Membrane Structure

Membrane Structure lipid and protein components of biological membranes Jim Huettner

Lecture Overview • History • Lipid components and asymmetry • Bioactive lipid metabolites • Membrane proteins • Amino acids in the membrane • Lipid modifications • Subdomains (lipid rafts)

Early History • The ability of oil to calm the surface of bodies of water was recognized in ancient times https://www.youtube.com/watch?v=f2H418M3V6M • 1700’s - Ben Franklin performed early experiments with olive oil spreading to form a monolayer on a pond surface • 1800’s - Agnes Pockels and Lord Rayleigh calculated the thickness of an oil monolayer at 13-16 Angstroms • 1900’s – Langmuir used a trough similar to Pockels’ home-made apparatus to analyze surface tension and confirm the earlier measurements Wang DN, Stieglitz H, Marden J, Tamm LK. (2013) Benjamin Franklin, Philadelphia's favorite son, was a membrane biophysicist. Biophys J. 104:287-91.

Bilayer History • 1925 - Gorter and Grendel calculate that RBC membranes have enough lipids for 2x the surface area • 1935-1960s – Davidson-Danielli-Robertson models envisioned a bilayer with proteins coating the inner and outer surface • 1972 – Singer and Nicholson – Fluid Mosaic Model

Summary of membrane models Robertson JL. (2018) The lipid bilayer membrane and its protein constituents. J Gen Physiol. 150:1472-1483.

Fluid Mosaic Model Lodish et al., 8th ed. Fig. 7.1

Lipid Components Amphipathic polar, hydrophilic non-polar, hydrophobic • Phosphoglycerides • PC – phosphatidylcholine • PE – phosphatidylethanolamine • PS – phosphatidylserine • PI - phosphatidylinositol • Sphingolipids • sphingomyelin • galactocerebroside (glycolipids) • Sterols • cholesterol

chemical structures Lodish et al., 8th ed. Fig. 7.8

phosphoglycerides Alberts et al., 6th ed.

w-6 and w-3 cis-unsaturated FAs

Sphingomyelin Luckey, 2nd ed. Figs. 2.7 & 2.8

Glycolipids Alberts et al., 6th ed.

Cholesterol Alberts et al., 6th ed. • Most prevalent in surface membrane (up to about 25%) • Precursor to bile acids, steroid hormones and vitamin D • Interacts with and stabilizes saturated acyl chains (sphingolipids) • Broadens temperature range of phase transition from gel state to fluid states

Lipid Composition Varies Alberts et al., 6th ed.

Lipid Mobility Alberts et al., 6th ed.

Distribution from MD simulation Luckey, 2nd ed. Figs. 8.7 & 8.8

Thickness and Curvature Lodish et al., 8th ed. Fig. 7.11

Bilayer Polarity Lodish et al., 8th ed. Fig. 7.5

Leaflet Asymmetry Anionic PS and PI are more prevalent in the cytoplasmic leaflet Luckey, 2nd ed. Fig. 2.14

Leaflet Composition Varies Composition (mol %) Lodish et al., 8th ed. Table 7.1

Enzymatic Modification

Phospholipase sites • Enzymatic release of bioactive metabolites: • IP3 and diacylglycerol by PLC • Lysophospholipids by PLA1 or PLA2 • Arachidonic or docosahexaenoic acid by PLA2 Lodish et al., 8th ed. Fig. 7.12

Membrane Proteins Alberts et al., 6th ed.

Acyl Modification C14 C16 C15 farnesyl or C20 geranylgeranyl Alberts et al., 6th ed.

GPI anchor Luckey, 2nd ed. Fig. 4.10

Additional Features • Oxidizing extracellular environment promotes inter- and intra-chain disulfide bonds • Reducing intracellular environment keeps cysteine side chains available for thioesterification • Extracellular oligosaccharide modification • Tendency for an excess of positively charged Arg and Lys side chains in the cytoplasmic segments Alberts et al., 6th ed.

TM Helix Amino Acid Prevalence Luckey, 2nd ed. Fig. 6.25

DG for membrane insertion Luckey, 2nd ed. Fig. 7.23

Insertion Signal Variety Luckey, 2nd ed. Fig. 7.25

Lipid subdomains (rafts) Alberts et al., 6th ed.

Lipid Rafts Alberts et al., 6th ed. Luckey, 2nd ed. Fig. 1.9

“Ballpark” Numbers • “Typical” membrane includes more than 100 distinct lipids • More than 500 different fatty acids have been identified • Lateral mobility in liposomes similar to olive oil and about 100 fold slower than for molecules in water • In animal cells about 50-90% of cholesterol is in the surface membrane • Lipid / Protein ratio varies: 80%L / 20%P in myelin, 25%L / 75%P in mitochondrial inner membrane

Additional Reading • van Meer G, Voelker DR, Feigenson GW. (2008) Membrane lipids: where they are and how they behave. Nat Rev Mol Cell Biol. 9:112-24. • van Meer G, de Kroon AI. (2011) Lipid map of the mammalian cell. J Cell Sci. 124:5-8. • SezginE, Levental I, Mayor S, Eggeling C. (2017) The mystery of membrane organization: composition, regulation and roles of lipid rafts. Nat Rev Mol Cell Biol. 18:361-374. • Jacobson K, Liu P, Lagerholm BC. (2019) The Lateral Organization and Mobility of Plasma Membrane Components. Cell. 177:806-819.

Discussion Paper for Wednesday Shi Z, Graber ZT, Baumgart T, Stone HA, Cohen E. (2018) Cell Membranes Resist Flow. Cell. 175:1769-1779. Groves JT. (2019) Membrane Mechanics in Living Cells. Dev Cell. 48:15-16. (a brief intro/review of the Shi et al. paper)

Membrane Structure