Frontiers in Molecular Biosciences

Ion mobility mass spectrometry (IM-MS) provides valuable structural information about protein shape and size through collision cross section (CCS). However, it lacks atomic level structural detail. While AlphaFold has been successful in predicting monomeric protein structure, it can struggle with modeling protein complexes. To address these limitations, we developed a method that integrates IM-MS data with AlphaFold and Rosetta to improve complex structure prediction. Our approach uses experimental CCS data to guide the assembly of AlphaFold predicted subunits using a Rosetta docking pipeline and evaluating the resulting complexes with a newly developed score. Using this strategy, we were able to improve root mean square deviation (RMSD) values for 26 of 38 (68%) complexes compared to AlphaFold-Multimer. Furthermore, 16 of these systems improved significantly from greater than 4 [A] RMSD to less than 4 [A]. This method demonstrates a robust approach to overcome limitations in complex assembly modeling. Table of Contents Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=182 SRC="FIGDIR/small/706193v1_ufig1.gif" ALT="Figure 1"> View larger version (68K): org.highwire.dtl.DTLVardef@cf71c3org.highwire.dtl.DTLVardef@135e09aorg.highwire.dtl.DTLVardef@2cc2fcorg.highwire.dtl.DTLVardef@b53feb_HPS_FORMAT_FIGEXP M_FIG In this integrative modeling work, protein complex structures were modeled by combining AlphaFold predicted subunits with Rosetta docking. Collision cross section data from ion-mobility mass spectrometry were used as evaluation constraints and docked models were scored using the IM-complex score. The best scoring models generally represent accurate protein complex structures. C_FIG

The protein caveolin-1 (cav1) is essential in the generation of caveolae, cup-like invaginations in the plasma membrane, but the mechanism of its action remains unclear. A recent cryo-EM structure revealed an 11-mer of cav1 (the 8S complex) forming a disk with a flat membrane-facing surface, raising the question of how a flat complex is able to generate membrane curvature. We previously conducted implicit-solvent and all-atom molecular dynamics simulations, which showed the 8S complex adopting a conical shape. The ability of the conical shape to remodel membranes was assumed but could not be confirmed. Here, we simulated the complex in discontinuous membrane patches of [~]30 nm diameter on the Anton 3 supercomputer. In a 2-s simulation, a flat POPC membrane patch acquired pronounced positive curvature (curved away from the complex), converting into a hemisphere of 21-nm outer diameter. However, when the complex was constrained to prevent the conversion into a conical shape, the membrane patch acquired slightly negative curvature. These results show that the conical shape of the 8S complex is essential for positive curvature generation. A homology model of cav3 behaved very similarly to cav1, but the two recently discovered nonvertebrate caveolins remained flat and generated pronounced negative curvature. Simulations of cav1 in 70:30 POPC:cholesterol and other cholesterol-containing mixtures showed significantly lower curvature than in the pure POPC membrane or in an E. coli membrane mimic. This appears to be caused by cholesterol flipping from the distal to the proximal leaflet. No specific binding of cholesterol to the cav1 CRAC motif was observed, nor significant enrichment of cholesterol in contact with the complex. These observations lead to the hypothesis that cholesterol is enriched in caveolae not because of specific binding to caveolin, but because it can alleviate curvature stress due to its negative spontaneous curvature and its ability to rapidly flip-flop.

Aiming to examine the effect of chronic alcohol exposure on the activity of CYP3A enzymes in human liver, we studied the metabolism of two CYP3A-specific substrates, 7-benzyloxyquinoline (7-BQ) and ivermectin, in 23 preparations of human liver microsomes (HLM) obtained from donors with documented alcohol exposure, grading from non-drinkers to heavy alcoholics. All HLM samples were characterized for the composition of the cytochrome P450 pool and the abundances of other drug-metabolizing and endoplasmic reticulum-stress-related enzymes by global proteomics. Our studies revealed a striking increase in the activities of CYP3A enzymes caused by chronic alcohol exposure. This effect is not associated with changes in CYP3A enzyme levels, which do not correlate with alcohol exposure. Instead, the rates of 7-BQ and ivermectin metabolism correlate with the content of alcohol-inducible CYP2E1. However, this enzyme does not metabolize ivermectin, and its activity with 7-BQ is negligible. These results suggest that the observed acceleration of the elimination of drugs metabolized by CYP3A enzymes by alcohol exposure is due to functional effects of the interaction between CYP3A and CYP2E1. To elucidate the potential mechanism of this effect, we studied the formation of CYP2E1-CYP3A4 complexes in CYP3A4-containing Supersomes with co-incorporated CYP2E1 using tag-transfer chemical crosslinking mass spectrometry (CX-MS). These experiments confirmed physical interactions between the proteins and allowed the identification of CYP3A4 residues at the sites of contact. This information was used to build structural models of the CYP2E1-CYP3A4 complex and to propose possible mechanisms for the observed effects.

Tyrosine Kinase inhibitors (TKIs) are widely used as effective chemotherapeutic agents for treating patients with EGFR-mutated NSCLC. Unfortunately, after treatment, patients eventually develop resistance to TKI therapy. The most common resistance mechanism for the TKI Osimertinib is the overexpression of the MET Proto-Oncogene, Receptor Tyrosine Kinase (MET). We previously demonstrated that miR-411-5p A-to-I edited at position 5 (miR-411ed) can directly target MET in A549 and H1299 cells. MiR-411ed in combination with Osimertinib reduced cell proliferation in two TKI resistant EGFR-mutated cell lines: HCC827R and PC9R. MiR-411ed did not downregulate MET expression in HCC827R, suggesting an alternative mechanism for TKI response. In this study, we aim to identify the mechanism of miR-411ed TKI response using a multi-omics approach of RNAseq and protein mass spectrometry. In our cellular model, we identified miR-411ed affected genes independent of MET activity, resulting in 211 genes (RNAseq) and 36 proteins (proteomics). Pathway analysis identified an increase in interferon signaling for RNAseq and combined omics, and a decrease in ERK/MAPK signaling in proteomics. Using the IsoTar target prediction tool, we identified STAT3 as a key regulator and confirmed STAT3 protein downregulation upon transfection with miR-411ed. We further investigated the effect of miR-411ed in vivo, observing a reduction in tumor size with miR-411ed in combination with Osimertinib but not with miR-411ed or Osimertinib treatment alone, confirming the effectiveness of miR-411ed in TKI response.

Nuclear Pore Complexes (NPCs) are large protein complexes in eukaryotic cells that span the double-membrane of the nucleus and regulate bi-directional transport between nucleus and cytoplasm. T h e NPC core is lined by intrinsically disordered protein chains called nucleoporins (Nups) which form a selective barrier where large macromolecules (cargoes) need to bind to nuclear transport receptors (NTRs) such as Karyopherins (Kaps) to cross. Previous experimental results have suggested that not only Nups but Kaps, too, are important in the transport process of other NTRs/NTR-cargo complexes. In this work, we assess the role of Kaps in the transport of other NTRs (specifically, NTF2s) through the NPC, a process referred to as the "Kap-centric transport model". Here, using coarse-grained MD simulation we show that Kaps are able to direct NTF2s into the Nup meshwork, which leads to their increased flow. Our results also suggest that NTRs follow specific lanes inside the pore to maximize efficient transport.

The 3D structure and mechanism of action are unknown for the integral plasma membrane transport protein Solute Carrier 4A10, which has been characterized functionally as an electroneutral Na+:HCO3- cotransporter. We used structure prediction and molecular dynamics simulations to study the binding of the transported ions to the Solute Carrier 4A10 protein and suggest a model of sequential binding of Na+ followed by HCO3- to the ion binding domain. The binding of HCO3- to the protein appears to depend absolutely on Na+ binding. Conversely, binding of HCO3- stabilizes the interaction between Na+ and its binding site. This allows the subsequent conformational changes of the Solute Carrier 4A10 protein and, thus, ion translocation. Measurements of intracellular pH and Na+ concentration revealed the dependence of Na+ on HCO3- transport. The study lays the necessary foundation for advanced analysis of ion translocation and the development of selective transport inhibitors of Solute Carrier 4A10 and other proteins of the protein family of HCO3- transporters.

Proteins with intrinsically disordered regions (IDRs) migrate at a higher apparent molecular weight in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) complicating their analysis and identification. Here, we investigate the sequence determinants of the hypomobility of IDRs using a series of synthetic low complexity domains. We find that negative charge increases the apparent molecular weight, but neutral polar tracts also have abnormally slow migration. Positive charge and hydrophobic residues decrease the apparent molecular weight, although lysine residues show a biphasic effect with decreased migration at high fractional contents. Combinations of residues show that different sequence contributions to the apparent molecular weight are not additive. The results can be rationalized by the protein-decorated micelle model by considering both SDS binding and the compaction of protein SDS-complexes.

Staphylococcus aureus (S. aureus) is an opportunistic pathogen that is a global health concern for its ability to cause a wide spectrum of clinical infections. Due to the emergence of resistance to commonly used antibiotics, there has been interest in exploring the use of antimicrobial peptides to treat S. aureus infections. However, changes in the lipid composition of the lipid bilayer membrane can alter the activity of peptides, and S. aureus is able to induce variations in lipid composition in response to environmental stress. Here, we explore how the main lipid components in S. aureus are altered when exposed to LL-37, a human cathelicidin involved in primary immune response, and ATRA-1, a short antimicrobial peptide derived from the snake Naja atra venom. A lipidomic study is conducted through HPLC-MS-MS (LC-ESI-MS/MS) to quantify phosphatidylglycerol, cardiolipin, lysyl-phosphatidylglycerol, monogalacto- and digalacto-diacylglycerol, and carotenoids. In addition, menaquinones, responsible for electron transport during oxidative phosphorylation, were also quantified. Biophysical properties such as membrane electric surface potential and lipid packing were assessed. We find that lipid adaptation is specific to the type of antimicrobial peptide, where ATRA-1 mainly induces changes in the electric surface potential through variations in Lysyl-PG, while exposure to LL-37 changes carotenoid levels, inducing an increase in membrane rigidity as measured by FTIR. In addition, both peptides induce a reduction in menaquinone and DGDG levels. These findings highlight the role of membrane lipid remodeling as a peptide-specific response mechanism in S. aureus, with implications for the development of AMP-based therapies. HighlightsO_LIStaphylococcus aureus responds through shifts in lipid composition and membrane biophysical properties to exposure to the antimicrobial peptides LL-37 and ATRA-1. C_LIO_LIBoth LL-37 and ATRA-1 lead to shifts in the glycolipids MGDG and DGDG; two lipids involved in regulating negative membrane curvature stress and responsible for shifting resistance to antimicrobial peptide activity in Staphylococcus aureus. C_LIO_LILL-37 treatment leads to an overall reduction in carotenoid content in Staphylococcus aureus, including the carotenoid end-product staphyloxanthin and the precursor 4,4-diaponeurosporenoic acid. Both lipids regulate membrane biophysical properties and protect Staphylococcus aureus from oxidative stress. C_LIO_LIBoth LL-37 and ATRA-1 lead to a reduction in menaquinone levels, which are involved in the electron transport chain during oxidative phosphorylation. Reduction in these menaquinones have been associated to the formation of small colony variants that are often observed in chronic Staphylococcus aureus infections. C_LI

A methodology for computationally unstructuring proteins is described and the results of its application to a variety of proteins analyzed and discussed. Some proteins prove more susceptible than others, and fold topology plays a part in this. Alpha helical structure is found to be generally somewhat robust, and, perhaps unsurprisingly, unstructuring often begins at exposed chain termini. Phosphofructokinase-1 and phosphofructokinase-2, which have similar sizes but different fold topologies, are found to differ markedly in their unstructuring behaviour.

Hepatocellular carcinoma (HCC) is one the most lethal cancer types. While infections with hepatitis B or hepatitis C viruses remain the primary risk factors for HCC development worldwide, metabolic disease-associated steatohepatitis has become the fastest growing etiology of HCC, particularly in the West. Our study characterizes, for the first time, the remodeling of the translation landscape in steatohepatitis-associated HCC using ribosome profiling and mass spectrometry techniques. In striking contrast with the transcriptional alterations, which do not show strong dependence on associated co-morbidity, translational alterations are broader in the steatohepatitis compared to the viral background. These alterations affect hundreds of genes, whose products are often involved in subcellular protein localization. By quantifying the relationship between the ribosome occupancy of upstream or downstream open reading frames (uORFs or dORFs) and coding regions (CDSs), we provide evidence of uORFs attenuating the translation efficiency of CDSs in HCC. We also identify numerous novel translated regions, including from RNAs currently annotated as non-coding, some yielding peptides that can be reproducibly identified in mass spectrometry datasets. Our study thus provides novel molecular data on HCC with a focus on the steatohepatitis etiology, reveals unexpectedly extensive translational control in this co-morbidity, and gives multiple hints and candidate targets for follow-up experiments.

Staphylococcus aureus (S. aureus) is a clinically relevant pathogen capable of adapting its membrane composition in response to environmental stress. In this adaptive process, bacterial carotenoids play a crucial role. Although staphyloxanthin (STX) is the main carotenoid produced by the bacterium, S. aureus also synthesizes other pigmented intermediates that play an unknown role in regulating membrane biophysical properties. In this study, we purified 4,4-diaponeurosporenoic acid (4,4'-DNPA) from S. aureus carotenoid extracts and evaluated its effect on the thermotropic and biophysical properties of representative membrane models. The highly rigid triterpenoid 4,4'-DNPA is one of the last precursors in the biosynthesis of STX and is found in high concentrations in the stationary phase of S. aureus. Phase transition temperatures were determined using infrared spectroscopy, while interfacial hydration and hydrophobic core dynamics were investigated using fluorescence spectroscopy through Laurdan generalized polarization and DPH anisotropy. The results show that 4,4'-DNPA increases the main phase transition temperature of lipid bilayers in a concentration-dependent manner. This is in contrast to STX that decreases the transition temperature. This difference is consistent with the additional fatty acid present in STX that changes its effect on the phase behavior. Furthermore, 4,4'-DNPA reduced the interfacial hydration levels and restricted hydrophobic-core dynamics at higher concentrations, consistent with increased molecular order and stability. 4,4'-DNPA therefore complements STX in increasing membrane order and lipid packing. These findings support the notion that the production of bacterial carotenoids functions as a biophysical regulatory mechanism of lipid packing in S. aureus membranes.

Under inflammatory conditions, the ubiquitin-like modifier FAT10 serves as a tag for protein degradation by the 26S proteasome. FAT10 is degraded along with its substrates and this process is independent of the segregase VCP/p97, which, in the regular ubiquitin pathway of degradation, is required if a substrate lacks a disordered initiation region. FAT10 itself is loosely folded and its tendency to aggregate has complicated investigations of its structure, interaction, and function. Recently hydrogen-deuterium exchange in combination with mass spectrometry has suggested that, in preparation of degradation by the proteasome, the adapter protein NUB1 traps FAT10 in a mostly unfolded state by capturing a {beta}-strand. {beta}-strand capture was subsequently confirmed by magic-angle spinning (MAS) NMR spectroscopy of a stabilized variant of the N-domain of FAT10 in complex with NUB1L, the longer splice variant of NUB1. MAS NMR, in addition, revealed that the N-domain of FAT10 and NUB1L form a fuzzy complex and that the N-terminus of FAT10 is positioned for initiation of degradation by specific non-covalent interaction with NUB1L. Here, we report the investigation of the wild-type N-domain of FAT10 by MAS NMR. Co-sedimentation with NUB1L yields high-quality spectra, which enable sequential assignment of resonances. Through the lens of MAS NMR, the complexes of the wild-type and stabilized N-domain of FAT10 with NUB1L are identical. The N-terminus of FAT10 again shows up prominently in the spectra, even though the residue is this time an Ala, not a Gly. Our experience suggests that co-sedimentation in combination with MAS NMR is generally helpful in the exploration of conditional folds of intrinsically disordered proteins.

The HIV-1 Vpu protein aids viral adaptation by influencing host cell pathways via protein interactions. While Vpu is mainly found in plasma and endomembranes, we recently discovered a soluble form that forms a stable, equimolar complex with Ca2+-bound calmodulin (Ca2+-CaM), potentially affecting Vpus cellular trafficking. Here, to determine the binding affinity and identify regions of soluble Vpu involved in CaM binding, we used ensemble Forster Resonance Energy Transfer (eFRET). We tested Cy3-labeled full-length (FL) Vpu, a C-terminal fragment (helices 2 and 3), and a Cy3-labeled FL Vpu V22A/W23Y mutant with substitutions in Vpus helix 1. All Vpus variants were labeled at residue L42C, and Ca2+-CaM was tagged with Cy5 at residue S39C. eFRET analysis of 100 nM Cy3-Vpu variants mixed with Cy5-Ca2+-CaM (in the range 100-600 nM) revealed dissociation constants (Kd) and binding energies ({Delta}G) for heterocomplexes. FL Vpu-Ca2+-CaM showed high stability (Kd [~]40 nM,{Delta} G [~]10.1 kcal/mol), while the truncated C-terminal region and V22A/W23Y mutant formed less stable complexes with Ca2+-CaM (Kd[~]200 nM and 800 nM,{Delta} G [~]9 kcal/mol and [~]8.3 kcal/mol). This, a binding hot spot in Vpus CaM-binding motif in helix 1 was identified, which may control the stability of Vpu-Ca2+-CaM complex and Vpus insertion in the membrane: We hypothesize that upon delivery to the membrane, the hydrophobic helix 1 of Vpu dissociates from Ca2+-CaM and inserts in the lipid bilayer; thereafter, CaM dissociates from Vpu facilitated by the reduced Vpu-Ca2+-CaM complex stability. The findings from this study advance our understanding of HIV-1 Vpu interactions with cellular components and may aid the development of antivirals.

Mitochondrial Complex III dysfunction is frequently associated with pathogenic variants in the MT-CYB gene, yet the functional consequences of many missense substitutions remain unresolved because they are classified as variants of uncertain significance (VUS). One such variant, m.14849T>C (p.Ser35Pro), has been reported in patients with multisystem mitochondrial phenotypes, including septo-optic dysplasia, cardiomyopathy, and exercise intolerance, although its structural impact on Cytochrome b function remains unclear. In this study, we employed 300 ns all-atom molecular dynamics simulations to assess structural and energetic consequences of the S35P substitution in the Cytochrome b subunit of human mitochondrial Complex III. The S35P variant did not induce global destabilization of the protein scaffold but instead promoted localized perturbations within the heme bL microenvironment. The mutation was associated with loss of a heme-proximal hydrogen-bonding network involving Ser35 and a decrease in electrostatic interaction energy between the protein matrix and the heme bL cofactor. Radial distribution function analysis further supported loosening of local packing around the prosthetic group. Consistent with these local changes, dynamics analyses indicated increased flexibility in distal transmembrane helices that form the heme-pocket scaffold and greater variability in the inter-heme Fe(bL)-Fe(bH) distance. Together, our findings suggest that S35P may exert functional effects by reorganizing the heme bL microenvironment rather than by inducing large-scale structural destabilization, underscoring the value of structure- and dynamics-based evaluation for mitochondrial VUS and suggesting a plausible mechanistic link to the pathophysiology of multisystem mitochondrial diseases.

Inflammasomes are cytosolic innate immune sensors that, once activated by a pathogenic threat, lead to activation of the inflammatory Caspase-1. Inflammasome activation and its consequences have been studied extensively in myeloid cells and in overexpression systems. Recent studies have identified cell type specific effects that are not fully explained by the known cleavage targets of Caspase-1. Here, we identified targets of caspase cleavage using mass spectrometry in primary intestinal epithelial cells by specifically activating the NAIP-NLRC4 inflammasome. We have taken an unbiased approach and developed a novel method for analyzing mass spectrometry data for evidence of caspase activity. Our approach can also be applied to existing proteomic datasets to establish the presence of caspase activity under various biological conditions. These results lay the groundwork for future studies on mechanisms of caspase-induced processes such as intestinal epithelial cell extrusion.

Although several ancillary tests are available in limited laboratories, diagnosis of microphthalmia (MiT)/TFE family translocation renal cell carcinoma (tRCC) could be challenging due to diverse and overlapping tumor morphology and the lack of reliable biomarkers. GPNMB has been recently identified as a diagnostic marker for various renal neoplasms with FLCN/TSC/mTOR-TFE alterations. However, the sensitivity and specificity of GPNMB immunostain are suboptimal and the result interpretation in ambiguous cases could be difficult. To search additional biomarkers that could improve the screening sensitivity and predict genetic aberrations in FLCN/TSC/mTOR-TFE pathway in renal tumors, we performed bioinformatic analysis of publicly available cancer databases and found GPR143, a transmembrane protein regulated by MiT transcription factors, was highly expressed in a subset of renal cell carcinomas (RCCs). In two the Cancer Genome Atlas (TCGA) kidney cancer cohorts, RCCs with high levels of GPR143 expression were enriched for renal neoplasms with FLCN/TSC/mTOR-TFE alterations. Similar to GPNMB labeling, GPR143 immunostain was positive in the majority of tRCC cases and renal tumors with FLCN/TSC/mTOR alterations, suggesting that GPR143 could function as another surrogate marker for FLCN/TSC/mTOR-TFE alterations in certain renal tumors. Interestingly, despite the concordant GPR143 and GPNMB immunoreactivity in most renal neoplasms with FLCN/TSC/mTOR-TFE alterations, diffuse GPR143 immunostain was observed in some cases with negative or focal GPNMB labeling. Taken together, our results indicate GPR143 could serve as a useful adjunct marker to improve the sensitivity for screening renal tumors with FLCN/TSC/mTOR-TFE alterations.

Lipoprotein composition is altered in sepsis, and supplementation with high-density lipoproteins has been reported to improve outcomes in experimental settings. In this study, we aimed to investigate the nature and inter-individual variability in the lipoprotein proteome to inform risk stratification and opportunities for precision medicine approaches. In a large proteomic dataset including 1134 patients (1781 samples) with sepsis and 149 healthy volunteers, we analysed 18 protein components of lipoproteins. We characterise heterogeneity of the lipoprotein proteome, defining three step-wise sub-phenotypes associated with increasing disease severity, one close to health, then an early phase patient group showing increased abundance of proteins that integrate HDL under inflammatory conditions (SAA1 and SAA2), then a group with decreased abundance of proteins that are components of HDL under healthy conditions that was associated with higher organ failure intensity (SOFA score) and increased mortality. We developed and externally validated a quantitative score reflective of lipoproteins alterations in sepsis, and machine learning predictive models to predict the LP class, advancing future individualised lipoproteins-based therapeutics in sepsis.

The structural and functional characteristics of membrane proteins can be influenced by the composition of the membrane. Consequently, native membranes are most relevant for the study of receptors and other membrane proteins. In this study, we investigated two types of cell-derived vesicles: natively shed extracellular vesicles (EVs) and mechanically derived vesicles (MVs). To this end, we utilized the human breast cancer cell line SKBR3, which strongly overexpresses the receptor HER2. We designed a protocol based on designed ankyrin repeat proteins (DARPins) to purify EVs and MVs enriched in HER2, and to ensure the native orientation of the HER2 receptors within the vesicle. The isolated HER2-containing EVs and MVs were characterized by cryo-EM, cryo-electron tomography (cryo-ET) and mass spectrometry (MS), which revealed fundamental differences between the different vesicle types. Our study highlights the greater structural diversity of EVs over MVs. A single particle cryo-EM analysis and classification of all visible receptors on the vesicle surface yielded electron density consistent with HER2 at modest resolution. Taken together, our results suggest that MVs can serve better than EVs as a suitable platform for the structure determination of membrane proteins within their native membrane environments.

Intercalation of small molecules between DNA base pairs affects DNA conformation, disrupting essential cellular processes including replication, transcription, and repair. We investigated conformational changes in 18-mer DNA upon intercalation of doxorubicin, SYBR Gold and YOYO-1 using extensive MD simulations. Two main patterns for the intercalation were identified: RISE-type intercalation occurs between adjacent base pairs and extends the DNA helix with decreased twist angles, while OPEN-type intercalation proceeds through base-pair opening without significant DNA extension. Kinetic analysis revealed that association rates for intercalation followed the order: first YO-moiety (mono-intercalation) > SYBR Gold > doxorubicin > YOYO-1 (bis-intercalation). Free energy landscape showed that forces at DNA termini reached up to 117 pN during stretching. Notably, base pairs adjacent to intercalators were protected from strand separation, accompanied by additional helical unwinding. MM-PBSA/GBSA analysis revealed that the driving force for intercalation is the stacking energy, and the binding affinity was highest for minor groove binding. Persistence length decreased with single molecule binding but recovered with two molecules due to their electrostatic repulsion. Mechanical properties of intercalated DNA showed position-dependence, demonstrating that multiple intercalation modes coexist in solution. The heterogeneous nature of intercalation explains why experimental measurements reflect ensemble averages rather than single binding configurations.

This study investigates the interaction between the cationic antimicrobial peptide protamine and bacterial porin OmpF (E. coli) at the single-molecule level. Using high-resolution conductance measurements in planar lipid bilayers, strong voltage- and concentration-dependent ion current blockages with OmpF, indicating significant protamine binding were observed. Further analysis revealed that peptide length influences binding kinetics, with longer peptides showing reduced affinity and slower exchange rates. These findings demonstrate that OmpF is a tractable model for studying cationic peptide-channel interactions and translocation mechanisms relevant to antimicrobial action.