Biology subjects

Veatch, S.

Publications and source records attributed to Veatch, S..

5 recordsLinked to original sources

bioRxiv2021

Membrane phase separation drives organization at B cell receptor clusters

Heterogeneity in intact cell plasma membranes has been explained by analogy to coexisting liquid-ordered and liquid-disordered phases, although models based on this idea fall short of describing the rich structure within cell membranes. Here, a new framework of lipid-driven plasma membrane heterogeneity is presented, drawing on quantitative measurements of protein partitioning and dynamics within B cell receptor clusters in live B lymphocyte plasma membranes, compared to coexisting phases in isolated plasma membranes. We propose that membrane domains in cells integrate the thermodynamic state of the membrane and the magnitude of the applied stimulus to give rise to a tunable response. This framework is supported through functional observations of B cell receptor phosphorylation in perturbed systems.

biophysics↗

bioRxiv2021

Surface Densities Prewet a Near-Critical Membrane

Recent work has highlighted roles for thermodynamic phase behavior in diverse cellular processes. Proteins and nucleic acids can phase separate into three-dimensional liquid droplets in the cytoplasm and nucleus and the plasma membrane of animal cells appears tuned close to a two-dimensional liquid-liquid critical point. In some examples, cytoplasmic proteins aggregate at plasma membrane domains, forming structures such as the post-synaptic density and diverse signaling clusters. Here we examine the physics of these surface densities, employing minimal simulations of co-acervating polymers coupled to an Ising membrane surface in conjunction with a complementary Landau theory. We argue that these surface densities are a novel phase reminiscent of pre-wetting, in which a molecularly thin three-dimensional liquid forms on a usually solid surface. However, in surface densities the solid surface is replaced by a membrane with an independent propensity to phase separate. We show that proximity to criticality in the membrane dramatically increases the parameter regime in which a pre-wetting-like transition occurs, leading to a broad region where coexisting surface phases can form even when a bulk phase is unstable. Our simulations naturally exhibit three surface phase coexistence even though both the membrane and the polymer bulk can only display two phase coexistence on their own. We argue that the physics of these surface densities enables diverse functions seen in Eukaryotic cells.

biophysics↗

bioRxiv2020

Mutant EGFR is a preferred SMURF2 substrate of ubiquitination: role in enhanced receptor stability and TKI sensitivity

We previously reported that differential protein degradation of TKI-sensitive [L858R, del(E746-A750)] and resistant (T790M) epidermal growth factor receptor (EGFR) mutants upon erlotinib treatment correlates with drug sensitivity. However, the molecular mechanism remains unclear. We also reported SMAD ubiquitination regulatory factor 2 (SMURF2) ligase activity is important in stabilizing EGFR. Here, using in vitro and in vivo ubiquitination assays, mass spectrometry, and super-resolution microscopy, we show SMURF2-EGFR functional interaction is critical in receptor stability and TKI sensitivity. We found that L858R/T790M EGFR is a preferred substrate of SMURF2-UBCH5 (an E3-E2) complex-mediated K63-linked polyubiquitination, which preferentially stabilizes mutant receptor. We identified four lysine (K) residues (K721, 846, 1037 and 1164) as the sites of ubiquitination and replacement of K to acetylation-mimicking asparagine (Q) at K1037 position in L858R/T790M background converts the stable protein sensitive to erlotinib-induced degradation. Using STochastic Optical Reconstruction Microscopy (STORM) imaging, we show that SMURF2 presence allows longer membrane retention of activated EGFR upon EGF treatment, whereas, siRNA-mediated SMURF2 knockdown fastens receptor endocytosis and lysosome enrichment. In an erlotinib-sensitive PC9 cells, SMURF2 overexpression increased EGFR levels with improved erlotinib tolerance, whereas, SMURF2 knockdown decreased EGFR steady state levels in NCI-H1975 and PC9-AR cells to overcome erlotinib and AZD-9291 resistance respectively. Additionally, by genetically altering the SMURF2-UBCH5 complex formation destabilized EGFR. Together, we propose that SMURF2-mediated preferential polyubiquitination of L858R/T790M EGFR may be competing with acetylation-mediated receptor internalization to provide enhanced receptor stability and that disruption of the E3-E2 complex may be an attractive alternate to overcome TKI resistance.

cancer biology↗

bioRxiv2019

Criticality of plasma membrane lipids reflects activation state of macrophage cells

Signalling is of particular importance in immune cells, and upstream in the signalling pathway many membrane receptors are functional only as complexes, co-locating with particular lipid species. Work over the last 15 years has shown that plasma membrane lipid composition is close to a critical point of phase separation, with evidence that cells adapt their composition in ways that alter the proximity to this thermodynamical point. Macrophage cells are a key component of the innate immune system, responsive to infections, regulating the local state of inflammation. We investigate changes in the plasma membranes proximity to the critical point, as a response to stimulation by various pro- and anti-inflammatory agents. Pro-inflammatory (IFN-{gamma}, Kdo-LipidA, LPS) perturbations induce an increase in the transition temperature of the GMPVs; anti-inflammatory IL4 has the opposite effect. These changes recapitulate complex plasma membrane composition changes, and are consistent with lipid criticality playing a master regulatory role: being closer to critical conditions increases membrane protein activity.

biophysics↗

bioRxiv2019

Membrane domains and phosphatase exclusion produce robust signaling responses in B cells engaged with natural ligands and artificial cross-linkers presented on lipid bilayers.

B cells respond to a wide variety of antigens with varying valiancy and mode of presentation to the highly expressed B cell receptor (BCR). We previously demonstrated that clustering the IgM isotype of BCR with an artificial soluble cross-linker led to the stabilization of an ordered phase-like domain. This domain sorted minimal peptides and full-length proteins to generate local hot-spots for tyrosine phosphorylation at receptor clusters, facilitating receptor activation. BCR can also be activated through interactions with natural ligands or artificial cross-linkers presented at bilayer surfaces, where it is proposed that alternate mechanisms play important roles in receptor clustering and activation, including one mechanism involving the exclusion phosphatases due to their bulky extracellular domains. The goals of the current study are to determine if markers of membrane phases are sorted by BCR clusters formed through engagement with bilayer-presented natural ligands or cross-linkers, and to estimate the contribution that membrane phase partitioning plays in organizing regulatory proteins with respect to BCR clusters under these stimulation conditions. We use super-resolution fluorescence localization microscopy to find that BCR engagement with either a bilayer-presented natural ligand or artificial cross-linker generates ordered phase-like domains that are more robust than those observed with soluble cross-linkers. In addition, we provide evidence that interactions between regulatory proteins and BCR are partially determined through their preference for ordered membrane domains and present a minimal model of receptor activation that incorporates both ordered domains and steric exclusion mechanisms to produce a more sensitive response. Overall, this work highlights that cells are capable of integrating multiple interaction modalities to give rise to cellular functions, likely conferring flexibility and robustness to cellular responses.

biophysics↗