Biology subjects

McDargh, Z. A.

Publications and source records attributed to McDargh, Z. A..

9 recordsLinked to original sources

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

Atomic-Resolution Prediction of Degrader-mediated Ternary Complex Structures by Combining Molecular Simulations with Hydrogen Deuterium Exchange

Targeted protein degradation (TPD) has emerged as a powerful approach in drug discovery for removing (rather than inhibiting) proteins implicated in diseases. A key step in this approach is the formation of an induced proximity complex, where a degrader molecule recruits an E3 ligase to the protein of interest (POI), facilitating the transfer of ubiquitin to the POI and initiating the proteasomal degradation process. Here, we address three critical aspects of the TPD process: 1) formation of the ternary complex induced by a degrader molecule, 2) conformational heterogeneity of the ternary complex, and 3) assessment of ubiquitination propensity via the full Cullin Ring Ligase (CRL) macromolecular assembly. The novel approach presented here combines experimental biophysical data--in this case hydrogen-deuterium exchange mass spectrometry (HDX-MS, which measures the solvent exposure of protein residues)--with all-atom explicit solvent molecular dynamics (MD) simulations aided by enhanced sampling techniques to predict structural ensembles of ternary complexes at atomic resolution. We present results demonstrating the efficiency, accuracy, and reliability of our approach to predict ternary structure ensembles using the bromodomain of SMARCA2 (SMARCA2BD) with the E3 ligase VHL as the system of interest. The simulations reproduce X-ray crystal structures - including prospective simulations validated on a new structure that we determined in this work (PDB ID: 7S4E) - with root mean square deviations (RMSD) of 1.1 to 1.6 [A]. The simulations also reveal a structural ensemble of low-energy conformations of the ternary complex within a broad energy basin. To further characterize the structural ensemble, we used snapshots from the aforementioned simulations as seeds for Hamiltonian replica exchange molecular dynamics (HREMD) simulations, and then perform 7.1 milliseconds of aggregate simulation time using Folding@home. The resulting free energy surface identifies the crystal structure conformation within a broad low-energy basin and the dynamic ensemble is consistent with solution-phase biophysical experimental data (HDX-MS and small-angle x-ray scattering, SAXS). Finally, we graft structures from the ternary complexes onto the full CRL and perform enhanced sampling simulations, where we find that differences in degradation efficiency can be explained by the proximity distribution of lysine residues on the POI relative to the E2-loaded ubiquitin. Several of the top predicted ubiquitinated lysine residues are validated prospectively through a ubiquitin mapping proteomics experiment.

biophysics↗

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

SNAREs and Synaptotagmin cooperatively determine the Ca2+ sensitivity of neurotransmitter release in fixed stoichiometry modules

Neurotransmitter release is accomplished by a multi-component machinery including the membrane-fusing SNARE proteins and Ca2+-sensing Synaptotagmin molecules. However, the Ca2+ sensitivity of release was found to increase or decrease with more or fewer SNARE complexes at the release site, respectively, while the cooperativity is unaffected (Acuna et al., 2014; Arancillo et al., 2013), suggesting that there is no simple division of labor between these two components. To examine the mechanisms underlying these findings, we developed molecular dynamics simulations of the neurotransmitter release machinery, with variable numbers of Synaptotagmin molecules and assembled SNARE complexes at the release site. Ca2+ uncaging simulations showed that increasing the number of SNARE complexes at fixed stoichiometric ratio of Synaptotagmin to SNAREs increased the Ca2+ sensitivity without affecting the cooperativity. The physiological cooperativity of ~4-5 was reproduced with 2-3 Synaptotagmin molecules per SNARE complex, suggesting that Synaptotagmin and SNAREs cooperate in fixed stoichiometry modules. In simulations of action potential-evoked release, increased numbers of Synaptotagmin-SNARE modules increased release probability, consistent with experiment. Our simulations suggest that the final membrane fusion step is driven by SNARE complex-mediated entropic forces, and by vesicle-tethering forces mediated by the long Synaptotagmin linker domains. In consequence, release rates are increased when more SNARE complexes and Synaptotagmin monomers are present at the fusion site.

neuroscience↗

bioRxiv2021

Coupling of Ca2+-triggered unclamping and membrane fusion during neurotransmitter release

Neurotransmitter (NT) release is accomplished by a machinery that unclamps fusion in response to calcium and then fuses the synaptic vesicle and plasma membranes. These are often thought of as distinct tasks assigned to non-overlapping components. Vesicle release rates have a power law dependence on [Ca2+] with an exponent of 3-5, long taken to indicate that 3-5 Ca2+ ions bind the calcium sensor Synaptotagmin to trigger release. However, dependencies at low [Ca] are inconsistent with simple sequential binding to a single Ca2+ sensor followed by a final fusion step. Here we developed coarse-grained molecular dynamics simulations of the NT release machinery accounting for Synaptotagmin-mediated unclamping and SNARE-mediated fusion. Calcium-triggered unclamping and SNARE-mediated fusion emerged from simulations as contemporaneous, coupled processes. Increasing cytosolic [Ca2+], the instantaneous fusion rate increased as SNAREpins were progressively and reversibly released by dissociation of Synaptotagmin-SNAREpin complexes. Simulations reproduced the observed dependence of release rates on [Ca2+], but the power law was unrelated to the number of Ca2+ ions required. Action potential-evoked vesicle release probabilities depended on the number of transiently unclamped SNAREpins, explaining experimental dependencies of release probabilities on both unclamping and membrane-fusing machinery components. These results describe a highly cooperative NT release machinery with intrinsically inseparable unclamping and membrane-fusing functionalities.

neuroscience↗

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

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↗