Wk2_Ch02,03_F09

8/8/2019 Wk2_Ch02,03_F09

http://slidepdf.com/reader/full/wk2ch0203f09 1/11

9/8/09

1

Chapter 2

Transport of ions and small molecules

across membranesBy

Stephan E. Lehnart & Andrew R. Marks

Cell Biology, Roosevelt University, Fall 2009

2.1 Introduction

• The membrane potential across a cell

membrane is due to:

– an electrochemical gradient across a membrane

– a membrane that is selectively permeable to ions

• Cells maintain a negative resting membrane

potential with the inside of the cell slightly

more negative than the outside.

• The membrane potential is a prerequisite for

electrical signals and for directed ion

movement across cellular membranes.

2.4 Electrochemical gradients across the cell membrane generate the membrane potential

Action Potentials

• Action potentials enable rapid communicationbetween cells.

• Na+, K+, and Ca2+ currents are key elementsof action potentials.

• Membrane depolarization is mediated by theflow of Na+ ions into cells through voltage-dependent Na+ channels.

• Repolarization is shaped by transport of K+

ions through several different types of K+

channels.

• The electrical activity of organs can be

measured as the sum of action potential

vectors.

• Alterations of the action potential can

predispose for arrhythmias or epilepsy.

2.12 Action potentials are electrical signals that depend on several types of ion channels

8/8/2019 Wk2_Ch02,03_F09


8/8/2019 Wk2_Ch02,03_F09


9/8/09

3

• The selectivity filter of K+ channels is an

evolutionarily conserved structure.

• The K+ channel selectivity filter catalyzes

dehydration of ions, which:

– confers specificity

– speeds up ion permeation

2.5 K+ channels catalyze selective and rapid ion permeation 2.5 K+ channels catalyze selective and rapid ion permeation

• Gating is an essential

property of ion

channels.

• Different gating

mechanisms definefunctional classes of K+

channels.

• The K+ channel gate is distinct from the

selectivity filter.

• K+ channels are regulated by the membrane

potential.

2.6 Different K+ channels use a similar gate coupled to different activating or inactivating mechanisms.

• The inwardly directed Na+ gradient maintained by the

Na+/K+-ATPase is required for the function of Na+

channels.

• Electrical signals at the cell membrane activate

voltage-dependent Na+ channels.

• The pore of voltage-dependent Na+ channels is

formed by one subunit, but its overall architecture is

similar to that of 6TM/1P K+ channels.

• Voltage-dependent Na+ channels are inactivated by

specific hydrophobic residues that block the pore.

2.7 Voltage-dependent Na+ channels are activated by membrane depolarization and translate electrical signals

8/8/2019 Wk2_Ch02,03_F09


9/8/09

4

• Cell surface Ca2+ channels translate membrane

signals into intracellular Ca2+ signals. Muscle Contraction

• The process of excitation-contraction

coupling, which is initiated by membrane

depolarization, controls muscle contraction.

• Ryanodine receptors and inositol 1,4,5-

trisphosphate receptors are Ca2+ channels

that release Ca2+ into the cytosol from

intracellular stores

• Intracellular Ca2+ release through ryanodine

receptors in the sarcoplasmic reticulummembrane stimulates contraction of the

myofilaments.

• Several different types of Ca2+ transport

proteins, including the Na+/Ca2+-exchanger

and Ca2+-ATPase are important for

– decreasing the cytosolic Ca2+ concentration

– controlling muscle relaxation

2.13 Cardiac and skeletal muscles are activated by excitation-contraction coupling 2.13 Cardiac and skeletal muscles are activated by excitation-contraction coupling

Aquaporins

• Aquaporins allow rapid

and selective water

transport across cellmembranes.

• Aquaporins aretetramers of four

identical subunits, with

each subunit forming a

pore.

• The aquaporin selectivity filter has three

major features that confer a high degree of

selectivity for water:

– size restriction

– electrostatic repulsion

– water dipole orientation

2.11 Selective water transport occurs through aquaporin channels

8/8/2019 Wk2_Ch02,03_F09


9/8/09

5

Glucose transport

• To cross the blood-brain

barrier, glucose is

transported acrossendothelial cells of

small blood vessels into

astrocytes.

• Glucose transporters

are uniporters

Symporters and antiporters mediate

coupled transport

• Bacterial lactose permease functions as a symporter .

– It couples lactose and proton transport across thecytoplasmic membrane.

• Lactose permease uses the electrochemical H+ gradient to drive lactose accumulation inside cells.

• Lactose permease can also use lactose gradients tocreate proton gradients across the cytoplasmicmembrane.

2.15 Symporters and antiporters mediate coupled transport

• The mechanism of transport by lactose permease

likely involves inward and outward configurations.

– They allow substrates to:

• bind on one side of the membrane and to

• be released on the other side

2.15 Symporters and antiporters mediate coupled transport

2.16 The transmembrane Na+ gradient is

essential for the function of many

transporters • The plasma membrane Na+

gradient is maintained by the

action of the Na+/K+-ATPase.

• The energy released bymovement of Na+ down its

electrochemical gradient is

coupled to the transport of a

variety of substrates.

– Ca2 +

– Mg2+

– Cl-

– glucose

2.18 The Ca2+-ATPase pumps Ca2+ into

intracellular storage compartments

• Ca2+-ATPases undergo areaction cycle involving

two major conformations,

similar to that of Na+/K+-

ATPases.

• Phosphorylation of Ca2+-ATPase subunits drives:

– conformational changes

– translocation of Ca2+ ionsacross the membrane

8/8/2019 Wk2_Ch02,03_F09


9/8/09

6

2.19 The Na+/K+-ATPase maintains the

plasma membrane Na+ and K+ gradients

• The Na+/K+-ATPase is a P-type ATPase that is

similar to the Ca2+-ATPase and the H+-ATPase.

• The Na+/K+-ATPase maintains the Na+ and K+

gradients across the plasma membrane.

• The plasma membrane Na+/K+-ATPase is

electrogenic:

– it transports three Na+ ions out of the cell for every two K+

ions it transports into the cell.

2.20 The F1Fo-ATP synthase couples H+

movement to ATP synthesis or hydrolysis

• The F1Fo-ATP synthase is a

key enzyme in oxidative

phosphorylation.

• The F1Fo-ATP synthase is a

multisubunit molecular

motor.

– It couples the energy

released by movement of protons down their

electrochemical gradient toATP synthesis.

2.21 H+-ATPases transport protons out of

the cytosol

• Proton concentrations affectmany cellular functions.

• Intracellular compartments areacidified by the action of V-ATPases.

• V-ATPases are proton pumpsthat consist of multiplesubunits, with a structure

similar to F1Fo-ATP synthases.

• V-ATPases in the

plasma membraneserve specialized

functions in:

– acidification of

extracellular fluids

– regulation of cytosolicpH

2.21 H+-ATPases transport protons out of the cytosol

Chapter 3

Membrane targeting of proteinsBy

D. Thomas Rutkowski & Vishwanath R. Lingappa

• Cells must localize proteins to

specific organelles and

membranes.

• Proteins are imported from the

cytosol directly into several types

of organelles.

• The endoplasmic reticulum (ER):

– is the entry point for proteinsinto the secretory pathway

– is highly specialized for that

purpose

• Several other organelles and theplasma membrane receive their

proteins by way of the secretory

pathway.

3.1 Introduction

8/8/2019 Wk2_Ch02,03_F09


9/8/09

7

• Signal sequences targetnascent secretory and

membrane proteins to theER for translocation.

• The only feature commonto all signal sequences is

a central, hydrophobic

core that is usually

sufficient to translocateany associated protein.

• Secretory proteins translocate completely across the ERmembrane; – transmembrane proteins are integrated into the membrane.

• Before leaving the ER, proteins are modified and folded byenzymes and chaperones in the lumen.

3.2 Proteins enter the secretory pathway by translocation across the ER membrane

• Docking of SRP with its

receptor brings the

ribosome and nascentchain into proximity with

the translocon.

• Docking requires theGTP binding and

hydrolysis activities of

SRP and its receptor.

• Proteins translocate through

an aqueous channel

composed of the Sec61complex, located within the

ER membrane.

• Numerous accessoryproteins that are involved in:

– Translocation

– Folding

– Modification associate with thechannel

• An interaction

between the

translocon and thesignal sequence

causes the channel

to open and initiates

translocation.

• The exact

mechanism of

translocation mayvary from one

protein to another.

• An interaction

between the

translocon and thesignal sequence

causes the channel

to open and initiates

translocation.

• The exact

mechanism of

translocation mayvary from one

protein to another.

8/8/2019 Wk2_Ch02,03_F09


9/8/09

8

• Posttranslational translocation proceeds

independently of both ribosomes and SRP

3.8 Some proteins target and translocate posttranslationally

3.9 ATP hydrolysis drives translocation

• The energy for

posttranslationaltranslocation comes fromATP hydrolysis by the BiP

protein within the ER

lumen.

• The energy source for

cotranslational

translocation is less clear,

but might be the same asfor posttranslational

translocation.

• The synthesis of

transmembrane

proteins requiresthat trans-

membrane

domains be

– recognized

– integrated into thelipid bilayer

• Transmembrane domains exit the transloconby moving laterally through a protein-lipidinterface.

3.10 Transmembrane proteins move out of the translocation channel and into the lipid bilayer

• Nascent chains are often

subjected to covalent

modification in the ER

lumen as they

translocate.

• The signal peptidase

complex cleaves signal

sequences.

• GPI addition covalently

tethers the C-termini of some proteins to the lipid

bilayer.

• Oligosaccharyltransferase (OST) catalyzes N-linked

glycosylation on many proteins as they are translocated

into the ER.

8/8/2019 Wk2_Ch02,03_F09


9/8/09

9

• Molecular chaperones associate with proteins in the

lumen and assist their folding.

• Subunits that have not yet assembled into complexes are

retained in the ER by interaction with chaperones.

• Protein disulfide isomerases (PDI) catalyze disulfide

bond formation and rearrangement in the ER.

• Calnexin and

calreticulin escortglycoproteins

through repeated

cycles of

chaperoning.

– The cycles arecontrolled by

addition and

removal of

glucose.

• Proteins are returned to the cytosol by the

process of retrograde translocation.

3.19 Terminally misfolded proteins in the ER are returned to the cytosol for degradation

• The major cellular

phospholipids are

synthesized predominantly

on the cytosolic face of the

ER membrane.

• The localization of enzymes

involved in lipid

biosynthesis can be

controlled by the cell to

regulate the generation of

new lipids.

• Cholesterol biosynthesis is

regulated by proteolysis of

a transcription factor

integrated into the ER

membrane.

• Each organelle has a unique composition of lipids.

– This requires that lipid transport from the ER toeach organelle be a specific process.

• The mechanisms of lipid transport betweenorganelles are unclear.

– They might involve direct contact between the ERand other membranes in the cell.

• Transbilayer movement of lipids establishesasymmetry of membrane leaflets.

8/8/2019 Wk2_Ch02,03_F09


Wk2_Ch02,03_F09

Documents

Transcript of Wk2_Ch02,03_F09