The document summarizes key aspects of cell membranes and membrane proteins. It discusses how cell membranes act as selective barriers and have three main roles: receiving information through signaling, importing and exporting materials through transportation, and enabling motility and cell growth. It then describes the lipid bilayer structure of membranes and different types of membrane proteins, focusing on their functions in transport, anchoring, receptor activity, and enzymatic processes. Finally, it discusses how membranes enclose intracellular compartments and how proteins are transported between membranes through various mechanisms.

1. Lecture 36: Membranes and Membrane Proteins
11/27/2011 10:24:00 PM
Cell membranes act as selective barriers
Prevent molecules from mixing
Three roles of plasma membrane
Receiving information (signaling)
Import/export (transportation)
Motility/cell growth
Membranes enclose many different compartments in a eukaryotic cell:
Nucleus (2x)
Mitochondria (2x)
ER, vesicles, golgi apparatus, lysosome, peroxisome
The Lipid Bilayer
Two-dimensional fluid
Fluidity depends on composition
Lipid bilayer is asymmetrical
Lipid asymmetry is generated inside the cell
Hydrophilic head, hydrophobic tail
The more unsaturated the tails are, fluidity is increased
Phosphatidylcholine is the most common phospholipid in cell
membranes
There are different types of membrane lipids and all are amphipathic
Hydrophilic molecules attract water, like dissolves in like
Hydrophobic molecules avoid water
Fats are hydrophobic, phospholipids are amphipathic (and form a
bilayer in water)
Pure phospholipids can form closed, spherical liposomes
Phospholipids can move
o Lateral
o Flexion
o Rotation
o Flip-flop
Fluidity depends on composition
o Cholesterol stiffens
o Low temperature, less unsaturation, long tails all reduce
fluidity.
Phospho and glycolipids are distributed asymmetrically in the plasma
membrane

2. o Glycolipids found on outside
o Phosphatidyl-serine, inositol, ethanolamine found on inside
usually.
Flippases transfer phospholipids to other side of membrane
New membranes are synthesized from ER.
o Form vesicles which fuse with other membranes
Membrane Proteins
Polypeptide chain usually crosses the bilayer as an α-helix
Proteins can be solubilized in detergents and purified
Plasma membrane is reinforced by the cell cortex
Cell surface is coated with carbohydrate
Cells can restrict movement of membrane proteins
Functions
o Transporters (ie Na pump)
o Anchors (integrins)
o Receptors (platelet-derived growth factor receptor)
o Enzymes (adenylcyclase)
50% of mass of plasma membranes, 50 times more lipid than protein
molecules.
Different ways of associating with membrane:
o Alpha helix, beta pleated sheets, transmembrane, lipid linked
o Can be peripheral (protein attached)
Folded up proteins traverse membrane easier because the polar
backbone is exposed
Multiple alpha helixes form a hydrophilic pore
Porin proteins form water-filled channels in the outer membrane of a
bacterium
o Formed by 16 strands of β-sheets
o Allows passage of ions and nutrients across outer membranes
of some bacteria and of mitochondria
Membranes are disrupted by detergents such as SDS and Triton X-100
o Have only one tail
Bacteriorhodopsin acts as a proton pump powered by light, drives ATP
synthase

3. Plasma membrane reinforced by cell cortex – imparts shape and
function
o Spectrin meshwork forms the cell cortex in red blood cells
Eukaryotic cells are sugar coated
o Absorb water for lubrication
o Cell-cell recognition
o Protect cell from physical, chemical, enzymatic damage
o Recognition of cell surface carb on neutrophils mediates
migration in infection
Movement can be restricted by cells
o Tethering to cell cortex, extracellular matrix, proteins on
surface of another cell, or by barriers of diffusion like tight
junctions.

4. Lecture 37
I. General Principles of Cell Signaling
Can act over long or short range
Each cell responds to limited set of signals
Signals relayed via intracellular signaling pathways
Nitric oxide crosses plasma membrane and activates intracellular
enzymes directly
Some hormones cross plasma membrane and bind to intracellular
receptors
There are three classes of cell surface receptors
Ion channel-linked receptors convert chemical into electrical signals
Intracellular signaling proteins act as molecular switches
Origins in unicellular organisms
o Yeast shows single cell-cell communication
o Two mating types, a and α plus a secreted mating factor
signal
Signals transduction: conversion of one type of signal into another
o Extracellular -> intracellular
4 ways animal cells signal
o Endocrine
o Paracrine
o Neuronal
o Contact-dependent
 Lateral inhibition: Unspecified epithelial cells, one cell is
dedicated to becoming a nerve cell and inhibits
surrounding cells by Delta-Notch signaling.
One signal molecule can induce different responses in different cells
o ie: acetylcholine: (time scale is seconds to minutes)
 In heart muscle cells, causes decreased rate and force
of contraction.
 In salivary gland cells, causes secretion.
 In skeletal muscle cells, causes contraction.
An animal cell depends on multiple extracellular signals
Extracellular signal molecules can alter activity of diverse cell proteins
which in turn alter cellular behavior

5. o The intracellular signaling proteins are involved in a signaling
cascade which ultimately reach the target proteins for altered
behavior like metabolism, gene expression, and cell shape or
movement.
Cellular signaling cascades can follow a complex path
o Primary transduction, relay, amplification, or branching to
different targets.
Extracellular signal molecules can either bind to cell surface receptors
or to intracellular enzymes or receptors (like nitric oxide)
o Nitric oxide is a product of nitroglycerin which is taken to
relax smooth muscle cells.
 Triggers smooth muscle relaxation in blood-vessel wall
Steroid hormones bind intracellular receptors that act as gene
regulatory proteins
o Cross plasma membrane, like NO
o Cholesterol does not cross membrane, rather inserts IN
membrane.
o Cortisol acts by activating a gene regulatory protein
Most signal molecules bind to receptor proteins on the target cell
surface
o Extracellular domains are the cell surface receptor
o Three basic classes:
 Ion channel linked -> nervous system, muscle
 G-protein linked -> all cells
 Enzyme-linked -> all cells
Many intracellular signaling proteins act as molecular switches
o Signaling by phosphorylation
 Signal in by phosphorylation, off by phosphatase
inactivation.
o Signaling by GTP-binding protein
 GTP binds to G-protein, turning it on.
 GTP hydrolysis inactivates by removing P.
II. G-protein-linked Receptors
Stimulation of G-protein linked receptors
G proteins can regulate ion channels

6. G proteins can activate membrane bound enzymes
Cyclic AMP pathway can activate downstream genes
Inositol phospholipid pathway triggers rise in Ca
Ca signal triggers many biological processes
Intracellular signaling cascades can achieve astonishing speed (ie
photoreceptors in the eye)
All G-protein linked receptors possess a similar structure
o 7 transmembrane protein
o Ligand binds to extracellular binding domain
o Cytoplasmic domain which binds to G-protein
o Tetramer is active, GDP can dissociate, GTP can bind, and
then complex dissociates into two activated parts.
o The alpha subunit switches itself off by hydrolyzing
bound GTP
G proteins couple receptor activation to opening of cardiomyocyte K
channels
o Acetylcholine binds to G protein linked receptor
o Beta gamma complex binds to closed K channel to open it
o Alpha subunit is inactivated (by hydrolysis) and inactive
complex reassociates with betta gamma complex to close K
channel.
Enzymes activated by G proteins catalyze synthesis of intracellular
second messengers
o Alpha subunit activates adenylyl cyclase which makes lots of
cylic AMP.
o Cyclic AMP concentration rises rapidly in response to
neurotransmitter serotonin
o Cyclic AMP is synthesized by adenylyl cyclase, degraded by
cAMPphosphodiesterase
Extracellular signals can act rapidly or slowly
Rise in intracellular cyclic AMP can activate gene transcription through
protein kinase A
o Translocates through nuclear pore, into nucleus,
phosphorylates gene regulatory protein to activate target
gene.

7. Membrane bound phospholipase C activates two small messenger
molecules: IP3, DAG
o Phospholipase C activated by alpha subunit, splits inositol
phospholipid into IP3 and DAG
o IP3 opens Ca channel in ER, Ca is released and works with
DAG to activate Protein Kinase C.
Fertilization of an egg by sperm triggers a rapid increase in cytosolic Ca
o Other processes triggered by Ca signal:
 Sperm entry -> embryonic development
 Skeletal muscle -> contraction
 Nerve cells -> secretion
Calcium/Calmodulin complex are what bind to proteins.
A rod photoreceptor cell from the retina is exquisitely sensitive to light
o G protein linked light receptor activates G protein
transducing, activated alpha subunit causes Na channels to
close.
o Light induced signaling cascade in rod photoreceptors greatly
amplifies light signals.
III. Enzyme linked receptors
Activated receptor tyrosine kinases assemble a complex of intracellular
signaling proteins
o Ligand brings two tyrosine kinase domains together,
phosphorylated to activate. Intracellular signaling proteins
bind to phosphorylated tyrosines.
o Activated complex includes Ras-activating protein, which is
anchored in membrane, transmits signal downstream.
 Ras is monomeric GTP-binding protein, not a trimeric G
protein, but resembles the alpha subunit and functions
as a molecular switch.
 30% of cancers arise from mutations in Ras.
 Ras activates a MAP-kinase phosphorylation cascade
Some enzyme-linked receptors activate a fast track to the nucleus
Protein kinase networks integrate info to control complex cell behaviors
Multicellularity and cell communication evolved independently in plants
and animals

8. Cytokine receptors are associated with cytoplasmic tyrosine kinases
o JAK kinases phosphorylate receptor which recruits
cytoplasmic proteins.
TGF-beta/BMP receptors activate gene regulatory proteins directly at
the plasma membrane
Signaling pathways can be highly interconnected: cross-talk
Lecture 38
General introduction:
Membrane enclosed organelles are distributed throughout the
cytoplasm
o Thousands of different reactions occur simultaneously, are
partitioned
o Cytosol is 54% of cell
o Mitochondria is 22% of cells
o ER is 12% of cell (1 per cell)
Nuclear membrane and ER may have evolved at the same time through
invagination of plasma membrane.
Mitochondria are thought to have originated from aerobic prokaryote
being engulfed by a larger anaerobic eukaryotic cell -> has it’s own
genome.
Nucleus is a double membrane organelle
o Encloses nuclear DNA, defines nuclear compartment and
contains most of the genetic information.
o Export, import through nuclear pore complex
 Contains about 100 proteins, two way gate, export of
mRNA, and ribosome subunits.
 Import of proteins requires a signal sequence called the
nuclear localization signal
 Requires energy (GTP) and special chaperone proteins
 Export of RNA from nucleus – RNA molecules are made
in the nucleus and exported to the cytoplasm as
processed mRNA
One Endoplasmic Reticulum

9. o System of interconnected sacs and tubes of membrane
o Extend throughout most of cell
o Major site of new membrane (lipid) synthesis
o With ribosomes on cytosolic side = rough ER
o Without ribosomes = smooth ER
o Most extensive network membrane in eukaryotic cells
Golgi apparatus
o Flattened sacs called cisternae which are piled like stacks of
plates
o Usually near nucleus
o Two faces:
 Cis face adjacent to ER
 Trans face towards plasma membrane (where post
translational modification occurs)
o Receives proteins and lipids
o Site of modification of proteins and lipids
o Dispatches proteins and lipids to final destinations
o Transport vesicles bud off
Other membrane enclosed organelles
o Endosomes – small membrane enclosed organelles that sort
ingested molecules in endocytosed materials. Passed to
lysosomes or recycled back to the plasma membrane.
o Lysosomes – small sacs containing digestive enzymes that
degrade organelles, macromolecules, and particles taken in
by endocytosis. “garbage disposal of the cell.” Ph about 7.2
o Peroxisomes – small membrane enclosed organelle containing
oxidative enzymes that break down lipids and destroy toxic
molecules
Protein transport
o Multiple modes of protein transport (import and export)
o Three mechanisms
 Transport through nuclear pores: protein with nuclear
localization signal enter through pores
 Across membranes: proteins moving from cytosol into
ER, mitochondria and peroxisomes transported across
organelle membrane by protein translocators

10.  By vesicles: from ER onward and from one
endomembrane compartment to another ferried by
transport vesicles
Protein sorting signals
o Specific amino acid sequence
o Directs protein to organelle
o Proteins without signals remain in cytosol
o Signal sequences direct proteins to different compoartments
 Continuous stretch of AA usually 15-20 residues in
length
 Usually removed after the protein reaches destination
 Organelles and signal sequences:
 ER import rich in V A L I and retendtion KDEL
 Mitochondria rich in R
 Nucleus PPKKKRKV
 Peroxisomes SKL
o Signal sequences are both necessary and sufficient to direct
protein to organelles
ER: entry point for protein distribution
o Proteins destined for golgi, lysosomes, endosomes and cell
surfaces first enter ER from cytosol
o Once inside ER or membrane, proteins do not reenter cytosol
o Water soluble proteins are completely translocated across ER
membrane and released into ER lumen
o Transmembrane proteins only partially translocated across ER
membrane and become embedded
Vesicular transport
o Entry into ER
o To golgi apparatus
o From er ->golgi -> other by continuous budding, fusion of
transport vesicles
o Vesicle transport provides routes of communication
Protein transport: quality control
o Most proteins that enter ER are destined for other locations

11. o Exit from the ER is highly selective: improperly modified and
or folded proteins are retained in lumen; dimeric or
multimeric proteins that fail to assemble are also retained
Exocytosis
o Constitutive: newly synthesized proteins, lipids, and carbs
delivered from ER via golgi to subcellular locations,
extracellularly to ECM via transport vesicles.
 Lipids and proteins supplied to plasma membrane
 Proteins secreted into ECM or onto the cell surface
o Regulated
 Specialized secretory cells synthesize high levels of
proteins such as hormones or digestive enzymes that
are stored in secretory vesicles for subsequent release
 Vesicles bud off from trans golgi network and
accumulate adjacent to plasma membrane until
mobilized by extracellular signal
Endocytosis
o Pinocytosis (drinking)
 Internalizes plasma membrane: as much membrane is
added to cell surface by exocytosis as is removed by
endocytosis – total surface area and volume remain
unchanged.
 Mainly carried out by transport vesicles: deliver
extracellular fluid and solutes to endosomes; fluid
intake is balanced by fluid loss during exocytosis
o Phagocytosis (eating)
 Specialized cells only

12. o

13. Lecture 39: Cytoskeleton
Roles of cytoskeletal filaments:
Intermediate – cell structure against mechanical stress
Microtubules – intracellular transport, railroad of cell
Actin – membrane mobility; cell movement
Intermediate filaments:
10 nm in diameter
Rope like structure composed of long polypeptides twisted together
Associated with cell junctions
Mechanical strength, cell shape, cell-cell contacts, and structure for
nuclear envelope
Monomers -> dimer -> tetramer -> 8 tetramers make one ropelike
filament
Different proteins:
o Epithelia – keratins
o Connective tissue, muscles, neuroglial cells – vimentin
o Nerve cells – neurofilaments
o Nuclear envelope in animal cells – nuclear lamins
Mutation in keratin genes = epidermolysisbullosa simplex
Networks of filaments connect across desmosomes in epithelia
Microtubules:
25 nm wide
Hollow, made of α and β tubulin anchored to γ tubulin
Have polarity – gives directionality
“Dynamic instability”: built or disassembled as needed
o Zip up to grow
o Unravel and tubulin molecules fall off if not needed
o This is done by GTP since tubulin are GTPases
 GTPases are the cell’s timers
 High energy phosphate bond. Molecules with GTPases
hydrolyze that bond, leaving GDP
 GTP between α and β tubulin molecules makes them
straighter, so they pack better. GTP hydrolysis makes
them kinked, so they fall off.

14. Organize cell organelles and control traffic of vesicles
Roles in interphase cell, dividing cell, ciliated cells, flagella.
The centrosome
o Centriles inside of centrosome, nobody knows what they do
o Centrosome is an envelope of tubulin where microtubules
extend out with plus end out.
Structure:
o α and β tubulin strands
Stabilizing or destabilizing MTs
o Microtubule associated proteins (MAPs)
 Bind to free ends of MTs and stabilize ends selectively
to polarize a cell
o Drugs can be used to change MT stability
 Colchicine binds free tubulin to prevent polymerization;
MTs disintegrate and mitosis stops
 Taxol prevents loss of subunits from MTs; MTs become
“frozen” in place and mitosis stops.
MT organized transport
o Anterograde transport, retrograde transport.
Motor proteins use ATP to power transport along the MT railroad
o Kinesin and dynein are dimers that walk along microtubule.
o One ATP is used per step.
Cilia and flagella are made of MTs
Actin Filaments:
Control cell movement
Found in:
o Epithelial cell microvilli
o Stress fibers in cultured cells
o Leading edge lamellipodia
o Contractile ring in dividing cells – cytokinesis
Actin polymerization requires ATP
o Free G-actin monomers use ATP to become F-actin to form
filaments. To uncoil, hydrolyze ATP and fall apart.
Actin dynamics provide force for membrane movement
o ARP complex create branches

15. o Depolymerizing protein promotes ATP hydrolysis
o Capping proteins cap the ends and stabilize ATP bound
monomer, stabilizing leading edge.
Actin binding proteins link actin fibers to the membrane and other
cellular components
Integrins link actin to focal adhesions
o Binds to extracellular structures, messages to actin.
Cells move by actin crawling (dynamics)
Axon growth cone crawling
Rho family GTPases control actin dynamics
o RhoA causes stress fibers
 Stabilize actin filaments
 Induces myosin phosphorylation and thus contractility
o Cdc432 causes filopodia extension
 Promotes actin nucleating by ARP complexes
o Rac promotes lamellipodia extension
 Promotes actin nucleation, but also uncapping to allow
more sites of nucleation
o Cell surface receptors modulate Rho family activity
 Attractive cues activate Rac and Cdc42 on area of
growth cone
 Repulsive cues activate RhoA
 Growth cone turns
Myosins: actin motor proteins
o Head, neck, tail
o Tails link up together
o Work as dimers
o Moves membranes or cell components
Muscle contraction by actin and myosin
o Myosin heads climb up actin filament
o Z disks move together, muscle contracts