Biology subjects

O'Shaughnessy, B.

Publications and source records attributed to O'Shaughnessy, B..

17 recordsLinked to original sources

bioRxiv2026

Drosophila germ band extension: a two-state reshaping mechanism

During embryogenesis tissues are reshaped by diverse forces that elicit fluid-like or solid-like response, but the underlying principles that determine tissue state are unclear. Here we study Drosophila germ-band extension (GBE), a classic example of convergent-extension that extends the ventral germ band (GB) to the dorsal embryo surface. Using experimentally constrained biophysical modeling we find fluctuating internal cell-cell junctional myosin stresses excite cell intercalations that weakly perturb the anterior GB tissue into a fluidized state. Planar-polarized intercalation bias drives weak convergent-extension of the fluid. In the posterior GB, by contrast, external stress from adjacent tissue induces crystal-like ordering of the cells. External stress-induced cell intercalations mediate crystal defect annealing and plastic flow of the solid that wraps the GB around the narrow embryo posterior. Thus, the inhomogeneous reshaping challenge of GBE is dealt with by a two-state remodeling strategy in which a slowly remodeled anterior fluid coexists with a posterior solid undergoing fast plastic flow. Similar strategies find use in other contexts such as wound healing, where actomyosin boundary stresses drive crystal-like order that imposes shape regularity on the inner wound boundary for successful wound closure.

developmental biology↗

bioRxiv2025

Why are so many fusogens rod-shaped?

Molecular fusogens catalyze membrane fusion for many basic biological processes. In eukaryotic cells, SNARE proteins drive membrane fusion for trafficking and for exocytic release in many contexts, from neurotransmission to enzymatic digestion, while other fusogens mediate cell-cell fusion for organ formation, placental development and gamete fusion. Enveloped viruses use glycoprotein fusogens for host cell entry and delivery of the viral genome. Despite this breadth of roles, a structural feature shared by many of these fusogens is their rod shape, conserved across the SNARE superfamily, the class I and II fusogen superfamilies and the class III fusogen family. Here we used highly coarse-grained molecular dynamics (MD) simulations to examine the collective behavior of rod-like fusogens on the microscopically long timescales of physiological membrane fusion. Rod-generated entropic forces maintained a cleared fusion site, squeezed and hemifused the membranes, and then expanded and ruptured the hemifused connection to yield fusion. More fusogens generated higher entropic forces and faster fusion, consistent with electrophysiological measurements at neuronal synapses. The required fusogenic feature was the rod shape, since simulated SNARE complexes, class II EFF-1 fusogens, and model rod-shaped complexes entropically drove fusion along similar pathways, whereas globular complexes failed. Thus, rod-like fusogens are optimally shaped generators of entropic forces that drive membrane fusion. These results suggest a universal rod-based fusion mechanism may have been the evolutionary driver of structural convergence among major classes of eukaryotic and viral fusogens. SignificanceLarge classes of eukaryotic and viral membrane fusion complexes are rod-shaped, a common structural feature despite a multiplicity of functions, from neurotransmission to organogenesis to cell entry. Here we used coarse-grained molecular dynamics to access the millisecond timescales of physiological fusion. Simulations revealed a universal mechanism whereby, regardless of structural details, rod-shaped fusogens generate entropic forces that drive fusion. This suggests the mechanism was a major evolutionary driver of structural convergence and multiple transmissions between eukaryotic hosts and viruses. Rod-mediated fusion is faster with more fusogens, explaining why release at presynaptic nerve terminals is enhanced or suppressed depending on the number of active SNARE fusogens. Thus, modulation of SNARE numbers and fusogenicities may represent a significant mechanism of synaptic plasticity.

biophysics↗

bioRxiv2023

Actomyosin pulsing rescues embryonic tissue folding from disruption by myosin fluctuations

During early development, myosin II mechanically reshapes and folds embryo tissue. A much-studied example is ventral furrow formation in Drosophila, marking the onset of gastrulation. Furrowing is driven by contraction of actomyosin networks on apical cell surfaces, but how the myosin patterning encodes tissue shape is unclear, and elastic models failed to reproduce essential features of experimental cell contraction profiles. The myosin patterning exhibits substantial cell-to-cell fluctuations with pulsatile time-dependence, a striking but unexplained feature of morphogenesis in many organisms. Here, using biophysical modeling we find viscous forces offer the principle resistance to actomyosin-driven apical constriction. In consequence, tissue shape is encoded in the direction-dependent curvature of the myosin patterning which orients an anterior-posterior furrow. Tissue contraction is highly sensitive to cell-to-cell myosin fluctuations, explaining furrowing failure in genetically perturbed embryos whose fluctuations are temporally persistent. In wild-type embryos, this catastrophic outcome is averted by pulsatile myosin time-dependence, a time-averaging effect that rescues furrowing. This low pass filter mechanism may underlie the usage of actomyosin pulsing in diverse morphogenetic processes across many organisms.

developmental biology↗

bioRxiv2022

Three membrane fusion pore families determine the pathways of pore dilation

During exocytosis secretory vesicles fuse with a target membrane and release neurotransmitters, hormones or other bioactive molecules through a membrane fusion pore. The initially small pore may subsequently dilate for full contents release, as commonly observed in amperometric traces. The size, shape and evolution of the pore is critical to the course of contents release, but exact fusion pore solutions accounting for membrane tension and bending energy constraints have not been available. Here we obtained exact solutions for fusion pores between two membranes. We find three families: a narrow pore, a wide pore and an intermediate tether-like pore. For high tensions these are close to the catenoidal and tether solutions recently reported for freely hinged membrane boundaries. We suggest membrane fusion initially generates a stable narrow pore, and the dilation pathway is a transition to the stable wide pore family. The unstable intermediate pore is the transition state that sets the energy barrier for this dilation pathway. Pore dilation is mechanosensitive, as the energy barrier is lowered by increased membrane tension. Finally, we study fusion pores in nanodiscs, powerful systems for the study of individual pores. We show that nanodiscs stabilize fusion pores by locking them into the narrow pore family. SignificanceDuring neurotransmission, hormone release and other fundamental processes, secretory vesicles fuse their membranes with target membranes to release contents through an initially small membrane fusion pore that subsequently dilates. Dilation is assisted by proteins such as SNAREs and synaptotagmin. While macroscopic soap film shapes are well characterized, finding exact solutions for microscopic cellular membrane surfaces is made more complex by bending energy constraints. Here, computational analysis revealed three families of fusion pores between two membranes. Our work suggests membrane fusion generates a member of the narrow pore family, and pore dilation is a transition to the wide pore family. The energy barrier that SNAREs or synaptotagmin must surmount to achieve dilation is set by a third unstable intermediate pore family.

biophysics↗

bioRxiv2022

Actin turnover protects the cytokinetic contractile ring from structural instability

In common with other actomyosin contractile cellular machineries, actin turnover is required for normal function of the cytokinetic contractile ring. Cofilin is an actin-binding protein contributing to turnover by severing actin filaments, required for cytokinesis by many organisms. In fission yeast cofilin mutants, contractile rings suffer bridging instabilities in which actin bundles peel away from the plasma membrane into straight bridges. The origin of this behaviour is unclear. Here we used molecularly explicit simulations of the fission yeast contractile ring to examine the role of cofilin. Simulations reproduced the experimentally observed cycles of bridging and reassembly during constriction, each lasting [~] 6 min, and the tendency for bridging to occur in ring segments with low myosin II Myo2 density. The lack of cofilin severing produced [~] 2-fold longer filaments and, consequently, [~] 2-fold higher ring tensions. Simulations identified bridging as originating in the boosted ring tension, which increased centripetal forces that detached actin from Myo2 that anchored actin to the membrane. Thus, cofilin serves a critical role in cytokinesis by protecting the contractile ring from bridging, the principal structural threat. Summary statementMolecularly explicit simulations showed that cofilin-mediated actin severing protects the fission yeast cytokinetic contractile ring from instabilities in which actin peels away into straight bridges.

cell biology↗

bioRxiv2021

Dynamical transitions of the actomyosin cortex can trigger single cell morphogenesis

Morphogenetic changes driven by actomyosin contractile forces are well-characterized at the tissue level. At the single cell level, shape changes steered by actomyosin contractile forces include mitotic rounding and cytokinetic furrow ingression. In some cases, more complex shape transitions associated with spatial patterning of the cortex were observed. The actomyosin cortex was widely studied using active gel frameworks, and stabilized contractile instabilities were shown to generate patterns, but whether complex shapes can emerge from these cortical patterns is not established. Here we show that complex morphogenetic changes at the single cell level can accompany cortical patterns, using a minimal active gel model. For sufficiently low membranecortex drag, an initially homogeneous cortex spontaneously develops stripes associated with stable furrows, similar to furrowing observed in cells. Our work suggests that controlled cortical instability can trigger morphogenesis at the cellular level.

biophysics↗

bioRxiv2021

Contractile ring constriction and septation in fission yeast are integrated mutually stabilizing processes

In common with other cellular machineries, the actomyosin contractile ring that divides cells during cytokinesis does not operate in isolation. Contractile rings in animal cells interact with contiguous actomyosin cortex, while ring constriction in many cell-walled organisms couples tightly to cell wall growth. In fission yeast, a septum grows in the wake of the constricting ring, ensuring cytokinesis leaves two daughter cells fully enclosed by cell wall. Here we mathematical modeled the integrated constriction-septation system in fission yeast, with a kinetic growth model evolving the 3D septum shape coupled to a molecularly explicit simulation of the contractile ring highly constrained by experimental data. Simulations revealed influences in both directions, stabilizing the ring-septum system as a whole. By providing a smooth circular anchoring surface for the ring, the inner septum leading edge stabilized ring organization and tension production; by mechanically regulating septum circularity and in-plane growth, ring tension stabilized septum growth and shape. Genetic or pharmacological perturbation of either subsystem destabilized this delicate balance, precipitating uncontrolled positive feedback with disastrous morphological and functional consequences. Thus, high curvature septum irregularities triggered bridging instabilities, in which contractile ring segments became unanchored. Bridging abolished the local tension-mediated septum shape regulation, exacerbating the irregularity in a mutually destabilizing runaway process. Our model explains a number of previously mysterious experimental observations, including unanchoring of ring segments observed in cells with mutations in the septum-growing {beta}-glucan synthases, and irregular septa in cells with mutations in the contractile ring myosin-II Myo2. Thus, the contractile ring and cell wall growth cellular machineries operate as a single integrated system, whose stability relies on mutual regulation by the two subsystems.

cell biology↗

bioRxiv2021

Why are SNAREpins rod-shaped?

SNARE proteins are the core components of the cellular machineries that fuse membranes for neurotransmitter or hormone release and other fundamental processes. Fusion is accomplished when SNARE proteins hosted by apposing membranes form SNARE complexes called SNAREpins, but the mechanism of fusion remains unclear. Computational simulations of SNARE-mediated membrane fusion are challenging due to the millisecond timescales of physiological membrane fusion. Here we used ultra-coarse-grained (UCG) simulations to investigate the minimal requirements for a molecular intracellular fusogen, and to elucidate the mechanisms of SNARE-mediated fusion. We find fusion by simple body forces that push vesicles together is highly inefficient. Inter-vesicle fusogens with different aspect ratios can fuse vesicles only if they are rodlike, of sufficient length to clear the fusogens from the fusion site by entropic forces. Simulations with rod-shaped SNAREpin-like fusogens fused 50-nm vesicles on ms timescales, driven by entropic forces along a reproducible fusion pathway. SNARE-SNARE and SNARE-membrane entropic forces cleared the fusion site and pressed the vesicles into an extended contact zone (ECZ), drove stalk nucleation at the high curvature ECZ boundary, and expanded the stalk into a long-lived hemifusion diaphragm in which a simple pore completed fusion. Our results provide strong support for the entropic hypothesis of SNARE-mediated membrane fusion, and implicate the rodlike structure of the SNAREpin complex as a necessity for entropic force production and fusion.

biophysics↗

bioRxiv2021

A hemifused complex is the hub in a network of pathways to membrane fusion

Membrane fusion is a critical step for many essential processes, from neurotransmission to fertilization. For over 40 years protein-free fusion driven by calcium or other cationic species has provided a simplified model of biological fusion, but the mechanisms remain poorly understood. Cation-mediated membrane fusion and permeation are essential in their own right to drug delivery strategies based on cell-penetrating peptides or cation-bearing lipid nanoparticles. Experimental studies suggest calcium drives anionic membranes to a hemifused intermediate which constitutes a hub in a network of pathways, but the pathway selection mechanism is unknown. Here we develop a mathematical model that identifies the network hub as a highly dynamical hemifusion complex. Multivalent cations drive expansion of this high tension hemifusion interface between interacting vesicles during a brief transient. The fate of this interface determines the outcome, either fusion, dead-end hemifusion or vesicle lysis. The model reproduces the unexplained finding that calcium-driven fusion of vesicles with planar membranes typically stalls at hemifusion, and we show the equilibrated hemifused state is a novel lens-shaped complex. Thus, membrane fusion kinetics follow a stochastic trajectory within a network of pathways, with outcome weightings set by a hemifused complex intermediate. SignificanceCells use multicomponent machineries to fuse membranes for neurotransmitter and hormone release and other fundamental processes. Protein-free fusion using calcium or other multivalent cationic fusogens has long been studied as a simplifying model. Cation-mediated membrane fusion or permeation are key events for a number of current drug delivery strategies. However, the mechanisms determining outcomes are unknown. Here we develop a mathematical model that identifies a dynamic hemifusion complex as the decision hub that stochastically sets the outcome in a network of pathways. Cations transiently grow a high tension hemifusion interface between membrane-enclosed compartments, whose fate governs whether fusion, dead-end hemifusion or vesicle lysis occurs. The model provides a systematic framework to predict outcomes of cationic fusogen-mediated interactions between membrane-enclosed compartments.

biophysics↗

bioRxiv2021

Myosins generate contractile force and maintain organization in the cytokinetic contractile ring

During cytokinesis cells assemble an actomyosin contractile ring whose tension constricts and divides cells, but the ring tension was rarely measured. Actomyosin force generation is well understood for the regular sarcomeric architecture of striated muscle, but recent super-resolution studies of fission yeast contractile rings revealed organizational building blocks that are not sarcomeres but irregularly positioned plasma membrane-anchored protein complexes called nodes. Here we measured contractile ring tensions in fission yeast protoplast cells. The myosin II isoforms Myo2 and Myp2 generated the tension, with a ~2-fold greater contribution from Myo2. Simulations of a molecularly detailed ring model revealed a sliding node mechanism for tension, where nodes hosting tense actin filaments were pulled bidirectionally around the ring. Myo2 and Myp2 chaperoned self-assembling components into the ring organization, and anchored the ring against bridging instabilities. Thus, beyond force production, Myo2 and Myp2 are the principal organizers, bundlers and anchors of the contractile ring.

cell biology↗

bioRxiv2021

Large fluctuations of the fusion intermediate help SARS-CoV-2 capture host cell membranes

Cell entry by SARS-CoV-2 is accomplished by the S2 subunit of the spike S protein on the virion surface by capture of the host cell membrane and fusion with the viral envelope. Capture and fusion require the prefusion S2 to transit to its potent, fusogenic form, the fusion intermediate (FI). However, the FI structure is unknown, detailed computational models of the FI are unavailable, and the mechanisms and timing of membrane capture and fusion are not established. Here, we constructed a full-length model of the CoV-2 FI by extrapolating from known CoV-2 pre- and postfusion structures. In atomistic and coarse-grained molecular dynamics simulations the FI was remarkably flexible and executed large bending and extensional fluctuations due to three hinges in the C-terminal base. Simulations suggested a host cell membrane capture time of [~] 2 ms. Isolated fusion peptide simulations identified an N-terminal helix that directed and maintained binding to the membrane but grossly underestimated the binding time, showing that the fusion peptide environment is radically altered when attached to its host fusion protein. The large configurational fluctuations of the FI generated a substantial exploration volume that aided capture of the target membrane, and may set the waiting time for fluctuation-triggered refolding of the FI that draws the viral envelope and host cell membrane together for fusion. These results describe the FI as a machinery designed for efficient membrane capture and suggest novel potential drug targets. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=117 SRC="FIGDIR/small/439051v3_ufig1.gif" ALT="Figure 1"> View larger version (29K): org.highwire.dtl.DTLVardef@1374b59org.highwire.dtl.DTLVardef@133d21forg.highwire.dtl.DTLVardef@127f2ddorg.highwire.dtl.DTLVardef@1b9ab8f_HPS_FORMAT_FIGEXP M_FIG C_FIG

biophysics↗

bioRxiv2021

Myosin turnover controls actomyosin contractile instability

Actomyosin contractile force is harnessed for diverse functions, from cell division to morphogenesis during development. However, actomyosin contractility is intrinsically unstable to self-reinforcing spatial variations that destroy actomyosin architecture if unopposed. The full instability was rarely observed, and how cells control the instability is not established. Here, we observed the instability run its full course in isolated cytokinetic contractile rings lacking component turnover. Myosin II aggregated hierarchically into aggregates of growing size and separation up to a maximum. Molecularly explicit simulations reproduced hierarchical aggregation that precipitated tension loss and ring fracture, and identified the maximum separation as the length of actin filaments mediating mechanical communication between aggregates. Late stage simulated aggregates had aster-like morphology with polarity sorted actin, similar to late stage actomyosin systems in vitro. Our results suggest myosin II turnover controls actomyosin contractile instability in normal cells, setting myosin aggregate size and intercepting catastrophic hierarchical aggregation and fracture.

cell biology↗

bioRxiv2021

Synaptotagmin rings as high sensitivity regulators of synaptic vesicle docking and fusion

Synchronous release at neuronal synapses is accomplished by a machinery that senses calcium influx and fuses the synaptic vesicle and plasma membranes to release neurotransmitters. Previous studies suggested the calcium sensor Synaptotagmin (Syt) is a facilitator of vesicle docking and both a facilitator and inhibitor of fusion. On phospholipid monolayers, the Syt C2AB domain spontaneously oligomerized into rings that are disassembled by Ca2+, suggesting Syt rings may clamp fusion as membrane-separating "washers" until Ca2+-mediated disassembly triggers fusion and release (Wang et al., 2014). Here we combined mathematical modeling with experiment to measure mechanical properties of Syt rings and to test this mechanism. Consistent with experiment, the model quantitatively recapitulates observed Syt ring-induced dome and volcano shapes on phospholipid monolayers, and predicts rings are stabilized by anionic phospholipid bilayers or bulk solution with ATP. The selected ring conformation is highly sensitive to membrane composition and bulk ATP levels, a property that may regulate vesicle docking and fusion in ATP-rich synaptic terminals. We find the Syt molecules hosted by a synaptic vesicle oligomerize into a halo, unbound from the vesicle, but in proximity to sufficiently PIP2-rich plasma membrane (PM) domains the PM-bound trans Syt ring conformation is preferred. Thus, the Syt halo serves as landing gear for spatially directed docking at PIP2-rich sites that define the active zones of exocytotic release, positioning the Syt ring to clamp fusion and await calcium. Our results suggest the Syt ring is both a Ca2+-sensitive fusion clamp and a high-fidelity sensor for directed docking. SignificanceSynchronous neurotransmitter release relies on directed docking of synaptic vesicles at active zones in axon terminals, where calcium influx activates membrane fusion and release. In vitro, the calcium sensor Synaptotagmin oligomerizes into rings disassembled by calcium. Here, experiment and modeling suggest the Synaptotagmin molecules hosted by an undocked vesicle oligomerize into a tethered, unbound halo in ATP-rich synaptic terminals. The halo directs vesicle docking to PIP2-rich plasma membrane domains in active zones, where the trans-bound ring conformation is favored, interposed between the membranes to clamp fusion until calcium triggers ring disassembly and neurotransmitter release. The mechanism exploits the extreme sensitivity of Synaptotagmin ring binding preferences to solution and membrane composition, with ~15 -fold-enhanced sensitivity for rings of ~15 molecules.

neuroscience↗

bioRxiv2020

Membrane pore energetics and the pathways to membrane rupture

Biological membranes owe their strength and low permeability to the phospholipid bilayers at their core. Membrane strength is determined by the energetics and dynamics of membrane pores, whose tension-dependent nucleation and growth leads to rupture. Creation of nanoscale membrane pores is central to exocytosis, trafficking and other processes fundamental to life that require breaching of secure plasma or organelle membranes, and is the basis for biotechnologies using drug delivery, delivery of genetic material for gene editing and antimicrobial peptides. A prevailing view from seminal electroporation and membrane rupture studies is that pore growth and bilayer rupture are controlled by macroscopically long-lived metastable defect states that precede fully developed pores. It was argued that defect nucleation becomes rate-limiting at high tensions, explaining the exponential tension-dependence of rupture times [E. Evans et al., Biophys. J. 85, 2342-2350 (2003)]. Here we measured membrane pore free energies and bilayer rupture using highly coarse-grained simulations that probe very long time scales. We find no evidence of metastable pore states. At lower tensions, small hydrophobic pores mature into large hydrophilic pores on the pathway to rupture, with classical tension dependence of rupture times. Above a critical tension membranes rupture directly from a small hydrophobic pore, and rupture times depend exponentially on tension. Thus, we recover the experimentally reported regimes, but the origin of the high tension exponential regime is unrelated to macroscopically long-lived pre-pore defects. It arises because hydrophilic pores cannot exist above a critical tension, leading to radically altered pore dynamics and rupture kinetics.Competing Interest StatementThe authors have declared no competing interest.View Full Text

biophysics↗

bioRxiv2019

The neuronal calcium sensor Synaptotagmin-1 and SNARE proteins cooperate to dilate fusion pores mechanically

All membrane fusion reactions proceed through an initial fusion pore, including calcium-triggered release of neurotransmitters and hormones. Expansion of this small pore to release cargo is energetically costly and regulated by cells, but the mechanisms are poorly understood. Here we show that the neuronal/exocytic calcium sensor Synaptotagmin-1 (Syt1) promotes expansion of fusion pores induced by SNARE proteins. Pore dilation relied on calcium-induced insertion of the tandem C2 domain hydrophobic loops of Syt1 into the membrane, previously shown to reorient the C2 domain. Mathematical modelling suggests that C2B reorientation rotates a bound SNARE complex so that it exerts force on the membranes in a mechanical lever action that increases the height of the fusion pore, provoking pore dilation to offset the bending energy penalty. We conclude that Syt1 exerts novel non-local calcium-dependent mechanical forces on fusion pores that dilate pores and assist neurotransmitter and hormone release. SIGNIFICANCE STATEMENTDuring neurotransmitter release, calcium-induced membrane insertion of the C2B domain of Synaptotagmin re-orients the bound SNARE complex which dilates the fusion pore in a mechanical lever action.

biophysics↗

bioRxiv2019

Anchoring of actin to the plasma membrane enables tension production in the fission yeast cytokinetic ring

The cytokinetic ring generates tensile force that drives cell division, but how tension emerges from the relatively disordered ring organization remains unclear. Long ago a muscle-like sliding filament mechanism was proposed, but evidence for sarcomeric order is lacking. Here we present quantitative evidence that in fission yeast ring tension originates from barbed-end anchoring of actin filaments to the plasma membrane, providing resistance to myosin forces which enables filaments to develop tension. The role of anchoring was highlighted by experiments on isolated fission yeast rings, where sections of ring unanchored from the membrane and shortened ~30-fold faster than normal [Mishra M., et al. (2013) Nat Cell Biol 15(7):853-859]. The dramatically elevated constriction rates are unexplained. Here we present a molecularly explicit simulation of constricting partially anchored rings as studied in these experiments. Simulations accurately reproduced the experimental constriction rates, and showed that following anchor release a segment becomes tensionless and shortens via a novel non-contractile reeling-in mechanism at about the load-free myosin-II velocity. The ends are reeled in by barbed-end-anchored actin filaments in adjacent segments. Other actin anchoring schemes failed to constrict rings. Our results quantitatively support a specific organization and anchoring scheme that generates tension in the cytokinetic ring.

cell biology↗

bioRxiv2017

Single-molecule force spectroscopy of protein-membrane interactions

Many biological processes rely on protein-membrane interactions in the presence of mechanical forces, yet high resolution methods to quantify such interactions are lacking. Here, we describe a single-molecule force spectroscopy approach to quantify membrane binding of C2 domains in Synaptotagmin-1 (Syt1) and Extended Synaptotagmin-2 (E-Syt2). Syts and E-Syts bind the plasma membrane via multiple C2 domains, bridging the plasma membrane with synaptic vesicles or endoplasmic reticulum to regulate membrane fusion or lipid exchange, respectively. In our approach single proteins attached to membranes supported on silica beads are pulled by optical tweezers, allowing membrane binding and unbinding transitions to be measured with unprecedented spatiotemporal resolution. C2 domains from either protein resisted unbinding forces of 2-7 pN and had binding energies of 4-14 kBT per C2 domain. Regulation by bilayer composition or Ca2+ recapitulated known properties of both proteins. The method can be widely applied to study protein-membrane interactions.

biophysics↗