Single molecule compression reveals intra-protein forces drive cytotoxin pore formation

Direct Visualization of the Dynamic Process of Epsilon Toxin on Hemolysis

Hemolysis is the process of rupturing erythrocytes (red blood cells) by forming nanopores on their membranes using hemolysins, which then impede membrane permeability. However, the self-assembly process before the state of transmembrane pores and underlying mechanisms of conformational change are not fully understood. In this work, theoretical and experimental evidence of the pre-pore morphology of Clostridium perfringens epsilon toxin (ETX), a typical hemolysin, is provided using in situ atomic force microscopy (AFM) complemented by molecular dynamics (MD) simulations to detect the conformational distribution of different states in Mica. The AFM suggests that the ETX pore is formed in two stages: ETX monomers first attach to the membrane and form a pre-pore in no special conditions required, which then undergo a conformational change to form a transmembrane pore at temperatures above the critical point in the presence of receptors. The authors' MD simulations reveal that initial nucleation occurs when specific amino acids adsorb to negatively charged mica cavities. This work fills the knowledge gap in understanding the early stage of hemolysis and the oligomerization of hemolysins. Moreover, the newly identified pre-pore of ETX holds promise as a candidate for nanopore applications.

Visualizing the Domino-Like Prepore-to-Pore Transition of Streptolysin O by High-Speed AFM

Pore-forming proteins (PFPs) are produced by various organisms, including pathogenic bacteria, and form pores within the target cell membrane. Streptolysin O (SLO) is a PFP produced by Streptococcus pyogenes and forms high-order oligomers on the membrane surface. In this prepore state, multiple α-helices in domain 3 of each subunit exist as unfolded structures and transiently interact with each other. They subsequently transition into transmembrane β-hairpins (TMHs) and form pores with diameters of 20-30 nm. However, in this pore formation process, the trigger of the transition in a subunit and collaboration between subunits remains elusive. Here, I observed the dynamic pore formation process using high-speed atomic force microscopy. During the oligomer transition process, each subunit was sequentially inserted into the membrane, propagating along the oligomer in a domino-like fashion (chain reaction). This process also occurred on hybrid oligomers containing wildtype and mutant subunits, which cannot insert into the membrane because of an introduced disulfide bond. Furthermore, propagation still occurred when an excessive force was added to hybrid oligomers in the prepore state. Based on the observed chain reactions, I estimate the free energies and forces that trigger the transition in a subunit. Furthermore, I hypothesize that the collaboration between subunits is related to the structure of their TMH regions and interactions between TMH-TMH and TMH-lipid molecules.

Cholesterol-dependent cytolysins: The outstanding questions

The cholesterol-dependent cytolysins (CDCs) are a major family of bacterial pore-forming proteins secreted as virulence factors by Gram-positive bacterial species. CDCs are produced as soluble, monomeric proteins that bind specifically to cholesterol-rich membranes, where they oligomerize into ring-shaped pores of more than 30 monomers. Understanding the details of the steps the toxin undergoes in converting from monomer to a membrane-spanning pore is a continuing challenge. In this review we summarize what we know about CDCs and highlight the remaining outstanding questions that require answers to obtain a complete picture of how these toxins kill cells.

Diversity of β-hemolysins produced by the human opportunistic streptococci

The genus Streptococcus infects a broad range of hosts, including humans. Some species, such as S. pyogenes, S. agalactiae, S. pneumoniae, and S. mutans, are recognized as the major human pathogens, and their pathogenicity toward humans has been investigated. However, many of other streptococcal species have been recognized as opportunistic pathogens in humans, and their clinical importance has been underestimated. In our previous study, the Anginosus group streptococci (AGS) and Mitis group streptococci (MGS) showed clear β-hemolysis on blood agar, and the factors responsible for the hemolysis were homologs of two types of β-hemolysins, cholesterol-dependent cytolysin (CDC) and streptolysin S (SLS). In contrast to the regular β-hemolysins produced by streptococci (typical CDCs and SLSs), genetically, structurally, and functionally atypical β-hemolysins have been observed in AGS and MGS. These atypical β-hemolysins are thought to affect and contribute to the pathogenic potential of opportunistic streptococci mainly inhabiting the human oral cavity. In this review, we introduce the diverse characteristics of β-hemolysin produced by opportunistic streptococci, focusing on the species/strains belonging to AGS and MGS, and discuss their pathogenic potential.

Spontaneous Rupture and Entanglement of Human Neuronal Tau Protein Induced by Piconewton Compressive Force

Mechanical force vector fluctuations in living cells can have a significant impact on protein behavior and functions. Here we report that a human tau protein tertiary structure can abruptly and spontaneously rupture, like a balloon, under biologically available piconewton compressive force, using a home-modified atomic force microscopy single-molecule manipulation. The rupture behavior is dependent on the physiological level of presence of ions, such as K⁺ and Mg²⁺. We observed rupture events in the presence of K⁺ but not in the presence of Mg²⁺ ions. We have also explored the entangled protein state formed following the events of the multiple and simultaneous protein ruptures under crowding. Crowded proteins simultaneously rupture and then spontaneously refold to an entangled folding state, different from either folded and unfolded states of the tau protein, which can be a plausible pathway for the tau protein aggregation that is related to a number of neurodegenerative diseases.

Kinetically Stable Triglyceride-Based Nanodroplets and Their Interactions with Lipid-Specific Proteins

Understanding of the interactions between proteins and natural and artificially prepared lipid membrane surfaces and embedded nonpolar cores is important in studies of physiological processes and their pathologies and is applicable to nanotechnologies. In particular, rapidly growing interest in cellular droplets defines the need for simplified biomimetic lipid model systems to overcome in vivo complexity and variability. We present a protocol for the preparation of kinetically stable nanoemulsions with nanodroplets composed of sphingomyelin (SM) and cholesterol (Chol), as amphiphilic surfactants, and trioleoylglycerol (TOG), at various molar ratios. To prepare stable SM/Chol-coated monodisperse lipid nanodroplets, we modified a reverse phase evaporation method and combined it with ultrasonication. Lipid composition, ζ-potential, gyration and hydrodynamic radius, shape, and temporal stability of the lipid nanodroplets were characterized and compared to extruded SM/Chol large unilamellar vesicles. Lipid nanodroplets and large unilamellar vesicles with theoretical SM/Chol/TOG molar ratios of 1/1/4.7 and 4/1/11.7 were further investigated for the orientational order of their interfacial water molecules using a second harmonic scattering technique, and for interactions with the SM-binding and Chol-binding pore-forming toxins equinatoxin II and perfringolysin O, respectively. The surface characteristics (ζ-potential, orientational order of interfacial water molecules) and binding of these proteins to the nanodroplet SM/Chol monolayers were similar to those for the SM/Chol bilayers of the large unilamellar vesicles and SM/Chol Langmuir monolayers, in terms of their surface structures. We propose that such SM/Chol/TOG nanoparticles with the required lipid compositions can serve as experimental models for monolayer membrane to provide a system that imitates the natural lipid droplets.

Probing Activated and Non-Activated Single Calmodulin Molecules under a Piconewton Compressive Force

Interrogating the protein structure-function inter-relationship under a piconewton force manipulation has been highly promising and informative. Although protein conformational changes under pulling force manipulations have been extensively studied, protein conformational changes under a compressive force have not been explored in detail. Using our home-modified sensitive and high signal-to-noise atomic force microscopy (AFM) approach, we have applied a piconewton compressive force, manipulating a Calmodulin (CaM) molecule to characterize two different forms of CaM, the Ca²⁺-ligated activated form and the Ca²⁺ free non-activated form (apo-CaM). We observed sudden and spontaneous structural rupture of apo-CaM under compressive force applied by an AFM tip, though no such events were recorded in the case of Ca²⁺-ligated activated CaM form. The sudden spontaneous structural rupture under a piconewton force compression has never been reported before, which presents an unexplored function that is likely important for protein-protein interactions and cell signaling functions.

Revealing Abrupt and Spontaneous Ruptures of Protein Native Structure under picoNewton Compressive Force Manipulation

Manipulating protein conformations for exploring protein structure-function relationship has shown great promise. Although protein conformational changes under pulling force manipulation have been extensively studied, protein conformation changes under a compressive force have not been explored quantitatively. The latter is even more biologically significant and relevant in revealing protein functions in living cells associated with protein crowdedness, distribution fluctuations, and cell osmotic stress. Here we report our experimental observations on abrupt ruptures of protein native structures under compressive force, demonstrated and studied by single-molecule AFM-FRET spectroscopic nanoscopy. Our results show that the protein ruptures are abrupt and spontaneous events occurred when the compressive force reaches a threshold of 12-75 pN, a force amplitude accessible from thermal fluctuations in a living cell. The abrupt ruptures are sensitive to local environment, likely a general and important pathway of protein unfolding in living cells.

Compressive Force Spectroscopy: From Living Cells to Single Proteins

One of the most successful applications of atomic force microscopy (AFM) in biology involves monitoring the effect of force on single biological molecules, often referred to as force spectroscopy. Such studies generally entail the application of pulling forces of different magnitudes and velocities upon individual molecules to resolve individualistic unfolding/separation pathways and the quantification of the force-dependent rate constants. However, a less recognized variation of this method, the application of compressive force, actually pre-dates many of these "tensile" force spectroscopic studies. Further, beyond being limited to the study of single molecules, these compressive force spectroscopic investigations have spanned samples as large as living cells to smaller, multi-molecular complexes such as viruses down to single protein molecules. Correspondingly, these studies have enabled the detailed characterization of individual cell states, subtle differences between seemingly identical viral structures, as well as the quantification of rate constants of functionally important, structural transitions in single proteins. Here, we briefly review some of the recent achievements that have been obtained with compressive force spectroscopy using AFM and highlight exciting areas of its future development.

Lysenin Toxin Membrane Insertion Is pH-Dependent but Independent of Neighboring Lysenins

Pore-forming toxins form a family of proteins that act as virulence factors of pathogenic bacteria, but similar proteins are found in all kingdoms of life, including the vertebrate immune system. They are secreted as soluble monomers that oligomerize on target membranes in the so-called prepore state; after activation, they insert into the membrane and adopt the pore state. Lysenin is a pore-forming toxin from the earthworm Eisenida foetida, of which both the soluble and membrane-inserted structures are solved. However, the activation and membrane-insertion mechanisms have remained elusive. Here, we used high-speed atomic force microscopy to directly visualize the membrane-insertion mechanism. Changing the environmental pH from pH 7.5 to below pH 6.0 favored membrane insertion. We detected a short α-helix in the soluble structure that comprised three glutamic acids (Glu92, Glu94, and Glu97) that we hypothesized may represent a pH-sensor (as in similar toxins, e.g., Listeriolysin). Mutant lysenin still can form pores, but mutating these glutamic acids to glutamines rendered the toxin pH-insensitive. On the other hand, toxins in the pore state did not favor insertion of neighboring prepores; indeed, pore insertion breaks the hexagonal ordered domains of prepores and separates from neighboring molecules in the membrane. pH-dependent activation of toxins may represent a common feature of pore-forming toxins. High-speed atomic force microscopy with single-molecule resolution at high temporal resolution and the possibility of exchanging buffers during the experiments presents itself as a unique tool for the study of toxin-state conversion.