Membrane Structure and Function Notes

Membranes

• Suggested Readings:
  • B-S 12.3 pp 359-362: Phospholipids and Glycolipids Readily Form Bimolecular Sheets in Aqueous Media
  • B-S 12.4 pp 362-369: Proteins Carry Out Most Membrane Processes
  • B-S 12.5 pp 369-373: Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
  • B-S 12.6 pp 373-375: Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
  • Alberts Chapter 10 Introduction, Membrane Structure pp 603: The Lipid Bilayer
  • Alberts Chapter 10 pp 604-615: The Lipid Bilayer
  • Alberts Chapter 10 pp 615-633: Membrane Proteins
• Movie 11.3 (Lipids), Movie 11.2 (Fluidity), Movie 11.7 (FRAP)

Introduction to Membranes

• Membranes provide the "walls" of the cell, including intracellular membranes and the plasma membrane.
• Membrane composition varies in lipid and protein content based on function.
• Membranes can have specialized domains for specific roles.

Plasma Membrane

• The plasma membrane is described as an asymmetric fluid mosaic.
• It is fluid because lipids are bound by noncovalent forces, allowing lateral movement.
• Integral proteins are embedded in the membrane.
• Peripheral proteins are loosely bound and easily removed.
• The plasma membrane is asymmetric due to different compositions on its two surfaces.

Membrane Fluidity

• Movie 11.2 illustrates the flexibility of the cell membrane.

Basic Bilayer Structure

• The bilayer consists of two sheets of phospholipids facing opposite directions.
• Phospholipids are represented as a circle (polar head group) and two tails (nonpolar hydrocarbon chains).
• The membrane has a hydrophobic core (hydrocarbon chains) and hydrophilic surfaces (polar head groups).

Membrane Composition

• Membranes are composed of a mixture of phospholipid molecules.
• Phospholipid molecules can be drawn as similar cartoons, with different colored circles representing the different polar head groups.
• Phosphatidylserine has a net negative charge.
• Sphingomyelin has a ceramide moiety but similar structure to phosphoglycerolipids. It has the same polar head group and two tailed structure as the phosphoglycerolipids.
• Hydrocarbon chains can be straight (saturated) or bent (unsaturated).
• Both chains of sphingolipids are usually saturated.

Asymmetry of Lipid Bilayer

• The outer leaflet contains more phosphatidylcholine and sphingomyelin (large polar head groups).
• The inner leaflet contains more ethanolamine and serine groups (smaller polar head groups).
• The outer leaflet also contains glycolipids (hexagons attached to the head group of a sphingolipid).

Fatty Acid Composition

• Membrane phospholipids are mixtures with respect to their fatty acid composition.
• Phosphatidylcholine molecules are a mixture of different molecular species, with fatty acids of different chain length and number of double bonds.
• Double bonds of physiological fatty acids are in the cis configuration, creating a bend in the molecule and limiting membrane packing.
• Double bonds increase membrane fluidity.

Phospholipid Movement

• Phospholipid molecules can move laterally and rotate within the membrane.
• Flip-flop (movement from one leaflet to the other) is rare because it requires transport of the polar head group through the nonpolar core.

Cholesterol

• Plasma membranes of mammalian cells contain cholesterol.
• Cholesterol fits within the phospholipid leaflet with its hydroxyl group adjacent to the polar head groups.
• The rigid sterol rings stiffen the adjacent portion of the hydrophobic core.
• The hydrocarbon tail is inserted closer to the methyl ends of the phospholipid fatty acyl chains, leaving the middle section of the bilayer relatively more fluid.

Membrane Proteins

• Plasma membranes have a variety of proteins associated with the lipid bilayer.
• Integral membrane proteins have one or more hydrophobic regions which are embedded in the lipid bilayer. Most pass completely through the membrane.
• Peripheral proteins are loosely associated with the membrane through noncovalent bonding to integral membrane proteins.

Functions of Membrane Proteins

• Proteins embedded in the plasma membrane have different functions.
• Transporters: sodium (Na+) pump pumps Na+ out of cells and potassium ions (K+) in.
• Anchors: integrins link intracellular actin filaments to extracellular matrix proteins.
• Receptors: bind extracellular hormones, generate intracellular signals. For example, the platelet-derived growth factor (PGDF) receptor binds PGDF and generates intracellular changes that cause the cell to grow and divide.
• Enzymes: adenylate cyclase catalyzes the synthesis of intracellular cyclic AMP in response to extracellular signals.

Transmembrane Proteins

• A common structural motif involves an alpha helical segment composed of nonpolar amino acids.
• Peptide bonds are coiled into the middle of the helix.
• Side groups on the surface are hydrophobic and associate with the lipid bilayer.
• Extracellular portions of proteins often have disulfide bonds contributing to their tertiary structure.
• The cytosol contains free sulfhydryl groups which break (reduce) disulfide bonds and maintain free –SH groups on proteins present on that side of the membrane.

Integral Membrane Protein Structures

• Some proteins have a single alpha-helix that passes through the membrane.
• Others pass through the membrane multiple times, with alpha-helical domains separated by hydrophilic loops.
• Multiple strands of a beta-pleated sheet can form a barrel or cylinder (Panel A).
• Multiple alpha helices can combine to form a transmembrane hydrophilic pore (Panel B).
• Hydrophobic amino acid side chains are in contact with the hydrophobic fatty acid chains of membrane phospholipids.
• Hydrophilic amino acids form a water-filled pore.
• Barrels formed by beta-pleated sheets can also form water-filled channels across the membrane.

Seven-Membrane-Spanning Alpha-Helical Domains

• One major family of integral membrane proteins has seven membrane-spanning alpha-helical domains.
• This family includes plasma membrane receptors for epinephrine and glucagon, and rhodopsin in the eye.

Integral Membrane Proteins - Exceptions

• Cyclooxygenase has a segment of alpha-helix inserted within the membrane, with both ends coming out on the same (cytoplasmic) face.

Mechanisms for Forming Integral Membrane Proteins

• Integral membrane proteins have one or more hydrophobic polypeptide domains which can associate with hydrophobic lipids.
• Lipid molecules can be covalently attached to the protein.
• The lipids are embedded within the lipid bilayer, leaving the protein on either the intracellular or extracellular surface.
• Integral proteins can only be removed by physically disrupting the membrane itself (with detergents).

Lipid Anchors

• Long chain fatty acids (myristate 14:0 and palmitate 16:0) are used to anchor proteins to membranes.
• Proteins with these additions are described as myristylated or palmitylated.
• Isoprenyl groups (polyprenylation) are another type of anchor.
• Farnesyl group has 15 carbons in a hydrocarbon chain with trans double bonds and methyl branches.
• Geranylgeranyl group has a similar structure with 20 carbons.
• Phospholipids with sugars attached to phosphatidylinositol are a third type of anchor.

Peripheral Proteins

• Peripheral proteins do not have a hydrophobic anchor to the membrane.
• They form noncovalent associations with the hydrophilic portions of membrane proteins.
• Hydrogen bonds and electrostatic bonds are disrupted by changes in pH and/or ion concentrations.

Intracellular Signaling

• Some proteins involved in intracellular signaling processes are associated with the membrane under certain conditions and not under others.
• Protein kinase C (PKC) is activated by diacylglycerol.
• The cellular signaling cascade generates diacylglycerol.
• Protein kinase A was activated allosterically by cyclic AMP.
• The otherwise cytosolic protein kinase C binds diacylglycerol and becomes anchored to (or translocated to) the membrane where it becomes active.
• Since the binding of diacylglycerol to PKC is noncovalent, the bound PKC is considered a peripheral protein.
• The activation of phospholipase A2 releases free arachidonic acid from membrane phospholipids and provides substrate for cyclooxygenase.

Specialized Domains

• Caveolae and rafts are specialized domains in the plasma membrane.
• Both are rich in sphingomyelin and cholesterol, and are more tightly packed, less fluid regions of the membrane bilayer.
• Certain proteins are specifically associated with these specialized lipid domains.
• Caveolae are involved in receptor-mediates endocytosis.
• Rafts contribute to processes requiring protein sorting and/or assembly of macromolecular complexes during signal transduction.

Lateral Movement

• Many integral proteins can move laterally within the membrane.
• This can be demonstrated when two separate cells (with tagged proteins) are fused to form a heterokaryon.
• Heterokaryons become hybrids when this process is followed by fusion of the two nuclei.

Restricted Movement

• Not all proteins are free to move within the membrane.
• Anchoring of cytoplasmic portions to the intracellular cytoskeleton restricts movement of integral proteins.
• This is well characterized in red blood cells where the specialized cytoskeleton maintains the characteristic discoid shape.
• The complex scaffold of spectrin and actin also contains a variety of other proteins such as ankyrin and protein 4.1, all held together by noncovalent forces.
• Tight junctions hold epithelial cells very closely together, contributing to the barrier mechanism of the epithelium.
• Integral proteins on the epithelial surface are free to move within that surface but not into the portion of the plasma membrane which abuts adjacent cells.

Fluorescence Recovery After Photobleaching (FRAP)

• Slide 21 showed movement of membrane proteins after heterokaryon formation.
• FRAP involves tagging a protein with a fluorescent marker.
• A small spot on the surface is exposed to light (bleached) and loses its fluorescence.
• Recovery occurs as unbleached molecules move into the region.
• Movie 11.7 (FRAP) shows this process for several different proteins, located in different parts of the cell.

Glycocalyx

• The glycocalyx is the term used to refer to the outer surface of the plasma membrane and its coat of carbohydrates.
• Some carbohydrates are bound covalently to the membrane as part of glycolipids and integral membrane glycoproteins.
• Other carbohydrates are components of peripheral membrane glycoproteins, labeled adsorbed glycoproteins.
• Epithelial cells can have a glycocalyx on the free epithelial surface, and a glycocalyx with a different mixture of glycolipids and glycoproteins facing the basal lamina.

Extracellular Matrix

• The extracellular matrix is a larger, more complex association of macromolecules than the glycocalyx.
• It includes the variety of proteins (both glycosylated and non-glycosylated) which fill the spaces between cells.
• The basal lamina below an epithelial layer is one type of extracellular matrix.
• The extracellular matrix influences the development, migration, proliferation, shape, and function of the cells that contact it.