shows structural polymorphism with nominal α-helicity, substantial β-propensity, and dominant random-coil features, irrespective of the extent of aggregation. Moreover, the individual features of the overall amphipathicity operate alternatively depending on the extent and organization of aggregation; the dominance gradually moves from charged to hydrophobic residues with the progressive generation of higher order aggregates (dimer to oligomer to fibril) and with increasing orderedness of the self-assembled construct (oligomer vs dimer/fibril). Similarly, the contribution of interchain salt bridges decreases with increasing order of aggregation (dimer to oligomer to fibril). However, the intrachain salt bridges consistently display their role in all phases of aggregation. Such phase-independent features also include equivalent roles of electrostatic and van der Waals forces on intrachain interactions, sole contribution of van der Waals forces on interchain cross-talk, and negligible peptide-water relationship. Finally, we propose a conjugate peptide-based aggregation suppressor having a single-point proline mutation.

Effect of Bacterial Amyloid Protein Phenol-Soluble Modulin Alpha 3 on the Aggregation of Amyloid Beta Protein Associated with Alzheimer's Disease

Since the proposal of the brainstem axis theory, increasing research attention has been paid to the interactions between bacterial amyloids produced by intestinal flora and the amyloid β-protein (Aβ) related to Alzheimer's disease (AD), and it has been considered as the possible cause of AD. Therefore, phenol-soluble modulin (PSM) α3, the most virulent protein secreted by Staphylococcus aureus, has attracted much attention. In this work, the effect of PSMα3 with a unique cross-α fibril architecture on the aggregation of pathogenic Aβ40 of AD was studied by extensive biophysical characterizations. The results proposed that the PSMα3 monomer inhibited the aggregation of Aβ40 in a concentration-dependent manner and changed the aggregation pathway to form granular aggregates. However, PSMα3 oligomers promoted the generation of the β-sheet structure, thus shortening the lag phase of Aβ40 aggregation. Moreover, the higher the cross-α content of PSMα3, the stronger the effect of the promotion, indicating that the cross-α structure of PSMα3 plays a crucial role in the aggregation of Aβ40. Further molecular dynamics (MD) simulations have shown that the Met1-Gly20 region in the PSMα3 monomer can be combined with the Asp1-Ala2 and His13-Val36 regions in the Aβ40 monomer by hydrophobic and electrostatic interactions, which prevents the conformational conversion of Aβ40 from the α-helix to β-sheet structure. By contrast, PSMα3 oligomers mainly combined with the central hydrophobic core (CHC) and the C-terminal region of the Aβ40 monomer by weak H-bonding and hydrophobic interactions, which could not inhibit the transition to the β-sheet structure in the aggregation pathway. Thus, the research has unraveled molecular interactions between Aβ40 and PSMα3 of different structures and provided a deeper understanding of the complex interactions between bacterial amyloids and AD-related pathogenic Aβ.

Hydrophobic-hydrophilic Alternation: An effective Pattern to de novo Designed Antimicrobial Peptides

The antimicrobial peptide (AMP) is a class of molecules that are active against a variety of microorganisms, from bacterial and cancer cells to fungi. Most AMPs are natural products, as part of an organism's own defense system against harmful microbes. However, the growing prevalence of drug resistance has forced researchers to design more promising engineered antimicrobial agents. Inspired by the amphiphilic detergents, the hydrophobic-hydrophilic alternation pattern was considered to be a simple but effective way to de novo design AMPs. In this model, hydrophobic amino acids (leucine, isoleucine etc.) and hydrophilic amino acids (arginine, lysine etc.) were arranged in an alternating way in the peptide sequence. The majority of this type of peptides have a clear hydrophilic-hydrophobic interface, which allows the molecules to have good solubility in both water and organic solvents. When they come into contact with hydrophobic membranes, many peptides undergo a conformational transformation, facilitating themself to insert into the cellular envelope. Moreover, positive-charged peptide amphiphiles tended to have an affinity with negatively-charged membrane interfaces and further led to envelope damage and cell death. Herein, several typical design patterns have been reviewed. Though varying in amino acid sequence, they all basically follow the rule of alternating arrangement of hydrophilic and hydrophobic residues. Based on that, researchers synthesized some lead compounds with favorable antimicrobial activities and preliminarily investigated their possible mode of action. Besides membrane disruption, these AMPs are proven to kill microbes in multiple mechanisms. These results deepened our understanding of AMPs' design and provided a theoretical basis for constructing peptide candidates with better biocompatibility and therapeutic potential.

Sappanwood-derived polyphenolic antidote of amyloidal toxins achieved detoxification via inhibition/reversion of amyloidal fibrillation

The formidable virulence of methicillin-resistant staphylococcus aureus (MRSA) have thrown great challenges to biomedicine, which mainly derives from their autocrine phenol-soluble modulins (PSMs) toxins, especially the most toxic member termed phenol-soluble modulins α3 (PSMα3). PSMα3 cytotoxicity is attributed to its amyloidal fibrillation and subsequent formation of cross-α sheet fibrils. Inspired by the multiple biological activity of Sappanwood, herein, we adopted brazilin, a natural polyphenolic compound originated from Caesalpinia sappan, as a potential antidote of PSMα3 toxins, and attempted to prove that the regulation of PSMα3 fibrillation was an effective alexipharmic way for MRSA infections. In vitro results revealed that brazilin suppressed PSMα3 fibrillation and disassembled preformed amyloidal fibrils in a dose-dependent manner, in which molar ratio (brazilin: PSMα3) of efficient inhibition and disassembly were both 1:1. These desired regulations dominated by brazilin benefited from its bonding to core fibrils-forming residues of PSMα3 monomers urged by hydrogen bonding and pi-pi stacking, and such binding modes facilitated brazilin-mediated inhibition or disruption of interactions between neighboring PSMα3 monomers. In this context, these inhibited and disassembled PSMα3 assemblies could not easily insert into cell membrane and subsequent penetration, and thus alleviating the membrane disruption, cytoplasmic leakage, and reactive oxygen species (ROS) generation in normal cells. As such, brazilin dramatically decreased the cytotoxicity borne by toxic PSMα3 fibrils. In addition, in vivo experiments affirmed that brazilin relieved the toxicity of PSMα3 toxins and thus promoted the skin wound healing of mice. This study provides a new antidote of PSMα3 toxins, and also confirms the feasibility of the assembly-regulation strategy in development of antidotes against supramolecular fibrillation-dependent toxins.

Salt-Inducing Assembly Polymorphism Strategy for Cytotoxicity Differentiation of Phenol-Soluble Modulin α3 Assemblies

Phenol-soluble modulin α3 (PSMα3) can self-assemble into fibrous assemblies with a unique "cross-α" sheet structure, which serves as a key virulence factor in the infection of Staphylococcus aureus. However, the structure-cytotoxicity relationships of PSMα3 still remain elusive. Herein, we utilized the strategy of salt-inducing assembly polymorphism to controllably prepare three PSMα3 assemblies with morphological and structural distinctions, including amorphous aggregates (AAs), rigid fibrils (RFs), and oligomers/curvilinear fibrils (OCFs), which provided a convincing method to facilitate the structure-cytotoxicity investigation of PSMα3 assemblies. Our results affirmed that amyloid fibrillation was essential for the enhancement of PSMα3 cytotoxicity, which was proved based on the evidence that RFs and OCFs both triggered more obvious cytotoxicity than AAs. Furthermore, our study also demonstrated that the cytotoxicity was severely dependent on the size and structure of PSMα3 fibrils. In detail, smaller OCFs rich in α-helices exhibited stronger virulence than RFs with larger sizes and low α-helical contents. The cytotoxicity caused by such fibrils was achieved via a membrane-disrupting mechanism, in which RFs and OCFs might be prone to membrane thinning and perforation, respectively. This strategy of salt-inducing PSMα3 assembly polymorphism facilitated the comprehension of the relationship between the characteristics of PSMα3 assemblies and their cytotoxicity and was also helpful to understanding the intrinsic assembly mechanism of the PSMα3.

Phenol-soluble modulins PSMα3 and PSMβ2 form nanotubes that are cross-α amyloids

Phenol-soluble modulins (PSMs) are peptide-based virulence factors that play significant roles in the pathogenesis of staphylococcal strains in community-associated and hospital-associated infections. In addition to cytotoxicity, PSMs display the propensity to self-assemble into fibrillar species, which may be mediated through the formation of amphipathic conformations. Here, we analyze the self-assembly behavior of two PSMs, PSMα3 and PSMβ2, which are derived from peptides expressed by methicillin-resistant Staphylococcus aureus (MRSA), a significant human pathogen. In both cases, we observed the formation of a mixture of self-assembled species including twisted filaments, helical ribbons, and nanotubes, which can reversibly interconvert in vitro. Cryo–electron microscopy structural analysis of three PSM nanotubes, two derived from PSMα3 and one from PSMβ2, revealed that the assemblies displayed remarkably similar structures based on lateral association of cross-α amyloid protofilaments. The amphipathic helical conformations of PSMα3 and PSMβ2 enforced a bilayer arrangement within the protofilaments that defined the structures of the respective PSMα3 and PSMβ2 nanotubes. We demonstrate that, similar to amyloids based on cross-β protofilaments, cross-α amyloids derived from these PSMs display polymorphism, not only in terms of the global morphology (e.g., twisted filament, helical ribbon, and nanotube) but also with respect to the number of protofilaments within a given peptide assembly. These results suggest that the folding landscape of PSM derivatives may be more complex than originally anticipated and that the assemblies are able to sample a wide range of supramolecular structural space.

Phenol-Soluble Modulins From Staphylococcus aureus Biofilms Form Complexes With DNA to Drive Autoimmunity

The bacterial amyloid curli, produced by Enterobacteriales including Salmonella species and Escherichia coli, is implicated in the pathogenesis of several complex autoimmune diseases. Curli binds to extracellular DNA, and these complexes drive autoimmunity via production of anti-double-stranded DNA autoantibodies. Here, we investigated immune activation by phenol-soluble modulins (PSMs), the amyloid proteins expressed by Staphylococcus species. We confirmed the amyloid nature of PSMs expressed by S. aureus using a novel specific amyloid stain, (E,E)-1-fluoro-2,5-bis(3-hydroxycarbonyl-4-hydroxy) styrylbenzene (FSB). Direct interaction of one of the S. aureus PSMs, PSMα3, with oligonucleotides promotes fibrillization of PSM amyloids and complex formation with bacterial DNA. Finally, utilizing a mouse model with an implanted mesh-associated S. aureus biofilm, we demonstrated that exposure to S. aureus biofilms for six weeks caused anti-double-stranded DNA autoantibody production in a PSM-dependent manner. Taken together, these results highlight how the presence of PSM-DNA complexes in S. aureus biofilms can induce autoimmune responses, and suggest an explanation for how bacterial infections trigger autoimmunity.

Rapid pathogen-specific recruitment of immune effector cells in the skin by secreted toxins

Swift recruitment of phagocytic leucocytes is critical in preventing infection when bacteria breach through the protective layers of the skin. According to canonical models, this occurs via an indirect process that is initiated by contact of bacteria with resident skin cells and which is independent of the pathogenic potential of the invader. Here we describe a more rapid mechanism of leucocyte recruitment to the site of intrusion of the important skin pathogen Staphylococcus aureus that is based on direct recognition of specific bacterial toxins, the phenol-soluble modulins (PSMs), by circulating leucocytes. We used a combination of intravital imaging, ear infection and skin abscess models, and in vitro gene expression studies to demonstrate that this early recruitment was dependent on the transcription factor EGR1 and contributed to the prevention of infection. Our findings refine the classical notion of the non-specific and resident cell-dependent character of the innate immune response to bacterial infection by demonstrating a pathogen-specific high-alert mechanism involving direct recruitment of immune effector cells by secreted bacterial products.

finDr: A web server for in silico D-peptide ligand identification

In the rapidly expanding field of peptide therapeutics, the short in vivo half-life of peptides represents a considerable limitation for drug action. D-peptides, consisting entirely of the dextrorotatory enantiomers of naturally occurring levorotatory amino acids (AAs), do not suffer from these shortcomings as they are intrinsically resistant to proteolytic degradation, resulting in a favourable pharmacokinetic profile. To experimentally identify D-peptide binders to interesting therapeutic targets, so-called mirror-image phage display is typically performed, whereby the target is synthesized in D-form and L-peptide binders are screened as in conventional phage display. This technique is extremely powerful, but it requires the synthesis of the target in D-form, which is challenging for large proteins. Here we present finDr, a novel web server for the computational identification and optimization of D-peptide ligands to any protein structure (https://findr.biologie.uni-freiburg.de/). finDr performs molecular docking to virtually screen a library of helical 12-mer peptides extracted from the RCSB Protein Data Bank (PDB) for their ability to bind to the target. In a separate, heuristic approach to search the chemical space of 12-mer peptides, finDr executes a customizable evolutionary algorithm (EA) for the de novo identification or optimization of D-peptide ligands. As a proof of principle, we demonstrate the validity of our approach to predict optimal binders to the pharmacologically relevant target phenol soluble modulin alpha 3 (PSMα3), a toxin of methicillin-resistant Staphylococcus aureus (MRSA). We validate the predictions using in vitro binding assays, supporting the success of this approach. Compared to conventional methods, finDr provides a low cost and easy-to-use alternative for the identification of D-peptide ligands against protein targets of choice without size limitation. We believe finDr will facilitate D-peptide discovery with implications in biotechnology and biomedicine.

Staphylococcus aureus N-terminus formylated δ-toxin tends to form amyloid fibrils, while the deformylated δ-toxin tends to form functional oligomer complexes

The community-associated Methicillin-resistant Staphylococcus aureus strain (CA-MRSA) is highly virulent and has become a major focus of public health professionals. Phenol-soluble modulins (PSM) are key factors in its increased virulence. δ-Toxin belongs to PSM family and has copious secretion in many S. aureus strains. In addition, δ-toxin exists in the S. aureus culture supernatant as both N-terminus formylated δ-toxin (fδ-toxin) and deformylated δ-toxin (dfδ-toxin) groups. Although δ-toxin has been studied for more than 70 years, its functions remain unclear. We isolated and purified PSMs from the supernatant of S. aureus MW2, and found fibrils and oligomers aggregates by Size Exclusion Chromatography. After analyzing PSM aggregates and using peptide simulations, we found that the difference in the monomer structure of fδ-toxin and dfδ-toxin might ultimately lead to differences in the aggregation ability: fδ-toxin and dfδ-toxin tend to form fibrils and oligomers respectively. Of note, we found that fδ-toxin fibrils enhanced the stability of biofilms, while dfδ-toxin oligomers promoted their dispersal. Additionally, oligomeric dfδ-toxin combined with PSMα to form a complex with enhanced functionality. Due to the different aggregation capabilities and functions of fδ-toxin and dfδ-toxin, we speculate that they may be involved in the regulation of physiological activities of S. aureus. Moreover, the dfδ-toxin oligomer not only provides a new form of complex in the study of PSMα, but also has significance as a reference in oligomer research pertaining to some human amyloid diseases.

Staphylococcus aureus Depends on Eap Proteins for Preventing Degradation of Its Phenol-Soluble Modulin Toxins by Neutrophil Serine Proteases

Neutrophil granulocytes act as a first line of defense against pathogenic staphylococci. However, Staphylococcus aureus has a remarkable capacity to survive neutrophil killing, which distinguishes it from the less-pathogenic Staphylococcus epidermidis. Both species release phenol-soluble modulin (PSM) toxins, which activate the neutrophil formyl-peptide receptor 2 (FPR2) to promote neutrophil influx and phagocytosis, and which disrupt neutrophils or their phagosomal membranes at high concentrations. We show here that the neutrophil serine proteases (NSPs) neutrophil elastase, cathepsin G and proteinase 3, which are released into the extracellular space or the phagosome upon neutrophil FPR2 stimulation, effectively degrade PSMs thereby preventing their capacity to activate and destroy neutrophils. Notably, S. aureus, but not S. epidermidis, secretes potent NSP-inhibitory proteins, Eap, EapH1, EapH2, which prevented the degradation of PSMs by NSPs. Accordingly, a S. aureus mutant lacking all three NSP inhibitory proteins was less effective in activating and destroying neutrophils and it survived less well in the presence of neutrophils than the parental strain. We show that Eap proteins promote pathology via PSM-mediated FPR2 activation since murine intraperitoneal infection with the S. aureus parental but not with the NSP inhibitors mutant strain, led to a significantly higher bacterial load in the peritoneum and kidneys of mFpr2^-/- compared to wild-type mice. These data demonstrate that NSPs can very effectively detoxify some of the most potent staphylococcal toxins and that the prominent human pathogen S. aureus has developed efficient inhibitors to preserve PSM functions. Preventing PSM degradation during infection represents an important survival strategy to ensure FPR2 activation.

Anti-Biofilm Molecules Targeting Functional Amyloids

The choice of an effective therapeutic strategy in the treatment of biofilm-related infections is a significant issue. Amyloids, which have been historically related to human diseases, are now considered to be prevailing structural components of the biofilm matrix in a wide range of bacteria. This assumption creates the potential for an exciting research area, in which functional amyloids are considered to be attractive targets for drug development to dissemble biofilm structures. The present review describes the best-characterized bacterial functional amyloids and focuses on anti-biofilm agents that target intrinsic and facultative amyloids. This study provides a better understanding of the different modes of actions of the anti-amyloid molecules to inhibit biofilm formation. This information can be further exploited to improve the therapeutic strategies to combat biofilm-related infections.

Computational Methods and Tools in Antimicrobial Peptide Research

The evolution of antibiotic-resistant bacteria is an ongoing and troubling development that has increased the number of diseases and infections that risk going untreated. There is an urgent need to develop alternative strategies and treatments to address this issue. One class of molecules that is attracting significant interest is that of antimicrobial peptides (AMPs). Their design and development has been aided considerably by the applications of molecular models, and we review these here. These methods include the use of tools to explore the relationships between their structures, dynamics, and functions and the increasing application of machine learning and molecular dynamics simulations. This review compiles resources such as AMP databases, AMP-related web servers, and commonly used techniques, together aimed at aiding researchers in the area toward complementing experimental studies with computational approaches.

Gene Regulation of Biofilm-Associated Functional Amyloids

Biofilms are bacterial communities encased in a rigid yet dynamic extracellular matrix. The sociobiology of bacterial communities within a biofilm is astonishing, with environmental factors playing a crucial role in determining the switch from planktonic to a sessile form of life. The mechanism of biofilm biogenesis is an intriguingly complex phenomenon governed by the tight regulation of expression of various biofilm-matrix components. One of the major constituents of the biofilm matrix is proteinaceous polymers called amyloids. Since the discovery, the significance of biofilm-associated amyloids in adhesion, aggregation, protection, and infection development has been much appreciated. The amyloid expression and assembly is regulated spatio-temporarily within the bacterial cells to perform a diverse function. This review provides a comprehensive account of the genetic regulation associated with the expression of amyloids in bacteria. The stringent control ensures optimal utilization of amyloid scaffold during biofilm biogenesis. We conclude the review by summarizing environmental factors influencing the expression and regulation of amyloids.

Modulating Kinetics of the Amyloid-Like Aggregation of S. aureus Phenol-Soluble Modulins by Changes in pH

The pathogen Staphylococcus aureus is recognized as one of the most frequent causes of biofilm-associated infections. The recently identified phenol-soluble modulin (PSM) peptides act as the key molecular effectors of staphylococcal biofilm maturation and promote the formation of an aggregated fibril structure. The aim of this study was to evaluate the effect of various pH values on the formation of functional amyloids of individual PSM peptides. Here, we combined a range of biophysical, chemical kinetics and microscopic techniques to address the structure and aggregation mechanism of individual PSMs under different conditions. We established that there is a pH-induced switch in PSM aggregation kinetics. Different lag times and growth of fibrils were observed, which indicates that there was no clear correlation between the rates of fibril elongation among different PSMs. This finding confirms that pH can modulate the aggregation properties of these peptides and suggest a deeper understanding of the formation of aggregates, which represents an important basis for strategies to interfere and might help in reducing the risk of biofilm-related infections.

Multifunctional Amyloids in the Biology of Gram-Positive Bacteria

Since they were discovered, amyloids have proven to be versatile proteins able to participate in a variety of cellular functions across all kingdoms of life. This multitask trait seems to reside in their ability to coexist as monomers, aggregates or fibrillar entities, with morphological and biochemical peculiarities. It is precisely this common molecular behaviour that allows amyloids to cross react with one another, triggering heterologous aggregation. In bacteria, many of these functional amyloids are devoted to the assembly of biofilms by organizing the matrix scaffold that keeps cells together. However, consistent with their notion of multifunctional proteins, functional amyloids participate in other biological roles within the same organisms, and emerging unprecedented functions are being discovered. In this review, we focus on functional amyloids reported in gram-positive bacteria, which are diverse in their assembly mechanisms and remarkably specific in their biological functions that they perform. Finally, we consider cross-seeding between functional amyloids as an emerging theme in interspecies interactions that contributes to the diversification of bacterial biology.

Emerging Roles of Functional Bacterial Amyloids in Gene Regulation, Toxicity, and Immunomodulation

Bacteria often reside in multicellular communities, called biofilms, held together by an extracellular matrix. In many bacteria, the major proteinaceous component of the biofilm are amyloid fibers. Amyloids are highly stable and structured protein aggregates which were known mostly to be associated with neurodegenerative diseases, such as Alzheimer's, Parkinson's, and Huntington's diseases. In recent years, microbial amyloids were identified also in other species and shown to play major roles in microbial physiology and virulence. For example, amyloid fibers assemble on the bacterial cell surface as a part of the extracellular matrix and are extremely important to the scaffolding and structural integrity of biofilms, which contribute to microbial resilience and resistance. Furthermore, microbial amyloids play fundamental nonscaffold roles that contribute to the development of biofilms underlying numerous persistent infections. Here, we review several nonscaffold roles of bacterial amyloid proteins, including bridging cells during collective migration, acting as regulators of cell fate, as toxins against other bacteria or against host immune cells, and as modulators of the hosts' immune system. These overall points on the complexity of the amyloid fold in encoding numerous activities, which offer approaches for the development of a novel repertoire of antivirulence therapeutics.

Exploring the candidates for a new protein folding - cross-α amyloid - in available protein databases

Amyloid fibrils are formed from the assembly of soluble proteins and are responsible for many diseases. They are known to have a cross-β structure, where the fibril runs perpendicular to the β-sheets. A new type of tertiary structure formed by the aggregation of peptides in their α-helical form, in naturally occurring as well as synthetic peptides, termed cross-α amyloid has been reported recently. We have studied the interactions responsible for the formation of these cross-α amyloids and proposed a model to determine the peptides that could form these structures. Eight such peptides obtained using the model have been shown to form a cross-α structure using molecular dynamics simulations. The formation of a cross-α structure from eight copies of a randomly chosen peptide and its stability over a microsecond simulation have been demonstrated. A software named Cross-Alpha-Det has been developed that can determine whether a protein can form a cross-α structure from its secondary structure.

Structural insight into the Staphylococcus aureus ATP-driven exporter of virulent peptide toxins

Staphylococcus aureus is a major human pathogen that has acquired alarming broad-spectrum antibiotic resistance. One group of secreted toxins with key roles during infection is the phenol-soluble modulins (PSMs). PSMs are amphipathic, membrane-destructive cytolytic peptides that are exported to the host-cell environment by a designated adenosine 5'-triphosphate (ATP)-binding cassette (ABC) transporter, the PSM transporter (PmtABCD). Here, we demonstrate that the minimal Pmt unit necessary for PSM export is PmtCD and provide its first atomic characterization by single-particle cryo-EM and x-ray crystallography. We have captured the transporter in the ATP-bound state at near atomic resolution, revealing a type II ABC exporter fold, with an additional cytosolic domain. Comparison to a lower-resolution nucleotide-free map displaying an "open" conformation and putative hydrophobic inner chamber of a size able to accommodate the binding of two PSM peptides provides mechanistic insight and sets the foundation for therapeutic design.

The Human LL-37(17-29) antimicrobial peptide reveals a functional supramolecular structure

Here, we demonstrate the self-assembly of the antimicrobial human LL-37 active core (residues 17–29) into a protein fibril of densely packed helices. The surface of the fibril encompasses alternating hydrophobic and positively charged zigzagged belts, which likely underlie interactions with and subsequent disruption of negatively charged lipid bilayers, such as bacterial membranes. LL-37_17–29 correspondingly forms wide, ribbon-like, thermostable fibrils in solution, which co-localize with bacterial cells. Structure-guided mutagenesis analyses supports the role of self-assembly in antibacterial activity. LL-37_17–29 resembles, in sequence and in the ability to form amphipathic helical fibrils, the bacterial cytotoxic PSMα3 peptide that assembles into cross-α amyloid fibrils. This argues helical, self-assembling, basic building blocks across kingdoms of life and points to potential structural mimicry mechanisms. The findings expose a protein fibril which performs a biological activity, and offer a scaffold for functional and durable biomaterials for a wide range of medical and technological applications.

The human antibacterial and immunomodulatory peptide LL-37 is a hCAP-18 protein cleavage product that self-assembles. Here, the authors present the human and gorilla LL-37 (17–29) crystal structures, revealing a self-assembly of amphipathic helices into a densely packed and elongated hexameric structure with a central pore and mutagenesis experiments support the role of self-assembly for antibacterial activity.

Use of a Stereochemical Strategy To Probe the Mechanism of Phenol-Soluble Modulin α3 Toxicity

Phenol-soluble modulin α3 (PSMα3) is a cytotoxic peptide secreted by virulent strains of Staphylococcus aureus. We used a stereochemical strategy to examine the mechanism of PSMα3-mediated toxicity. One hypothesis is that PSMα3 toxicity requires fibril formation; an alternative is that toxicity is caused by soluble forms of PSMα3, possibly oligomeric. We find that the unnatural enantiomer (D residues) displays cytotoxicity comparable to that of L-PSMα3. Racemic PSMα3 is similarly toxic to enantiopure PSMα3 (L or D) under some conditions, but the toxicity is lost under conditions that cause racemic PSMα3 to aggregate. A crystal structure of racemic PSMα3-NH₂ displays an α-helical secondary structure and a packing pattern that is reminiscent of the cross-α arrangement recently discovered in crystals of L-PSMα3. Our data suggest that the cytotoxicity of PSMα3 does not depend on stereospecific engagement of a target protein or other chiral macromolecule, an observation that supports a mechanism based on membrane disruption. In addition, our data support the hypothesis that toxicity is exerted by a soluble form rather than an insoluble fibrillar form.

The Mechanism behind Bacterial Lipoprotein Release: Phenol-Soluble Modulins Mediate Toll-Like Receptor 2 Activation via Extracellular Vesicle Release from Staphylococcus aureus

Our study highlights the roles of surfactant-like molecules in bacterial inflammation with important implications for the prevention and therapy of inflammatory disorders. It describes a potential pathway for the transfer of hydrophobic bacterial lipoproteins, the major TLR2 agonists, from the cytoplasmic membrane of Gram-positive bacteria to the TLR2 receptor at the surface of host cells. Moreover, our study reveals a molecular mechanism that explains how cytoplasmic and membrane-embedded bacterial proteins can be released by bacterial cells without using any of the typical protein secretion routes, thereby contributing to our understanding of the processes used by bacteria to communicate with host organisms and the environment.

The innate immune system uses Toll-like receptor (TLR) 2 to detect conserved bacterial lipoproteins of invading pathogens. The lipid anchor attaches lipoproteins to the cytoplasmic membrane and prevents their release from the bacterial cell envelope. How bacteria release lipoproteins and how these molecules reach TLR2 remain unknown. Staphylococcus aureus has been described to liberate membrane vesicles. The composition, mode of release, and relevance for microbe-host interaction of such membrane vesicles have remained ambiguous. We recently reported that S. aureus can release lipoproteins only when surfactant-like small peptides, the phenol-soluble modulins (PSMs), are expressed. Here we demonstrate that PSM peptides promote the release of membrane vesicles from the cytoplasmic membrane of S. aureus via an increase in membrane fluidity, and we provide evidence that the bacterial turgor is the driving force for vesicle budding under hypotonic osmotic conditions. Intriguingly, the majority of lipoproteins are released by S. aureus as components of membrane vesicles, and this process depends on surfactant-like molecules such as PSMs. Vesicle disruption at high detergent concentrations promotes the capacity of lipoproteins to activate TLR2. These results reveal that vesicle release by bacterium-derived surfactants is required for TLR2-mediated inflammation.

Extreme amyloid polymorphism in Staphylococcus aureus virulent PSMα peptides

Members of the Staphylococcus aureus phenol-soluble modulin (PSM) peptide family are secreted as functional amyloids that serve diverse roles in pathogenicity and may be present as full-length peptides or as naturally occurring truncations. We recently showed that the activity of PSMα3, the most toxic member, stems from the formation of cross-α fibrils, which are at variance with the cross-β fibrils linked with eukaryotic amyloid pathologies. Here, we show that PSMα1 and PSMα4, involved in biofilm structuring, form canonical cross-β amyloid fibrils wherein β-sheets tightly mate through steric zipper interfaces, conferring high stability. Contrastingly, a truncated PSMα3 has antibacterial activity, forms reversible fibrils, and reveals two polymorphic and atypical β-rich fibril architectures. These architectures are radically different from both the cross-α fibrils formed by full-length PSMα3, and from the canonical cross-β fibrils. Our results point to structural plasticity being at the basis of the functional diversity exhibited by S. aureus PSMαs.

Do amyloid structures formed by Staphylococcus aureus phenol-soluble modulins have a biological function?

Phenol-soluble modulins (PSMs) are alpha-helical, amphipathic peptides that have multiple functions in staphylococcal physiology and virulence. Recent research has suggested that PSMs form amyloid fibrils and amyloids are involved in PSM-mediated phenotypes such as cytolysis and biofilm stability. While we observed PSM amyloid formation using electron microscopy and dye assays, there were no apparent differences in the production of extracellular fibrous material between a PSM-deficient strain and the isogenic wild-type strain. Furthermore, we detected no correlation between cytolytic or pro-inflammatory activities with the propensity of PSM derivatives to form amyloids. In addition, we propose a model based on our finding of non-specific attachment of PSMs to DNA, which we here report results in resistance to DNase digestion, explaining previous findings on PSM-mediated biofilm stability without the necessity to assume amyloid involvement. Collectively, our results indicate that PSM amyloid formation may not be of major relevance for known key biological functions of PSMs. Intriguingly, however, we found that amyloid-forming capacity of PSMalpha3 allows almost no amino acid exchanges, suggesting importance of amyloid formation in possibly yet unknown functions of PSMs.

Activity of Antimicrobial Peptide Aggregates Decreases with Increased Cell Membrane Embedding Free Energy Cost

Antimicrobial peptides (AMPs) are a promising alternative to antibiotics for mitigating bacterial infections, in light of increasing bacterial resistance to antibiotics. However, predicting, understanding, and controlling the antibacterial activity of AMPs remain a significant challenge. While peptide intramolecular interactions are known to modulate AMP antimicrobial activity, peptide intermolecular interactions remain elusive in their impact on peptide bioactivity. Herein, we test the relationship between AMP intermolecular interactions and antibacterial efficacy by controlling AMP intermolecular hydrophobic and hydrogen bonding interactions. Molecular dynamics simulations and Gibbs free energy calculations in concert with experimental assays show that increasing intermolecular interactions via interpeptide aggregation increases the energy cost for the peptide to embed into the bacterial cell membrane, which in turn decreases the AMP antibacterial activity. Our findings provide a route for predicting and controlling the antibacterial activity of AMPs against Gram-negative bacteria via reductions of intermolecular AMP interactions.

Non-classical Protein Excretion Is Boosted by PSMα-Induced Cell Leakage

Release of cytoplasmic proteins into the supernatant occurs both in bacteria and eukaryotes. Because the underlying mechanism remains unclear, the excretion of cytoplasmic proteins (ECP) has been referred to as "non-classical protein secretion." We show that none of the known specific protein transport systems of Gram-positive bacteria are involved in ECP. However, the expression of the cationic and amphipathic α-type phenol-soluble modulins (PSMs), particularly of PSMα2, significantly increase ECP, while PSMβ peptides or δ-toxin have no effect on ECP. Because psm expression is strictly controlled by the accessory gene regulator (agr), ECP is also reduced in agr-negative mutants. PSMα peptides damage the cytoplasmic membrane, as indicated by the release of not only CPs but also lipids, nucleic acids, and ATP. Thus, our results show that in Staphylococcus aureus, PSMα peptides non-specifically boost the translocation of CPs by their membrane-damaging activity.

The cytotoxic Staphylococcus aureus PSMα3 reveals a cross-α amyloid-like fibril

Amyloids are ordered protein aggregates, found in all kingdoms of life, and are involved in aggregation diseases as well as in physiological activities. In microbes, functional amyloids are often key virulence determinants, yet the structural basis for their activity remains elusive. We determined the fibril structure and function of the highly toxic, 22-residue phenol-soluble modulin α3 (PSMα3) peptide secreted by Staphylococcus aureus PSMα3 formed elongated fibrils that shared the morphological and tinctorial characteristics of canonical cross-β eukaryotic amyloids. However, the crystal structure of full-length PSMα3, solved de novo at 1.45 angstrom resolution, revealed a distinctive "cross-α" amyloid-like architecture, in which amphipathic α helices stacked perpendicular to the fibril axis into tight self-associating sheets. The cross-α fibrillation of PSMα3 facilitated cytotoxicity, suggesting that this assembly mode underlies function in S. aureus.

Phenol-Soluble Modulin Toxins of Staphylococcus haemolyticus

Coagulase-negative staphylococci (CoNS) are important nosocomial pathogens and the leading cause of sepsis. The second most frequently implicated species, after Staphylococcus epidermidis, is Staphylococcus haemolyticus. However, we have a significant lack of knowledge about what causes virulence of S. haemolyticus, as virulence factors of this pathogen have remained virtually unexplored. In contrast to the aggressive pathogen Staphylococcus aureus, toxin production has traditionally not been associated with CoNS. Recent findings have suggested that phenol-soluble modulins (PSMs), amphipathic peptide toxins with broad cytolytic activity, are widespread in staphylococci, but there has been no systematic assessment of PSM production in CoNS other than S. epidermidis. Here, we identified, purified, and characterized PSMs of S. haemolyticus. We found three PSMs of the β-type, which correspond to peptides that before were described to have anti-gonococcal activity. We also detected an α-type PSM that has not previously been described. Furthermore, we confirmed that S. haemolyticus does not produce a δ-toxin, as results from genome sequencing had indicated. All four S. haemolyticus PSMs had strong pro-inflammatory activity, promoting neutrophil chemotaxis. Notably, we identified in particular the novel α-type PSM, S. haemolyticus PSMα, as a potent hemolysin and leukocidin. For the first time, our study describes toxins of this important staphylococcal pathogen with the potential to have a significant impact on virulence during blood infection and sepsis.

The Formyl Peptide Receptors: Diversity of Ligands and Mechanism for Recognition

The formyl peptide receptors (FPRs) are G protein-coupled receptors that transduce chemotactic signals in phagocytes and mediate host-defense as well as inflammatory responses including cell adhesion, directed migration, granule release and superoxide production. In recent years, the cellular distribution and biological functions of FPRs have expanded to include additional roles in homeostasis of organ functions and modulation of inflammation. In a prototype, FPRs recognize peptides containing N-formylated methionine such as those produced in bacteria and mitochondria, thereby serving as pattern recognition receptors. The repertoire of FPR ligands, however, has expanded rapidly to include not only N-formyl peptides from microbes but also non-formyl peptides of microbial and host origins, synthetic small molecules and an eicosanoid. How these chemically diverse ligands are recognized by the three human FPRs (FPR1, FPR2 and FPR3) and their murine equivalents is largely unclear. In the absence of crystal structures for the FPRs, site-directed mutagenesis, computer-aided ligand docking and structural simulation have led to the identification of amino acids within FPR1 and FPR2 that interact with several formyl peptides. This review article summarizes the progress made in the understanding of FPR ligand diversity as well as ligand recognition mechanisms used by these receptors.

Dissecting the contribution of Staphylococcus aureus α-phenol-soluble modulins to biofilm amyloid structure

The opportunistic pathogen Staphylococcus aureus is recognized as one of the most frequent causes of biofilm-associated infections. The recently discovered phenol soluble modulins (PSMs) are small α-helical amphipathic peptides that act as the main molecular effectors of staphylococcal biofilm maturation, promoting the formation of an extracellular fibril structure with amyloid-like properties. Here, we combine computational, biophysical and in cell analysis to address the specific contribution of individual PSMs to biofilm structure. We demonstrate that despite their highly similar sequence and structure, contrary to what it was previously thought, not all PSMs participate in amyloid fibril formation. A balance of hydrophobic/hydrophilic forces and helical propensity seems to define the aggregation propensity of PSMs and control their assembly and function. This knowledge would allow to target specifically the amyloid properties of these peptides. In this way, we show that Epigallocatechin-3-gallate (EGCG), the principal polyphenol in green tea, prevents the assembly of amyloidogenic PSMs and disentangles their preformed amyloid fibrils.

Amyloid Structures as Biofilm Matrix Scaffolds

Recent insights into bacterial biofilm matrix structures have induced a paradigm shift toward the recognition of amyloid fibers as common building block structures that confer stability to the exopolysaccharide matrix. Here we describe the functional amyloid systems related to biofilm matrix formation in both Gram-negative and Gram-positive bacteria and recent knowledge regarding the interaction of amyloids with other biofilm matrix components such as extracellular DNA (eDNA) and the host immune system. In addition, we summarize the efforts to identify compounds that target amyloid fibers for therapeutic purposes and recent developments that take advantage of the amyloid structure to engineer amyloid fibers of bacterial biofilm matrices for biotechnological applications.

Phenol-soluble modulin α4 mediates Staphylococcus aureus-associated vascular leakage by stimulating heparin-binding protein release from neutrophils

Vascular leakage frequently occurs in patients with severe Staphylococcus aureus infection. However, the mechanism underlying S. aureus infection-induced vascular leakage remains unclear. Here, we identified the S. aureus virulence factor phenol-soluble modulin (PSM)α4 from the culture supernatant of strain USA300 as a stimulator of heparin-binding protein (HBP) release from polymorphonuclear neutrophils (PMNs) and demonstrated that PSMα4-induced HBP release from PMNs leads to vascular leakage. PSMα4 appeared less cytolytic than PSMα1-3 and was insensitive to lipoproteins; it significantly increased myeloperoxidase and elastase release from PMNs and cell surface CD63 expression in PMNs. PSMα4-induced HBP release required formyl peptide receptor 2 (FPR2) and phosphoinositide 3-kinase (PI3K) and depended on Ca(2+) influx and cytoskeleton rearrangement. Thus, PSMα4 may stimulate HBP release by activating FPR2 and PI3K to initiate PMN degranulation. PSMα4-induced HBP release from PMNs increased endothelial cell monolayer permeability in vitro and induced vascular leakage in mice. This novel function of PSMα4 may contribute to the pathogenesis of S. aureus and may be a potential therapeutic target.

The Role of Staphylococcus aureus Virulence Factors in Skin Infection and Their Potential as Vaccine Antigens

Staphylococcus aureus (S. aureus) causes the vast majority of skin and soft tissue infections (SSTIs) in humans. S. aureus has become increasingly resistant to antibiotics and there is an urgent need for new strategies to tackle S. aureus infections. Vaccines offer a potential solution to this epidemic of antimicrobial resistance. However, the development of next generation efficacious anti-S. aureus vaccines necessitates a greater understanding of the protective immune response against S. aureus infection. In particular, it will be important to ascertain if distinct immune mechanisms are required to confer protection at distinct anatomical sites. Recent discoveries have highlighted that interleukin-17-producing T cells play a particularly important role in the immune response to S. aureus skin infection and suggest that vaccine strategies to specifically target these types of T cells may be beneficial in the treatment of S. aureus SSTIs. S. aureus expresses a large number of cell wall-anchored (CWA) proteins, which are covalently attached to the cell wall peptidoglycan. The virulence potential of many CWA proteins has been demonstrated in infection models; however, there is a paucity of information regarding their roles during SSTIs. In this review, we highlight potential candidate antigens for vaccines targeted at protection against SSTIs.

Peptide length and folding state govern the capacity of staphylococcal β-type phenol-soluble modulins to activate human formyl-peptide receptors 1 or 2

Most staphylococci produce short α-type PSMs and about twice as long β-type PSMs that are potent leukocyte attractants and toxins. PSMs are usually secreted with the N-terminal formyl group but are only weak agonists for the leukocyte FPR1. Instead, the FPR1-related FPR2 senses PSMs efficiently and is crucial for leukocyte recruitment in infection. Which structural features distinguish FPR1 from FPR2 ligands has remained elusive. To analyze which peptide properties may govern the capacities of β-type PSMs to activate FPRs, full-length and truncated variants of such peptides from Staphylococcus aureus, Staphylococcus epidermidis, and Staphylococcus lugdunensis were synthesized. FPR2 activation was observed even for short N- or C-terminal β-type PSM variants once they were longer than 18 aa, and this activity increased with length. In contrast, the shortest tested peptides were potent FPR1 agonists, and this property declined with increasing peptide length. Whereas full-length β-type PSMs formed α-helices and exhibited no FPR1-specific activity, the truncated peptides had less-stable secondary structures, were weak agonists for FPR1, and required N-terminal formyl-methionine residues to be FPR2 agonists. Together, these data suggest that FPR1 and FPR2 have opposed ligand preferences. Short, flexible PSM structures may favor FPR1 but not FPR2 activation, whereas longer peptides with α-helical, amphipathic properties are strong FPR2 but only weak FPR1 agonists. These findings should help to unravel the ligand specificities of 2 critical human PRRs, and they may be important for new, anti-infective and anti-inflammatory strategies.

Staphylococcal Immune Evasion Proteins: Structure, Function, and Host Adaptation

Staphylococcus aureus is a successful human and animal pathogen. Its pathogenicity is linked to its ability to secrete a large amount of virulence factors. These secreted proteins interfere with many critical components of the immune system, both innate and adaptive, and hamper proper immune functioning. In recent years, numerous studies have been conducted in order to understand the molecular mechanism underlying the interaction of evasion molecules with the host immune system. Structural studies have fundamentally contributed to our understanding of the mechanisms of action of the individual factors. Furthermore, such studies revealed one of the most striking characteristics of the secreted immune evasion molecules: their conserved structure. Despite high-sequence variability, most immune evasion molecules belong to a small number of structural categories. Another remarkable characteristic is that S. aureus carries most of these virulence factors on mobile genetic elements (MGE) or ex-MGE in its accessory genome. Coevolution of pathogen and host has resulted in immune evasion molecules with a highly host-specific function and prevalence. In this review, we explore how these shared structures and genomic locations relate to function and host specificity. This is discussed in the context of therapeutic options for these immune evasion molecules in infectious as well as in inflammatory diseases.

Excretion of cytosolic proteins (ECP) in bacteria

Excretion of cytosolic proteins (ECP) has been reported in bacteria and eukaryotes. As none of the classical signal peptide (SP) dependent or SP-independent pathways could be associated with ECP, it has been also referred to as 'non-classical protein export'. When microbiologists first began to study this subject in 1990, mainly singular cytoplasmic proteins were investigated, such as GAPDH at the cell surface and in the supernatant of pathogenic streptococci or glutamine synthetase (GlnA) as a major extracellular protein in pathogenic mycobacteria. Later, with the rising popularity of proteomics, it became obvious that the secretome of most bacteria contained a copious amount of cytosolic proteins. In particular ancient proteins such as glycolytic enzymes, chaperones, translation factors or enzymes involved in detoxification of reactive oxygen were found in the supernatants. As the excreted proteins do not possess a common motive, the most widespread opinion is that ECP is due to cell lysis. Indeed, upregulation of autolysins or distortion of the murein structure increased ECP, suggesting that enhanced ECP is some sort of survival strategy to counteract osmotic stress. However, in the meantime there are mounting evidences and hints that speak against cell lysis as a primary mechanism for ECP. Very likely, ECP belongs to the normal life cycle of bacteria and involves a programmed process. This review provides a brief overview of the 'non-classical protein export'.

Enemy attraction: bacterial agonists for leukocyte chemotaxis receptors

The innate immune system recognizes conserved microorganism-associated molecular patterns (MAMPs), some of which are sensed by G protein-coupled receptors (GPCRs), and this leads to chemotactic leukocyte influx. Recent studies have indicated that these processes are crucial for host defence and rely on a larger set of chemotactic MAMPs and corresponding GPCRs than was previously thought. Agonists, such as bacterial formyl peptides, enterococcal pheromone peptides, staphylococcal peptide toxins, bacterial fermentation products and the Helicobacter pylori peptide HP(2-20), stimulate specific GPCRs. The importance of leukocyte chemotaxis in host defence is highlighted by the fact that some bacterial pathogens produce chemotaxis inhibitors. How the various chemoattractants, receptors and antagonists shape antibacterial host defence represents an important topic for future research.