membrane function
DESCRIPTION
Membrane Function. Signal Transduction. I. Introduction to Receptors & Signal Transduction. The Players. Signaling molecules Receptors G-proteins Second messenger systems Effector proteins. Signaling Molecules. Neurotransmitters Hormones Growth factors Drugs Other nomenclature - PowerPoint PPT PresentationTRANSCRIPT
Membrane FunctionMembrane Function
Signal Transduction
I. Introduction to I. Introduction to Receptors & Signal Receptors & Signal
TransductionTransduction
The PlayersThe Players
Signaling molecules Receptors G-proteins Second messenger systems Effector proteins
Signaling MoleculesSignaling Molecules
Neurotransmitters Hormones Growth factors Drugs Other nomenclature
Ligand Agonist / Antagonist
ReceptorsReceptors
Receptors are proteins associated with cell membranes
Receptors “recognize” signaling molecules by binding to them.
Binding of receptors by signaling molecules ---> Cell behavior change
Figure 1: Overview of Figure 1: Overview of SignalingSignaling
TyrosineKinase
mRNASynthesis
Protein Synthesis
SecondMessangers
Protein Kinases
IonChannels
Hormones:SteroidsThyroid
GrowthFactors
TransmittersTransmitters
Hormones
Neurotansmitters: Neurotansmitters: Biogenic Amines.Biogenic Amines.
Catecholamines Epinephrine Norepinephrine Dopamine
Esters: Acetylcholine Indolamines
Histamine 5-HT
Neurotransmitters: Neurotransmitters: PeptidesPeptides
Substance P Neuropeptide Y (NPY) Enkephalins Somatostatin VIP
Neurotransmitters: Amino Neurotransmitters: Amino AcidsAcids
Excitatory Glutamate Aspartate
Inhibitory -aminobutyric acid (GABA) Glycine
Neurotranmitters: OtherNeurotranmitters: Other
Nitric Oxide Arachadonic acid Carbon Monoxide PAF Zinc
The G-ProteinsThe G-Proteins
Involved in most signaling processes
Link receptor proteins to effector proteins.
Trimeric proteins composed of , , and -subunits.
Figure 2: G-Protein CyclingFigure 2: G-Protein Cycling
Adenylate CyclasePhospholipase C
Ion ChannelsPhospholipase A2
Phosphodiesterase
A
A
A
A
R
R
R
R
GTP(GTPase)
-Pi
GTP
GTP
GDP
GDP
Functional G-Protein UnitsFunctional G-Protein Units
GTP-activated -subunit produce second messenger and/or opens ion channels.
-complexes Initially thought to be inert. Probably not inert Exact role currently ill-defined.
Second messengers Second messengers produced by G-protein produced by G-protein
activation.activation. Adenylate Cyclase cAMP
Phospholipase C (PLC) Inositol triphosphate (IP3) Diacylglycerol (DAG)
Ion Channel Activity
Families of G-proteinsFamilies of G-proteins
Unique structure of their -subunits. subunits appear to be similar
across families. Main families:
Gs
Gi
Gq
II. cAMP Second II. cAMP Second Messenger SystemMessenger System
Figure 3: Adenylate Figure 3: Adenylate CyclaseCyclase
AdenylateCyclaseR1 R2
As
Gs Gi
Ai
GTPGDP
GTPGDP
PDEAMP cAMP
ATP-Mg++
Reg RegC
C
C
C
Protein
Protein-PProtein Kinase A
(PKA)PKA
Summary of Adenylate Summary of Adenylate Cyclase ActivationCyclase Activation
Receptors which associate with Gs -type G-protein Stimulates adenylate cyclase. Increases cAMP
Receptors which associate with Gi -type G-protein Inhibit adenylate cyclase. Decreases cAMP
Summary of cAMP actionSummary of cAMP action
cAMP exerts its effect by activating protein kinase A (PKA)
PKA phosphorylates proteins Enzymes, pumps, and channels Phosphorylation can either increase
or decrease activity depending on the protein.
Adenylate CyclaseAdenylate Cyclase
Family of membrane spanning enzymes.
Types I through IV have been well characterized. Additional types probably exist.
Types differ with respect to activity modulation by other second messenger systems
Adenylate Cyclase Activity Adenylate Cyclase Activity and Other Messenger and Other Messenger
SystemsSystems Kinases (PKA, PKC, other) can phosphorylate adenylate cyclase in some cells.
Binding of adenylate cyclase by: -subunits of other G-proteins Ca++/calmodulin complexes
Allows other second messenger systems to interact with cAMP system
III. The Phospholipase C III. The Phospholipase C Second Messenger Second Messenger
System:System:IPIP33 and DAG and DAG
Figure 4: Phospholipase C Figure 4: Phospholipase C SystemSystem
R
Ca++
PKC
Ca++Endoplasmic Reticulum
Gq
PLC
ProteinProtein-P
A
DAG
IP3
PIP2
Summary of the Summary of the Phospholipase C Phospholipase C
MessengersMessengers Agonist binds receptor Occupied Receptor ---> activation of
PLC (Gq -mediated) PLC Produces second messengers:
IP3 and DAG PLC activation associated with
Ca++-channel activation
Action of IPAction of IP33
IP3 binds to IP3-receptors on the endoplasmic reticulum
Releases intracellular Ca++ stores.
Action of DAGAction of DAG
Remains membrane associated. Activates Protein kinase C (PKC)
which translocates from the cytosol to the membrane
Activated PKC phosphorylates other proteins and alters their function state.
PLC System and CalciumPLC System and Calcium
PLC causes the IP3-mediated Calcium
PLC also causes the influx of Ca++. Ca++ binds one of a family of Ca++
binding proteins (calmodulin). Ca++/calmodulin complex
binds to yet other proteins and changes their functional activity.
IV. Guanylate Cyclase: IV. Guanylate Cyclase: cGMP and Nitric Oxide As cGMP and Nitric Oxide As
Second MessengersSecond Messengers
Figure 5: Nitric Oxide and Figure 5: Nitric Oxide and cGMPcGMP
cGMP
NO
Ca++
GTP
GMP
IntracellularCa++ Stores
Ca++
Ca++
Arginine
+Citrulline GTP
NO
PDE
Membrane BoundGuanylate Cyclase
SolubleGuanylate Cyclase
C.M.
Ion ChannelscGMP-Dependent PK
PDEase Activity
NO Synthetase
NO is Membrane Soluble.NO is Membrane Soluble.
Diffusion to nearby cells Increase cGMP levels in nearby
cells Vascular endothelial cells and
nearby smooth muscle cells.
V. SIGNALING BY V. SIGNALING BY ACETYLCHOLINEACETYLCHOLINE
Acetylcholine As a Acetylcholine As a NeurotransmitterNeurotransmitter
Both the central and peripheral nervous systems.
Binds two broad classes of receptors: Nicotinic receptors Muscarinic receptors.
Nicotinic Receptor Nicotinic Receptor FeaturesFeatures
Composed of 5 subunits: 2 , , and .
Subunits are arranged to form a central cavity that extends across the membrane.
Nicotinic receptors are also channels ACh-binding opens gates and allows
ion fluxes across the channel
Figure 6: Nicotinic Figure 6: Nicotinic ReceptorReceptorChannel
AgonistBinding
Site
Gate
Subclasses of Nicotinic Subclasses of Nicotinic ReceptorsReceptors
Skeletal muscle (N1 or Nm) Unique and subunits
Autonomic ganglia (N2 or Ng).
Both N1 and N2 are gene-product families not single receptor types.
Other Ligand-Gated Other Ligand-Gated ChannelsChannels
Structural and sequence similarity to nicotinic receptors.
Example agonists for these channels include: Serotonin (5-HT) Glutamate GABA Glycine
Muscarinic receptorsMuscarinic receptors
Muscarinic receptors are not channels.
Operate through G-proteins to alter second messenger systems.
5 muscarinic subtypes have been cloned and sequenced (M1, M2, M3, M4, M5).
Grouping Muscarinic Grouping Muscarinic ReceptorsReceptors
M1, M3, and M5 receptors: Activate Phospholipase C through Gq. PLC activation ---> increased IP3 -->
increased intracellular Ca++
Increased intracellular Ca++ --->Activation of Ca++-sensitive K+ & Cl- channels.
Grouping Muscarinic Grouping Muscarinic ReceptorsReceptors
M2 and M4 receptors Gi -coupled inhibition of adenylate
cyclase Go or Gi -coupled regulation of certain
Ca++ & K+ channels.
VI. Signaling by VI. Signaling by Epinephrine and Epinephrine and
Norepinephrine and Norepinephrine and Coupling Through Coupling Through
Adrenergic ReceptorsAdrenergic Receptors
Three Families of Three Families of Adrenergic Receptors:Adrenergic Receptors:
-receptors: Three subtypes and .
-receptors: Three subtypes AB and C
-receptors: Three subtypes A
B and C
..
All adrenergic receptors All adrenergic receptors appear to be coupled to appear to be coupled to
cellular processes cellular processes through G-proteinsthrough G-proteins
Occupation of Occupation of Adrenergic ReceptorsAdrenergic Receptors
Gs-mediated stimulation of adenylate cyclase
Increased cAMP Increased PKA activity.
Occupation of Occupation of - -Adrenergic ReceptorsAdrenergic Receptors
Mechanistic details sketchy Possibly Gq-mediated PLC
activation Increases IP3 and DAG for some
subtypes (1B)? Activates Ca++-channels for other
subtypes (1A)?
Occupation of Occupation of - -Adrenergic ReceptorsAdrenergic Receptors
Gi -mediated inhibition of adenylate cyclase.
Decreased cAMP Decreased PKA activity.