Membrane Domains Research Articles

Differential pairing of transmembrane domain GxxxG dimerization motifs defines two HLA-DR MHC class II conformers

MHC class II molecules function to present exogenous antigen-derived peptides to CD4 T cells to both drive T cell activation and to provide signals back into the class II antigen-presenting cell. Previous work established the presence of multiple GxxxG dimerization motifs within the transmembrane domains of MHC class II α and β chains across a wide range of species and revealed a role for differential GxxxG motif pairing in the formation of two discrete mouse class II conformers with distinct functional properties (i.e., M1-and M2-paired I-Ak class II). Biochemical and mutagenesis studies detailed herein extend this model to human class II by identifying an anti-HLA-DR mAb (Tü36) that selectively binds M1-paired HLA-DR molecules. Analysis of the HLA-DR allele reactivity of the Tü36 mAb helped define other HLA-DR residues involved in mAb binding. In silico modeling of both TM domain interactions and whole protein structure is consistent with the outcome of biochemical/mutagenesis studies and provides insight into the possible structural differences between the two HLA-DR conformers. Cholesterol depletion studies indicate a role for cholesterol-rich membrane domains in the formation/maintenance of Tü36 mAb reactive DR molecules. Finally, phylogenetic analysis of the amino acid sequences of Tü36-reactive HLA-DR β chains reveals a unique pattern of both Tü36 mAb reactivity and key amino acid polymorphisms. In total, these studies bring the paradigm M1/M2-paired MHC class II molecules to the human HLA-DR molecule and suggest that the functional differences between these conformers defined in mouse class II extend to the human immune system.

Antibody binding reports spatial heterogeneities in cell membrane organization

The spatial organization of cell membrane glycoproteins and glycolipids is critical for mediating the binding of ligands, receptors, and macromolecules on the plasma membrane. However, we currently do not have the methods to quantify the spatial heterogeneities of macromolecular crowding on live cell surfaces. In this work, we combine experiment and simulation to report crowding heterogeneities on reconstituted membranes and live cell membranes with nanometer spatial resolution. By quantifying the effective binding affinity of IgG monoclonal antibodies to engineered antigen sensors, we discover sharp gradients in crowding within a few nanometers of the crowded membrane surface. Our measurements on human cancer cells support the hypothesis that raft-like membrane domains exclude bulky membrane proteins and glycoproteins. Our facile and high-throughput method to quantify spatial crowding heterogeneities on live cell membranes may facilitate monoclonal antibody design and provide a mechanistic understanding of plasma membrane biophysical organization.

CryoEM structure of a post-assembly MS-ring reveals plasticity in stoichiometry and conformation.

The flagellar motor supports bacterial chemotaxis, a process that allows bacteria to move in response to their environment. A central feature of this motor is the MS-ring, which is composed entirely of repeats of the FliF subunit. This MS-ring is critical for the assembly and stability of the flagellar switch and the entire flagellum. Despite multiple independent cryoEM structures of the MS-ring, there remains a debate about the stoichiometry and organization of the ring-building motifs (RBMs). Here, we report the cryoEM structure of a Salmonella MS-ring that was purified from the assembled flagellar switch complex (MSC-ring). We term this the 'post-assembly' state. Using 2D class averages, we show that under these conditions, the post-assembly MS-ring can contain 32, 33, or 34 FliF subunits, with 33 being the most common. RBM3 has a single location with C32, C33, or C34 symmetry. RBM2 is found in two locations with RBM2inner having C21 or C22 symmetry and an RBM2outer-RBM1 having C11 symmetry. Comparison to previously reported structures identifies several differences. Most strikingly, we find that the membrane domain forms 11 regions of discrete density at the base of the structure rather than a contiguous ring, although density could not be unambiguously interpreted. We further find density in some previously unresolved areas, and we assigned amino acids to those regions. Finally, we find differences in interdomain angles in RBM3 that affect the diameter of the ring. Together, these investigations support a model of the flagellum with structural plasticity, which may be important for flagellar assembly and function.

DNA-Origami Line-Actants Control Domain Organization and Fission in Synthetic Membranes.

Cells can precisely program the shape and lateral organization of their membranes using protein machinery. Aiming to replicate a comparable degree of control, here we introduce DNA-origami line-actants (DOLAs) as synthetic analogues of membrane-sculpting proteins. DOLAs are designed to selectively accumulate at the line-interface between coexisting domains in phase-separated lipid membranes, modulating the tendency of the domains to coalesce. With experiments and coarse-grained simulations, we demonstrate that DOLAs can reversibly stabilize two-dimensional analogues of Pickering emulsions on synthetic giant liposomes, enabling dynamic programming of membrane lateral organization. The control afforded over membrane structure by DOLAs extends to three-dimensional morphology, as exemplified by a proof-of-concept synthetic pathway leading to vesicle fission. With DOLAs we lay the foundations for mimicking, in synthetic systems, some of the critical membrane-hosted functionalities of biological cells, including signaling, trafficking, sensing, and division.

Genome-Wide Identification and Expression Analysis of the SWEET Gene Family in Annual Alfalfa (Medicago polymorpha).

SWEET (Sugars will eventually be exported transporter) proteins are a group of sugar transporters that are involved in sugar efflux, phloem loading, reproductive development, plant senescence, and stress responses. In this study, 23 SWEET transporter members were identified in the Medicago polymorpha genome, heterogeneously distributed on seven chromosomes. These MpSWEET genes were divided into four subfamilies, which showed similar gene structure and motif composition within the same subfamily. Seventeen MpSWEET genes encode seven transmembrane helices (TMHs), and all MpSWEET proteins possess conserved membrane domains and putative serine phosphorylation sites. Four and three pairs of MpSWEET genes were predicted to be segmentally and tandemly duplicated, respectively, which may have contributed to their evolution of M. polymorpha. The results of microarray and RNA-Seq data showed that some MpSWEET genes were specifically expressed in disparate developmental stages (including seedling stage, early flowering stage, and late flowering stage) or tissues such as flower and large pod. Based on protein network interaction and expression patterns of MpSWEET genes, six MpSWEET genes were selected for further quantitative real-time PCR validation in different stress treatments. qRT-PCR results showed that MpSWEET05, MpSWEET07, MpSWEET12, MpSWEET15, and MpSWEET21 were significantly upregulated for at least two of the three abiotic stress treatments. These findings provide new insights into the complex transcriptional regulation of MpSWEET genes, which facilitates future research to elucidate the function of MpSWEET genes in M. polymorpha and other legume crops.

The Air-Liquid Interface Reorganizes Membrane Lipids and Enhances the Recruitment of Slc26a3 to Lipid-Rich Domains in Human Colonoid Monolayers.

Cholesterol-rich membrane domains, also called lipid rafts (LRs), are specialized membrane domains that provide a platform for intracellular signal transduction. Membrane proteins often cluster in LRs that further aggregate into larger platform-like structures that are enriched in ceramides and are called ceramide-rich platforms (CRPs). The role of CRPs in the regulation of intestinal epithelial functions remains unknown. Down-regulated in adenoma (DRA) is an intestinal Cl-/HCO3- antiporter that is enriched in LRs. However, little is known regarding the mechanisms involved in the regulation of DRA activity. The air-liquid interface (ALI) was created by removing apical media for a specified number of days; from 12-14 days post-confluency, Caco-2/BBe cells or a colonoid monolayer were grown as submerged cultures. Confocal imaging was used to examine the dimensions of membrane microdomains that contained DRA. DRA expression and activity were enhanced in Caco-2/BBe cells and human colonoids using an ALI culture method. ALI causes an increase in acid sphingomyelinase (ASMase) activity, an enzyme responsible for enhancing ceramide content in the plasma membrane. ALI cultures expressed a larger number of DRA-containing platforms with dimensions >2 µm compared to cells grown as submerged cultures. ASMase inhibitor, desipramine, disrupted CRPs and reduced the ALI-induced increase in DRA expression in the apical membrane. Exposing normal human colonoid monolayers to ALI increased the ASMase activity and enhanced the differentiation of colonoids along with basal and forskolin-stimulated DRA activities. ALI increases DRA activity and expression by increasing ASMase activity and platform formation in Caco-2/BBe cells and by enhancing the differentiation of colonoids.

Dynamic Regulation of Order Lipid Meso-domains in the Capillary Endothelial Cell Membrane

The structure and function of ion channels are sensitive to direct interactions with lipids of the membrane and to the biophysical properties of the membrane microenvironment, such that membrane fluidity, lateral pressure profile, and thickness can modulate the channels. Cholesterol plays a significant role in regulating membrane fluidity and is capable of phase separation where cholesterol will spontaneously separate into its own domain, which is partially responsible for developing two distinct domains within the membrane. These ordered domains are regions of high cholesterol and low fluidity, and the disordered domains are regions of low cholesterol and high fluidity. Here we examine and provide evidence of the presence of ordered cholesterol membrane domains in capillary endothelial cells. We will test the hypothesis that capillary endothelial cell ordered domains function as a signaling hub for different phospholipids regulating ion channel function. Using the isolated retinal vasculature, confocal microscopy, and fluorescent Bodipy-cholesterol, a marker of ordered lipid domains, we observed large regions approximately 2.3± 0.9 μm long of cholesterol ordered meso-domains within capillary endothelial cells and not the feeding arteriole. To determine the dynamic nature of these meso-domains, we used membrane-disordering drugs to disrupt the organization of membrane lipids. The ordered meso-domains display dynamic membrane disordering by ethanol and can re-cluster into the large ordered meso-domains. In addition, we observed that the critical signaling phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) co-localized with the ordered meso-domains. To determine the cooperativity between these lipid molecules, we examined the effects of ethanol-dependent disordering of cholesterol on the membrane localization of PIP2. In addition, we examined the effects of PIP2 depletion with the G-protein couple receptor agonist, U46619, on the disordered state of the cholesterol meso-domains. These data suggest that ordered meso-domains might play a role as a signaling hub affecting the membrane fluidity and bioavailability of signaling lipids modulating ion channel function in capillary endothelial cell. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A new antibody for the study of human CD157 expression and function

Abstract CD157 is an ectoenzyme that has both cyclic ADP-ribose hydrolase and ADP-ribosyl cyclase activities. In the human hematopoietic system, CD157 is prevalently expressed by cells of the myelomonocytic lineage, and interacts with extracellular matrix components and regulates leukocyte diapedesis via integrin-mediated signaling. CD157 is implicated in the pathophysiology of several neurological disorders. Although it lacks a cytoplasmatic domain, CD157 is able to transduce intracellular signals by establishing functional and structural crosstalk with β1 (CD29) and β2 (CD18) integrins. CD157 ligation causes integrin– dependent cytoskeletal remodeling and increase in F-actin content. Activation of downstream signaling pathways leads to cell polarization, with CD157 migrating prevalently to the rear of the cells whereas F-actin is localized at the opposite pole. However, the molecular and cellular functions of CD157 still remain poorly understood. Here, we describe the generation of a monoclonal antibody specific for human CD157 (clone W21007F) that can be used in flow cytometry, immunofluorescence and blocking of CD157 mediated adhesion of monocytes to fibronectin. Treatment of the pro-monocytic, myeloid leukemia cell line U937 with clone W21007F induces cell polarization and a marked increase in filamentous actin, with CD157 clustering at the uropod. The majority of F-Actin fibers are located at the opposite end of CD157, and CD157 co-localizes with integrin beta-1 in distinct membrane domains. The reference clone SY/11B5 and isotype control antibody do not show this effect. In conclusion, anti-human CD157 (clone W21007F) is a valuable tool for the study of CD157 function and expression in human cells.

Coupling of protein condensates to ordered lipid domains determines functional membrane organization.

During T cell activation, the transmembrane adaptor protein LAT (linker for activation of T cells) forms biomolecular condensates with Grb2 and Sos1, facilitating signaling. LAT has also been associated with cholesterol-rich condensed lipid domains; However, the potential coupling between protein condensation and lipid phase separation and its role in organizing T cell signaling were unknown. Here, we report that LAT/Grb2/Sos1 condensates reconstituted on model membranes can induce and template lipid domains, indicating strong coupling between lipid- and protein-based phase separation. Correspondingly, activation of T cells induces cytoplasmic protein condensates that associate with and stabilize raft-like membrane domains. Inversely, lipid domains nucleate and stabilize LAT protein condensates in both reconstituted and living systems. This coupling of lipid and protein assembly is functionally important, as uncoupling of lipid domains from cytoplasmic protein condensates abrogates T cell activation. Thus, thermodynamic coupling between protein condensates and ordered lipid domains regulates the functional organization of living membranes.

Dynamic assembly and interaction of glycosphingolipids in cholesterol-containing model membranes

Abstract Glycosphingolipids (GSLs) in the mammalian plasma membrane are essential for various biological events that occur on cell membranes by forming glycolipid-rich membrane domains, such as lipid rafts. Lactosylceramide (LacCer) forms a highly ordered phase in model and biological membranes. However, the details of this domain remain unclear. We examined the dynamic assembly of LacCer in cholesterol-containing phase-separated membranes by fluorescence imaging and solid-state NMR. Solid-state 2H NMR of the deuterated LacCer probes in the membranes indicated that the potent LacCer–LacCer interaction significantly reduced the incorporation of cholesterol in the LacCer domains. Moreover, increasing the amount of cholesterol induced the formation of nanometer-scale LacCer domains, which can be biologically relevant. The lateral interaction of ganglioside GM3 with the epidermal growth factor receptor (EGFR) transmembrane segment was examined using fluorescence spectroscopy under membrane conditions. Lateral FRET between the NBD fluorophore on the EGFR transmembrane segment and the ATTO594 fluorophore on the GM3 head group suggested that GM3 interacts with the transmembrane segment and partially inhibits the EGFR dimerization by stabilizing the peptide monomer. More precise investigations of GSL–GSL and GSL-protein interactions occurring in membrane environments can lead to the elucidation of the cellular functions of GSL from structural points of view.

Phosphatidylinositol 4,5-Bisphosphate Sensing Lipid Raft via Inter-Leaflet Coupling Regulated by Acyl Chain Length of Sphingomyelin

Phosphatidylinositol 4,5-bisphosphate (PIP2) is an important molecule located at the inner leaflet of cell membrane, where it serves as anchoring sites for a cohort of membrane-associated molecules and as a broad-reaching signaling intermediate. The lipid raft is thought as the major platform recruiting proteins for signal transduction and also known to mediate PIP2 accumulation across the membrane. While the significance of this cross-membrane coupling is increasingly appreciated, it remains unclear whether and how PIP2 senses the dynamic change of the ordered lipid domains over the packed hydrophobic core of the bilayer. Herein, by means of molecular dynamic simulation, we reveal that inner PIP2 molecules can sense the outer lipid domain via inter-leaflet coupling, and the coupling manner is dictated by the acyl chain length of sphingomyelin (SM) partitioned to the lipid domain. Shorter SM promotes membrane domain registration, whereby PIP2 accumulates beneath the domain across the membrane. In contrast, the anti-registration is thermodynamically preferred if the lipid domain has longer SM due to the hydrophobic mismatch between the corresponding acyl chains in SM and PIP2. In this case, PIP2 is expelled by the domain with a higher diffusivity. These results provide molecular insights into the regulatory mechanism of correlation between the outer lipid domain and inner PIP2, both of which are critical components for cell signal transduction.

Inducing Lipid Domains in Membranes by Self‐Assembly of DNA Origami

AbstractSelf‐assembly of biological molecules and structures is a fundamental property of life. Whereas most biological functions are based on self‐assembled proteins and protein complexes, the self‐assembly of lipids is important for the spatial organization of heterogeneous cellular reaction environments and to catalyze cooperative interactions on/with membranes. Lipid domains or “rafts”, which are known to selectively recruit proteins, play an important functional role in sorting and trafficking of membrane components between subcellular organelles. However, how the recruitment and interactions of proteins in turn contributes to the formation and turnover of these structures has not been systematically addressed, due to the large variety in membrane–protein features and their spatiotemporal dynamics. The small size and transient nature of lipid domains adds to the complexity in visualizing them in living cells. Here, DNA origami is presented as a programmable tool to mimic protein clustering and assembly on membranes and illustrate how nanometer sized lipid domains coalesce into visible domains upon origami self‐assembly in defined patterns. Hence, the local membrane composition can be efficiently regulated by the self‐assembly of peripheral membrane binders. This reinforces the hypothesis that lipid rafts in cells occur as a result of membrane–protein interactions and, in particular, protein self‐assembly.

Vacuole membrane contact sites regulate liquid-ordered domain formation during glucose starvation.

Liquid-ordered (Lo) membrane domains have been proposed to play important roles in a wide variety of biological processes, such as protein sorting and cell signaling. However, the mechanisms by which they are formed and maintained remain poorly understood. Lo domains are formed in the vacuolar membrane of yeast in response to glucose starvation. Here, we show that the deletion of proteins that localize to vacuole membrane contact sites (MCSs) caused a marked decrease in the number of cells with Lo domains. In addition to Lo domain formation, autophagy is induced upon glucose starvation. However, the deletion of core autophagy proteins did not inhibit Lo domain formation. Thus, we propose that vacuolar Lo domain formation during glucose restriction is regulated by MCSs but not by autophagy.

Laurdan Discerns Lipid Membrane Hydration and Cholesterol Content.

Studies of biological membrane heterogeneity particularly benefit from the use of the environment-sensitive fluorescent probe Laurdan, for which shifts in the emission, produced by any stimulus (e.g., fluidity variations), are ascribed to alterations in hydration near the fluorophore. Ironically, no direct measure of the influence of the membrane hydration level on Laurdan spectra has been available. To address this, we investigated the fluorescence spectrum of Laurdan embedded in solid-supported lipid bilayers as a function of hydration and compared it with the effect of cholesterol─a major membrane fluidity regulator. The effects are illusively similar, and hence the results obtained with this probe should be interpreted with caution. The dominant phenomenon governing the changes in the spectrum is the hindrance of the lipid internal dynamics. Furthermore, we unveiled the intriguing mechanism of dehydration-induced redistribution of cholesterol between domains in the phase-separated membrane, which reflects yet another regulatory function of cholesterol.

Abstract 8: Differential flt3 mutational status in acute myeloid leukemia predicts sensitivity to flt3 inhibitor gilteritinib in vitro and in vivo

Abstract Acute Myeloid Leukemia (AML) is a heterogenous hematopoietic cancer characterized by abnormal differentiation and uncontrolled proliferation of hematopoietic progenitor cells. Recent approaches to treatment, which aim to improve outcomes in refractory AML, include strategies to molecularly target specific receptor-mediated signaling pathways which regulate cell growth and proliferation. One target-specific approach is the inhibition of fms-like tyrosine kinase 3 (FLT3). FLT3 is expressed on the surface of hematopoietic progenitor cells and plays an important role in regulating proliferation, survival, and differentiation of multipotent stem cells. The presence of internal tandem duplications (ITDs) at the juxta membrane domain within the FLT3 receptor are common driver mutations in 30% of AML patients. In this study, we utilized 3 luciferase-expressing human AML models, MV4-11, MOLM-13, and, OCI-AML3 to determine the prognostic impact of FLT3 inhibitor Gilteritinib on the allelic ratio of ITD versus wildtype allele. OCI-AML3 expresses wild-type FLT3 whereas MOLM-13 is heterozygous for FLT3-ITD mutations, and MV4-11 possesses a homozygous in-frame tandem repeat insertion in the FLT3 allele. Here we demonstrate that FLT3-ITD mutational status impacts the efficacy of Gilteritinib in both in vitro and in vivo AML models. The in vitro assays showed that treatment with Gilteritinib resulted in significant dose dependent reduction in overall cell viability and bioluminescence in the MV4-11 cell line, while MOLM-13 was only moderately sensitive, and OCI-AML3 was least sensitive to Gilteritinib treatment. Additionally, we evaluated the downstream MAPK and PI3K signaling pathways as FLT3-ITD oncogenic mutations result in ligand-independent constitutive activation of signaling pathways mediating leukemic transformation. For in vivo validation, MV4-11, MOLM-13, and OCI-AML3 luciferase expressing cells were implanted systemically into NCG mice and once tumors were established, we evaluated the survival benefit in response to Gilteritinib and various standard of care agents. In accordance with our in vitro findings, the quantification of tumor progression by bioluminescent imaging showed the highest therapeutic activity of Gilteritinib in MV4-11 tumor bearing mice through selective targeting of FLT3-ITD mutations. Taken together, our in vitro and in vivo results demonstrate that differential FLT3-ITD mutational status in AML models impacts Gilteritinib treatment response and downstream signaling events with the highest activity in MV4-11 followed by MOLM-13, and lack of activity in OCI-AML3 harboring wild-type FLT3. Citation Format: Bincy John, Pasupathi Sundaramoorthy, Elizabeth Rainbolt, Andrew Wong, Zachary Ward, Justin Avery, Savannah Todd, Shasta Kidder, Diana Gietl, Chassidy Hall, David Harris. Differential flt3 mutational status in acute myeloid leukemia predicts sensitivity to flt3 inhibitor gilteritinib in vitro and in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 8.

Functional organization of extracellular hyaluronan, CD44, and RHAMM

AbstractThe integral membrane protein CD44 is constitutively expressed in most tissues, and is the primary cell surface receptor for hyaluronan (HA). HA is the key scaffolding organizer of proteoglycans and receptors in the extracellular matrix. CD44 is expressed as cell type‐specific and context‐specific alternatively spliced isoforms with variable glycosylation and interreceptor interactions. CD44 plays a critical role in the organization and patterning of domains in the cell membrane, achieved through linking the cortical actin cytoskeleton with the pericellular HA matrix. This functional and spatial organization of CD44 and HA maintains a protective and homeostatic cellular microenvironment. Tissue injury and cellular stress result in HA fragmentation and unconventional export of the CD44 coreceptor, RHAMM, which collectively alter the homeostatic organization of CD44/HA. The resulting interplay among HA, HA fragments, CD44 and RHAMM triggers cellular responses such as cell motility that contribute to tissue repair, and disease processes. Here we review current understanding of the structural biology of HA, CD44, and RHAMM, and provide an overview of their known functional organization and patterning at the cell surface. Changes in their surface organization act as sensors for detecting danger, and are critical to the understanding of consequent intracellular signaling and response mechanisms to cellular stress.

Membrane phase separation drives responsive assembly of receptor signaling domains.

Plasma membrane heterogeneity has been tied to a litany of cellular functions and is often explained by analogy to membrane phase separation; however, models based on phase separation alone fall short of describing the rich organization available within cell membranes. Here we present comprehensive experimental evidence motivating an updated model of plasma membrane heterogeneity in which membrane domains assemble in response to protein scaffolds. Quantitative super-resolution nanoscopy measurements in live B lymphocytes detect membrane domains that emerge upon clustering B cell receptors (BCRs). These domains enrich and retain membrane proteins based on their preference for the liquid-ordered phase. Unlike phase-separated membranes that consist of binary phases with defined compositions, membrane composition at BCR clusters is modulated through the protein constituents in clusters and the composition of the membrane overall. This tunable domain structure is detected through the variable sorting of membrane probes and impacts the magnitude of BCR activation.

Variation of existence and location of aquaporin 3 in relation to cryoresistance of ram spermatozoa.

Osmotic changes during the process of freeze-thawing involve changes in the location of aquaporins (AQPs) in membrane domains of spermatozoa. Some AQPs, like aquaporin 3 (AQP3), are linked to sperm cryotolerance in the porcine species. Conspicuous individual variability exists between rams and their ejaculates, which may be classified as displaying good freezability (GFE) or poor freezability (PFE), depending on several endogenous and environmental factors. The present work aimed to examine whether differences in freezability could even involve changes in location and expression of AQP3 in ram spermatozoa. Thirty ejaculates from 10 rams (three of each) were evaluated and subsequently classified as GFE (n = 13) or PFE (n = 17) through a principal component analysis (PCA) and k-means cluster analysis. Spermatozoa were examined for the presence, abundance and distribution of AQP3 by western blot and immunocytochemistry, employing a commercial rabbit polyclonal antibody (AQP3 - ab125219). Although AQP3 was found in the sperm acrosome, midpiece, principal and end piece of the tail in both fresh and after frozen-thawed samples, its highest immunolabeling was found in the mid- and principal piece. In the GFE group, the expression of AQP3 in the mid- and principal piece was greater (P < 0.05) in frozen-thawed samples than in fresh specimens while such differences were not detected in the PFE group. Sperm cryotolerance relates to changes in AQP3 expression and thus AQP3 could be used as a biomarker for cryotolerance. A greater capacity of AQP3 localization in mid- and principal piece of the spermatozoa could be linked to an increase the osmo-adaptative capacity of ejaculates with better capacity to withstand freeze-thawing processes.

Tuberculostearic Acid Controls Mycobacterial Membrane Compartmentalization.

The intracellular membrane domain (IMD) is a laterally discrete region of the mycobacterial plasma membrane, enriched in the subpolar region of the rod-shaped cell. Here, we report genome-wide transposon sequencing to discover the controllers of membrane compartmentalization in Mycobacterium smegmatis. The putative gene cfa showed the most significant effect on recovery from membrane compartment disruption by dibucaine. Enzymatic analysis of Cfa and lipidomic analysis of a cfa deletion mutant (Δcfa) demonstrated that Cfa is an essential methyltransferase for the synthesis of major membrane phospholipids containing a C19:0 monomethyl-branched stearic acid, also known as tuberculostearic acid (TBSA). TBSA has been intensively studied due to its abundant and genus-specific production in mycobacteria, but its biosynthetic enzymes had remained elusive. Cfa catalyzed the S-adenosyl-l-methionine-dependent methyltransferase reaction using oleic acid-containing lipid as a substrate, and Δcfa accumulated C18:1 oleic acid, suggesting that Cfa commits oleic acid to TBSA biosynthesis, likely contributing directly to lateral membrane partitioning. Consistent with this model, Δcfa displayed delayed restoration of subpolar IMD and delayed outgrowth after bacteriostatic dibucaine treatment. These results reveal the physiological significance of TBSA in controlling lateral membrane partitioning in mycobacteria. IMPORTANCE As its common name implies, tuberculostearic acid is an abundant and genus-specific branched-chain fatty acid in mycobacterial membranes. This fatty acid, 10-methyl octadecanoic acid, has been an intense focus of research, particularly as a diagnostic marker for tuberculosis. It was discovered in 1934, and yet the enzymes that mediate the biosynthesis of this fatty acid and the functions of this unusual fatty acid in cells have remained elusive. Through a genome-wide transposon sequencing screen, enzyme assay, and global lipidomic analysis, we show that Cfa is the long-sought enzyme that is specifically involved in the first step of generating tuberculostearic acid. By characterizing a cfa deletion mutant, we further demonstrate that tuberculostearic acid actively regulates lateral membrane heterogeneity in mycobacteria. These findings indicate the role of branched fatty acids in controlling the functions of the plasma membrane, a critical barrier for the pathogen to survive in its human host.

Directed growth and fusion of membrane-wall microdomains requires CASP-mediated inhibition and displacement of secretory foci

Casparian strips (CS) are aligned bands of lignin-impregnated cell walls, building an extracellular diffusion barrier in roots. Their structure profoundly differs from tight junctions (TJ), analogous structures in animals. Nonetheless, CS membrane domain (CSD) proteins 1-5 (CASP1-5) are homologues of occludins, TJ components. CASP-marked membranes display cell wall (matrix) adhesion and membrane protein exclusion. A full CASP knock-out now reveals CASPs are not needed for localized lignification, since correctly positioned lignin microdomains still form in the mutant. Ultra-structurally, however, these microdomains are disorganized, showing excessive cell wall growth, lack of exclusion zone and matrix adhesion, and impaired exocyst dynamics. Proximity-labelling identifies a Rab-GTPase subfamily, known exocyst activators, as potential CASP-interactors and demonstrate their localization and function at the CSD. We propose that CASP microdomains displace initial secretory foci by excluding vesicle tethering factors, thereby ensuring rapid fusion of microdomains into a membrane-cell wall band that seals the extracellular space.