#membrane curvature

Plasma Membrane

Which of the following influences membrane curvature?

Intrinsic lipid shape contributes substantially to membrane bending energy as described by Helfrich elastic theory and packing parameter concepts. Phospholipids with small headgroup relative to bulky unsaturated tail volume possess negative intrinsic curvature favoring concave bending of monolayer. Phosphatidylethanolamine possesses ethanolamine headgroup substantially smaller than choline of phosphatidylcholine conferring cone shape with packing parameter greater than one, preferring inverted hexagonal HII phases under stress and reducing bilayer stability. Enrichment of PE in cytoplasmic leaflet promotes formation of fusion intermediates like stalks and hemifusion diaphragms required for SNARE-mediated exocytosis, supports high curvature of endocytic buds and cristae junctions stabilized by ATP synthase dimers and OPA1. Phosphatidylethanolamine is generated from phosphatidylserine decarboxylation by phosphatidylserine decarboxylase PSD in mitochondria inner membrane and via CDP-ethanolamine Kennedy pathway in ER. Cholesterol modulates bending rigidity but induces less curvature than PE, sphingomyelin cylindrical shape stabilizes flat domains. Therefore PE distribution is major lipid factor shaping vesicle budding and organelle morphology during trafficking.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane Curvature and PE Shape.

View Details

Glycolipid and Phospholipid

Which lipid modification helps maintain membrane curvature?

Maintenance high curvature membranes such as thirty nanometer synaptic vesicles tubular ER sheets requires conical lipids complementing protein scaffolds BAR ENTH and reticulons. Phosphatidylethanolamine ethanolamine headgroup cross section smaller than diacylglycerol tail volume adopts molecular cone favoring negative mean curvature concave cytosolic leaflet facilitating fission fusion stalk formation hemifusion diaphragm expansion. Synthesis via Kennedy pathway CDP ethanolamine ethanolamine phosphotransferase EPT1 and mitochondrial phosphatidylserine decarboxylase PSD converting PS PE enriched contact sites generating local PE pools ER mitochondria associated membranes. Flippases ATP11 transport PE inward increasing inner leaflet concentration; BAR domain proteins amphiphysin endophilin sense curvature recruit dynamin for scission constriction. PE depletion knockdown PCYT2 impairs cytokinesis nuclear envelope breakdown Golgi fragmentation ER exit site formation. Cholesterol cylindrical near zero curvature does not generate curvature alone, sphingomyelin lamellar stabilizer prevents bending, phosphatidylinositol signaling mainly IP3 DAG generation not curvature. Hence PE uniquely supports curvature shape coupling reducing bending modulus twenty to ten kBT mechanism central organelle morphology questions examined cell biology biophysics and trafficking exams linking lipid shape physics biology membrane remodeling processes.

Ref: Vance JE & Tasseva G., Biochim Biophys Acta 2013, PE cone shape maintains curvature and fusion.

View Details

Properties of Plasma Membrane

What is the primary determinant of lipid bilayer curvature?

Spontaneous curvature stems from imbalance between optimal headgroup repulsion and hydrocarbon chain pressure described by Israelachvili packing parameter P equals v divided by a times l. When headgroup area a large relative to volume v molecule resembles inverted cone P less than one yielding positive curvature forming micelles as lysophosphatidylcholine with one chain generating outward bulges. Small headgroup like PE PA DAG yields cone P greater than one favoring negative curvature invaginations and HII tubes needed for fusion fission. PC sphingomyelin balanced a and v give cylinder P about one stabilizing flat bilayers low curvature elastic stress. Cells manipulate curvature enzymatic remodeling PC to PA via phospholipase D reducing head size or insertion ENTH BAR amphipathic helices wedging into leaflet creating asymmetry. Cholesterol modestly increases order but near cylindrical shape contributes little curvature per se. Glycolipid large oligosaccharide exerts steric pressure outside. Linking head size to curvature clarifies tubular ER reticulons caveolae caveolin scaffolding and budding mechanisms examined in cell biology and biophysics exams requiring shape understanding and bending energetics quantitative analysis for vesicle formation.

Ref: McMahon & Gallop, Nature 2005, Mechanisms membrane curvature determinants headgroup size.

View Details

Properties of Plasma Membrane

Which of the following lipid modifications increases curvature in membranes?

Membrane bending energy is governed by molecular geometry quantified as shape parameter defined as volume over area times length. Phosphatidylethanolamine possesses small ethanolamine headgroup about zero point two eight square nanometers versus larger hydrocarbon volume due to two acyl chains, creating conical shape with parameter greater than one favoring negative spontaneous curvature typical of inner leaflet concave tubules and fusion stalk intermediates. Enrichment of PE in cytoplasmic leaflet at SNARE assembly sites reduces hydration repulsion and elastic cost of merging outer leaflets first forming hemifusion diaphragm. Influenza hemagglutinin fusion peptide inserts and recruits PE; reconstituted PE containing liposomes fuse rapidly upon calcium addition while pure phosphatidylcholine liposomes are fusogen resistant. Enzymes phosphatidylserine decarboxylase PSD in mitochondria inner membrane generate curvature for cristae formation. Flippases maintain PE asymmetry. Lysophospholipids opposite shape induce positive curvature inhibiting stalk formation. Cholesterol buffers rigidity but modest curvature effect. Appreciating cone versus cylinder concept connects lipid metabolism to exocytosis cytokinesis and viral entry mechanisms thoroughly asked in examinations.

Ref: Chernomordik & Kozlov, Nature Structural Biology 2003, PE and membrane curvature in fusion.

View Details

Properties of Plasma Membrane

Which phospholipid is conical in shape and contributes to membrane curvature?

Intrinsic lipid curvature concept explains membrane remodeling during endocytosis exocytosis and organelle shaping. Lipid shape analyzed via Israelachvili packing parameter p equals v over a times l where v volume tail a head area l length: cylindrical p approximately one forms flat bilayers as phosphatidylcholine with two acyl chains head size similar to tail cross-section; conical p greater than one small head large tails such as phosphatidylethanolamine and diacylglycerol tends negative curvature monolayers bending toward water favoring inverted hexagonal HII phase; inverted conical p less than one third large head single chain lysophosphatidylcholine tends positive curvature micelles. PE ethanolamine headgroup small poorly hydrated versus two bulky eighteen carbon tails diacylglycerol causes conical geometry inducing negative spontaneous curvature stabilizing fusion intermediates stalk formation where contacting monolayers merge reducing fusion barrier energy estimated twenty to forty kilocalories. Concentrated inner leaflet at sites synapse vesicle budding, nuclear pore formation, facilitated by BAR and ENTH domains sensing curvature. Thus conical PE contributes curvature elasticity vital for dynamic remodeling and fusion competence.

Ref: McMahon and Gallop, Membrane Curvature and PE Conical Shape, Nature 2005.

View Details