Membrane-protein interactions play a central role in membrane mediated cellular processes ranging from signaling, budding, and fusion, to transport across the cell membrane. Of particular significance is the process of efficient protein olgomerization and transmembrane pore formation on the membrane surface; the primary virulent pathway for the action of antimicrobial peptides and pore forming toxins (PFTs). The suggested nanoscopic length scales and dynamic nature of such membrane lipid-protein interactions makes their detection extremely challenging. Using a combination of super-resolution stimulated emission depletion nanoscopy with fluorescence correlation spectroscopy (STED-FCS) we unravel the emergence of nanoscale lateral heterogeneity in supported bilayer membranes made up of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol upon interaction with the PFT, listeriolysin O (LLO). A distinct length scale-dependent dynamical crossover (<200 nm) from a Brownian diffusive regime is observed at 33 and 50% cholesterol compositions, indicating the partitioning of lipids into domains with variable cholesterol content. At 25% cholesterol content, this dyamical crossover is observed only in bilayers incubated with LLO providing evidence for the existence of sub ∼100 nm dynamical lipid nanodomains bound to LLO pore assemblies. By introducing asymmetry in cholesterol composition across the bilayer leaflets we infer that this domain formation is driven largely due to active cholesterol sequestration and transient trapping of lipids to the membrane bound motifs present in the toxins, en route to LLO oligomerization and subsequent pore formation. Bilayers prepared with labeled lipids present in either the proximal or distal leaflet allow us to track the dynamical perturbation in a leaflet-dependent manner upon LLO incubation. From the differences in the extent and intensity of the dynamical crossover as observed with STED-FCS, these experiments reveal that the affinity for cholesterol in the membrane binding motifs of the LLO subdomains induce cholesterol and lipid reorganization to a greater extent in the distal (upper) leaflet when compared with the proximal (lower) leaflet. The observed length scale-dependent membrane reorganization that occurs due to invasion by LLO could be generalized to other cholesterol-dependent cytolysins and emphasizes the significant advantage of using super-resolution STED nanoscopy to unravel complex lipid-protein interactions in membrane and cellular biophysics.

Microcavity supported lipid bilayers (MSLB) are contact-free membranes suspended across aqueousfilled pores that maintain the lipid bilayer in a highly fluidic state and free from frictional interactions with substrate. Such platforms offer the prospect of liposome-like fluidity with the compositional versatility and addressability of supported lipid bilayers and thus offer significant opportunity for modelling membrane asymmetry, protein-membrane interactions and aggregation at the membrane interface. Herein, we evaluate their performance by studying the effect of transmembrane lipid asymmetry on lipid diffusivity, membrane viscosity and cholera toxin-ganglioside recognition across six symmetric and asymmetric membranes including binary compositions containing both fluid and gel phase, and ternary phase separated membrane compositions. Fluorescence lifetime correlation spectroscopy (FLCS) was used to determine the lateral mobility of lipid and protein, and electrochemical impedance spectroscopy (EIS) enabled detection of protein-membrane assembly over the nanomolar range. Transmembrane leaflet asymmetry was observed to have profound impact on membrane electrochemical resistance where the resistance of a ternary symmetric phase separated bilayer was found to be at least 2.6 times higher than the asymmetric bilayer with analogous composition at the distal leaflet but where the lower leaflet comprised only 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Similarly, the diffusion coefficient for MSLBs was observed to be 2.5 fold faster for asymmetric MSLBs where the lower leaflet is DOPC alone. Our results demonstrate that interplay of lipid packing across both membrane leaflets and concentration of GM1 both affect the extent of cholera toxin aggregation and consequent diffusion of the cholera-GM1 aggregates. Given that true biomembranes are both fluidic and asymmetric, MSLBs offer the opportunity for building greater biomimicry into biophysical models and the approach described demonstrates the value of MSLBs in studying aggregation and membrane associated multivalent interactions prevalent in many carbohydrates mediated processes.

Many drugs have intracellular or membrane-associated targets thus understanding their interaction with the cell membrane is of value in drug development. Cell-free tools used to predict membrane interactions should replicate the molecular organization of the membrane. Microcavity array supported lipid bilayer (MSLB) platform are versatile biophysical models of the cell membrane that combine liposome-like membrane fluidity with stability and addressability. We used an MSLB herein to interrogate drugmembrane interactions across seven drugs from different classes, including non-steroidal antiinflammatories; Ibuprofen (Ibu) and Diclofenac (Dic), antibiotics; Rifampicin (Rif), Levofloxacin (Levo) and Pefloxacin (Pef), and bisphosphonates; Alendronate (Ale) and Clodronate (Clo). Fluorescence lifetime correlation spectroscopy (FLCS) and electrochemical impedance spectroscopy (EIS) were used to evaluate the impact of drug on DOPC and binary bilayers over physiologically relevant drug concentrations. Whereas FLCS data revealed Ibu, Levo, Pef, Ale and Clo had no impact on lipid lateral

Raft-like functional domains with putative sizes of 20–200 nm and which are evolving dynamically are believed to be the most crucial regions in cellular membranes which determine cell signaling and various functions of cells.

Cell membranes are believed to be highly complex dynamical systems having compositional heterogeneity involving several types of lipids and proteins as the major constituents. This dynamical and compositional heterogeneity is suggested to be critical to the maintenance of active functionality and response to chemical, mechanical, electrical and thermal stresses. However, delineating the various factors responsible for the spatio-temporal response of actual cell membranes to stresses can be quite challenging. In this work we show how biomimetic phospholipid bilayer membranes with variable lipid fluidity determine the optimal assembly mechanism of the pore-forming protein, listeriolysin O (LLO), belonging to the class of cholesterol dependent cytolysins (CDCs). By combining atomic force microscopy (AFM) and super-resolution stimulated emission depletion (STED) microscopy imaging on model membranes, we show that pores formed by LLO in supported lipid bilayers can have variable conformation and morphology depending on the fluidity of the bilayer. At a fixed cholesterol concentration, pores formed in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) membranes were larger, flexible and more prone to coalescence when compared with the smaller and more compact pores formed in the lower fluidity 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes. In contrast, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes did not show any evidence of pore formation. Fluorescence correlation spectroscopy (FCS) in STED mode revealed the appearance of a length scale, ξ, below which lipid dynamics, under the influence of LLO protein binding and assembly, becomes anomalous. Interestingly, the magnitude of ξ is found to correlate with both lipid fluidity and pore dimensions (and flexibility) in DOPC and POPC bilayers. However this length scale dependent crossover, signalling the onset of anomalous diffusion, was not observed in DMPC bilayers. Our study highlights the subtle interplay of lipid membrane mediated protein assembly and lipid fluidity in determining proteo-lipidic complexes formed in biomembranes and the significant insight that STED microscopy provides in unraveling critical aspects of nanoscale membrane biophysics.