Cell Membranes and Vesicular Transport

structural lipids

• phospholipids

• cholesterol

• sphingolipids

biologically-active lipids

• steroid hormones

• platelet activating factor

• sphingolipids

storage lipids

• fatty acids

• triglycerides

more lipid bilayer fluidity

short, unsaturated hydrocarbon tails w/ less cholesterol

asymmetric composition of lipid bilayer

• more phosphatidylcholine on outside

• more phosphatidylserine phosphatidylethanolamine on inside

carbohydrate component of membranes

• short oligosaccharide chains (glycolipids and glycoproteins)

• long polysaccharide chains (proteglycans)

• outside only

integral membrane proteins

• transmembrane

• monolayer associated a-helix

• lipid-linked

peripheral membrane proteins

protein-attached

protein placement in membranes

• tethered to cell cortex

• tethered to ECM molecules

• tethered to proteins on surface of another cell

• diffusion barriers restrict proteins to membrane domain

tight junction

seals gap between epithelial cells

adherens junction

connects actin filament bundles in one cell with next cell

desmosome

connects intermediate filaments in one cell to those in next cell

gap junction

allows passage of small water-soluble molecules from cell to cell

hemidesmosome

anchors intermediate filaments in cell to ECM

actin-linked cell-matrix junction

anchors actin filaments in cell to ECM

cadherin

for cell-cell linkages

integrin

for cell-matrix linkages

transmembrane protein of adherens

classical cadherin

transmembrane protein of desmosome

nonclassical cadherin: desmoglein and desmocolin

transmembrane protein of actin-linked junction

integrin

transmembrane protein of hemidesmosome

alpha-beta integrin, collagen type 17

desmosomes give epithelia

mechanical strength; spots of cell-cell adhesion on lateral surfaces

intracellular cytoskeletal attachment of adherens

actin filaments

intracellular cytoskeletal attachment of actin-linked junction

actin filaments

intracellular cytoskeletal attachment of desmosome

intermediate filaments

intracellular cytoskeletal attachment of hemidesmosome

intermediate filaments

autoantibodies to desmogleins found in

pemphigus

autoantibodies to BP180, BP230, and specific integrins found in

pemphigoid

endomembrane system

interconnected network of ER, golgi, lysosome, and endosome

proteins on cytosolic surface of budding vesicle

• clatherin: from trans-golgi movement

• coat protein I: moves golgi to ER

• coat protein II: moves ER to golgi

how vesicle is formed

1) cargo receptors bind to what cells want inside

2) adaptins trap receptors in patch

3) clatherin coats and bends membrane inward to create a pocket

4) dynamin forms ring to pinch off clatherin-coated vesicle

5) clatherin coat sheds once vesicle pinches off to be recycled and so membrane can touch target membrane and fuse

vesicle origin and cargo is identified by

Rab proteins on surface of vesicle

target membrane has tethering proteins to

recognize correct surface identifier protein (Rab)

transmembrane proteins SNAREs on vesicle and target membrane

• enables docking of transport vesicles

• twist together tightly to exclude water so membranes will fuse

• once lipid bilayers fuse, cargo is released

constitutive secretion

• happens continuously

• adds new lipids and proteins to membrane for maintenance or cell growth

regulated secretion

• release signal causes exocytosis

• release hormones, mucus, digestive enzymes, and neurotransmitters

pinocytosis

endocytosis of fluid and macromolecules

phagocytosis

endocytosis of large particles or cells