Cell Membranes and Vesicular Transport

Membrane thickness

5-8 nm thick

Membrane self-sealing property

Tough but flexible; no free edges allowed, seals itself

Membrane barrier property

Barrier to polar/charged solutes; permeable to non-polar compounds

Protein to lipid ratio

Varies widely by cell type

4 major membrane functions

Forms external cell boundary, regulates traffic (active/passive), divides cell into compartments, important for cell-cell communication

Single membrane organelles (examples)

Peroxisome, lysosome

Double membrane organelles

Nucleus, mitochondria

3 main chemical components of membranes

Lipids, carbohydrates, proteins

Membrane lipid types

Phospholipids and cholesterol

Where are membrane carbohydrates located

On the outer surface only

Membrane protein functions (general)

Selective exchange between inside/outside, receive external signals

Lipid characteristics

Chemically diverse, insoluble in water, NOT polymers, high C/H relative to O/N/P/S, small molecules that associate with other lipids

Structural lipids

Phospholipids, cholesterol, sphingolipids

Biologically-active lipids

Steroid hormones, platelet-activating factor, sphingolipids

Storage lipids

Fatty acids, triglycerides (universal stored energy form)

Amphipathic

Having both hydrophobic and hydrophilic regions (describes phospholipids)

Phospholipid polar head made of

Choline, phosphate, glycerol

Phospholipid nonpolar tails

Two hydrocarbon tails

Why phospholipids form bilayers

Their amphipathic (dual) nature favors bilayer formation

Cholesterol structure

Rigid planar steroid ring structure with a nonpolar hydrocarbon tail and small polar head group

Cholesterol's role in the bilayer

Fills gaps between phospholipid tails; amphipathic

Effect of a double bond in a phospholipid tail

Creates a kink (unsaturation) that fills gaps and increases fluidity

Fluid Mosaic Model

Model describing the membrane in which lipids/proteins (mixed components) move around within the membrane

3 factors controlling membrane fluidity

Hydrocarbon tail length, number of double bonds (saturation), cholesterol content

Effect of shorter hydrocarbon tails on fluidity

More fluid

Effect of more double bonds (unsaturation) on fluidity

More fluid

Effect of cholesterol on fluidity

Makes membrane less flexible/more rigid (short and rigid)

Lateral diffusion

Movement of lipids within the same monolayer

Flip-flop

Movement of lipids between the inner and outer membrane layers; rarely occurs on its own

Flipase

Enzyme that catalyzes flip-flop movement between inside and outside membrane layers

Phosphatidylcholine location

More concentrated on the OUTSIDE (outer) monolayer

Phosphatidylserine location

More concentrated on the INSIDE (inner) monolayer

Phosphatidylethanolamine location

More concentrated on the INSIDE (inner) monolayer

Sphingomyelin location

More concentrated on the OUTSIDE (outer) monolayer

Glycolipids

Carbohydrate-containing lipids found only on the outer (extracellular) surface of the membrane

Short oligosaccharide chains found on

Glycolipids and glycoproteins

Long polysaccharide chains found on

Proteoglycans

Integral membrane proteins

Proteins embedded within the lipid bilayer (transmembrane, monolayer-associated, or lipid-linked)

Peripheral membrane proteins

Proteins attached to the surface of other membrane proteins, not embedded in the bilayer

4 ways proteins associate with membranes

Transmembrane, monolayer-associated alpha helix, lipid-linked, protein-attached

4 categories of membrane protein function

Transporters/channels, anchors, receptors, enzymes

Anchor protein function

Connects cell's cytoskeleton to outside molecules; provides strength to the thin fragile membrane

Membrane protein placement

Not random; can be localized to form distinct membrane domains; proteins can move within a domain

Ways membrane proteins are localized/tethered

Tethered to cell cortex, tethered to extracellular matrix, tethered to proteins on another cell, restricted by diffusion barriers

Diffusion barriers

Structures that restrict membrane proteins to a particular membrane domain

Apical plasma membrane

Top/luminal surface of an epithelial cell

Lateral plasma membrane

Side surface of an epithelial cell, facing neighboring cells

Basal plasma membrane

Bottom surface of an epithelial cell, facing the basal lamina

Tight junction

Seals the gap between epithelial cells

Adherens junction

Connects actin filament bundle in one cell to that in the next cell

Desmosome

Connects intermediate filaments in one cell to those in the next cell

Gap junction

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

Actin-linked cell-matrix junction

Anchors actin filaments in a cell to the extracellular matrix

Hemidesmosome

Anchors intermediate filaments in a cell to the extracellular matrix

Basic cell linkage mechanism (3 parts)

Transmembrane adhesion protein (cadherin or integrin) + adaptor protein(s) + actin or intermediate filaments

Cadherin

Transmembrane adhesion protein used for CELL-CELL linkages

Integrin

Transmembrane adhesion protein used for CELL-MATRIX linkages

Adherens junction transmembrane protein

Classical cadherin (links to actin filaments)

Desmosome transmembrane protein

Nonclassical cadherins: desmoglein and desmocollin (links to intermediate filaments)

Actin-linked junction transmembrane protein

Integrin (links to actin filaments)

Hemidesmosome transmembrane protein

alpha6beta4 integrin and collagen type XVII (links to intermediate filaments)

Desmosome cytoplasmic plaque adaptor proteins

Desmoplakin, plakoglobin, plakophilin

Desmosome intermediate filament type

Keratin (in integument or oral mucous membranes)

Pemphigus

Autoimmune disease caused by autoantibodies against desmogleins (desmosome proteins)

Hemidesmosome key transmembrane integrin

alpha6beta4 integrin, binds laminin and collagen XVII (BP180)

Hemidesmosome intracellular adaptor

BP230, binds keratin (intermediate filament)

Pemphigoid

Autoimmune skin condition with blisters, caused by autoantibodies to BP180, BP230, and specific integrins

Pemphigus vs pemphigoid

Pemphigus = autoantibodies to desmosome proteins (desmogleins); Pemphigoid = autoantibodies to hemidesmosome proteins (BP180/BP230/integrins)

Endomembrane system components

ER, Golgi, lysosome, endosome

Endosome

Membrane compartment formed from endocytosed material

Vesicular transport purpose

Carries soluble proteins and membrane between compartments

Vesicle orientation rule

Cytosolic side of membrane always faces cytosol; non-cytosolic side always faces lumen or outside of cell

Why protein sorting happens first in vesicular transport

So only appropriate proteins move to their specific destination

Donor compartment

Where a vesicle buds off from (via budding)

Target compartment

Where a vesicle fuses to deliver its cargo (via fusion)

Clathrin

Coat protein involved in trans-Golgi movement (exocytosis/endocytosis)

COPI (Coat Protein I)

Coats vesicles that move material around the Golgi and back to the ER (retrograde)

COPII (Coat Protein II)

Coats vesicles that bud off the ER and deliver cargo to the cis face of the Golgi

Purpose of vesicle coat specificity

Acts like a "return address label," specific for particular origins/destinations

Order of clathrin-coated vesicle formation

Coat assembly and cargo selection, then bud formation, then vesicle formation (pinching off), then uncoating

Cargo receptors

Bind to specific molecules to be packaged into a vesicle

Adaptins

Trap cargo receptors in a patch during coat assembly; signal clathrin to associate with GTP

Dynamin

Forms a ring around the vesicle neck that pinches off the vesicle from the membrane

Uncoating

Shedding of the clathrin coat so the vesicle membrane can touch and fuse with the target membrane

Rab proteins

Identify vesicle origin and cargo type; located on the vesicle surface

Tethering proteins

Located on target membrane; recognize the correct Rab protein and bring the vesicle closer to the target membrane

v-SNARE

SNARE protein located on the vesicle membrane

t-SNARE

SNARE protein located on the target membrane

SNARE function

v-SNARE and t-SNARE interact/twist tightly together, excluding water so membranes can fuse; cargo released after fusion

Order of vesicle targeting steps

Tethering, then docking, then fusion

Membrane fusion process

SNAREs expel water at interface by pulling membranes together, then each lipid leaflet fuses to form a continuous bilayer, allowing contents to mix

Exocytosis

Process of vesicle docking and fusion with plasma membrane to release contents outside the cell

Constitutive secretion

Continuous, unregulated exocytosis; provides new lipids/proteins to plasma membrane and releases soluble proteins outside the cell

Regulated secretion

Exocytosis that only occurs with a specific signal; controls release of hormones, mucus, digestive enzymes, neurotransmitters

Why regulated secretory vesicles are packed extra full

Acidic pH and high Ca++ in the vesicle cause aggregation of contents

Examples of processes requiring exocytosis

Bone/dentin/cementum/enamel protein production, tooth development hormones, lamellar granules for skin/oral mucosa, salivary gland products

Endocytosis

Process where plasma membrane buds inward around material, pinches off, and delivers ingested material to an endosome

Pinocytosis

Endocytosis of fluid and macromolecules ("cell drinking")

Phagocytosis

Endocytosis of large particles or cells ("cell eating")

Early endosome

Endosome located near the plasma membrane