Chapter 15: Signal transduction
Know the terminology:Enzyme-linked receptor, G-protein linked receptor, nuclear hormone receptor, G-protein, adaptor protein, scaffolding protein, SH2 domain, MAPK, Ras, protein kinase, MAPK, protein phosphatase, phospholipase, phosphodiesterase, cAMP, crosstalk,
Chapter 15: Signal transduction
Outline:General principles of signal transductionOverview of:
SignalingReceptorsTransducersTargets
Major types of cell-surface receptorsRTK signalingG-protein signaling
General Principles of Signal Transduction
1. Communication usually involves a (i) signaling molecule, (ii) a receptor, (iii) intracellular signal transducers and (iv) targets
General Principles of Signal Transduction
2. Each cell responds to a complex profile of signaling molecules (crosstalk)
General Principles of Signal Transduction
3. Different cells respond differently to a particular signaling molecule
General Principles of Signal Transduction
4. Cells can remember the effects of some signals
5. Cells can adjust their sensitivity to a signal
General Principles of Signal Transduction
4. Cells can remember the effects of some signals
5. Cells can adjust their sensitivity to a signal
General Principles of Signal Transduction
6. Signal can exhibit complex responses to signal concentration
Signaling molecules
Signaling molecules come in many chemical forms:
• Proteins: insulin, glucagon• Steroids et al.: testosterone, estradiol,
cortisol• Amines: thyroxine, catecholamines,
acetylcholine • Gases: nitric oxide
Signaling pathways require molecules with rapid rates of synthesis and degradation
Typically released from one cell and recognized by another cell
Signaling molecules
Secretory signals:• Autocrine-signals affect same cell or cell
type• Paracrine-signals affect neighbouring cell• Endocrine-signals affect distant cells
Contact-dependent signals:-signals are not released but affect other cells
in contact through protein-protein interactions
Autocrine signaling
Signals released by one cell affect other cells in the immediate vicinity
Amplify a response by inducing many “like-cells” to respond in the same way
Allows cells to exhibit a coordinated response (a community effect)
Autocrine signaling
Paracrine signaling
Signals released by one cell affect different cells in the immediate vicinity
Synaptic transmission resembles paracrine stimulation but the response is limited to cells in very close proximity
The outward propagation of the signal is limited by cellular uptake, extracellular degradation, and binding
Endocrine signaling
Signals released by one cell affect different cells far away
Endocrine signaling often exerts multiple effects on the organism by affecting many different tissues
Receptors
Proteins that bind signals and initiate a signaling cascade
Cell membrane receptors-integral membrane proteins that bind an extracellular signal and start a signal cascade
Intracellular receptors-nuclear hormone receptors
Nuclear hormone receptors
Examples that we have already discussed include steroid hormone receptor and thyroid hormone receptor
NHRs are transcription factors that respond to specific ligands
Ligands alter the ability to bind to specific DNA regulatory elements
Intracellular signal transduction
Once the receptor is activated, the signal is propagated by proteins that act as:
–Relay proteins–Messenger proteins–Adaptor proteins–Amplifier proteins–Transducer proteins–Bifurcation proteins–Integrator proteins–Latent gene regulatory proteins
Intracellular signal transduction
Activated cell membrane receptors can alter the activity of intracellular enzymes including:
–Protein modifying enzymes•kinases/ phosphatases•acetylases/ deacetylases
–Lipid modifying enzymes•Phospholipases•Phosphotidyl inositol kinase
–Nucleotide modifying enzymes•cyclases/ phosphodiesterases
Protein kinases
Phospholipases
PLC generates DAG and phosphoinositides, such as IP3 (inositol 1, 4, 5- triphosphate)
Targets
The final targets of signaling cascades are usually proteins: •Regulators of gene expression (transcription factors, histone remodeling enzymes)•Enzymes (metabolic enzymes)•Structural proteins (cytoskeletal proteins)
•Effects alter activity (catalytic, DNA binding) or the ability to interact with other proteins (structural proteins, subcellular localization).
Cell surface receptors
3 main classes of cell surface receptors:
Ion-channel linked receptorsEnzyme linked receptors may possess intrinsic enzyme activity or, once ligands bind, activate enzyme activity
G-protein linked receptors are trimeric GTP-binding protein (G-protein) that regulate the activity of other proteins
Enzyme-linked receptors5 main classes distinguished by:•type of effector (e.g. kinase vs. phosphatase) •target (serine/threonine, tyrosine, histidine)•type of linkage between receptor and enzyme
1. Receptor tyrosine kinase (-RTK)2. Tyrosine kinase linked receptor3. Receptor serine/threonine kinase4. Receptor guanylyl cyclase5. Histidine-kinase associated receptors
Receptor tyrosine kinasesMost common type of receptor for many common protein hormones including EGF, PDGF, FGF, HGF, IGF-1, VEGF, NGF.
Receptor tyrosine kinasesReceptor itself possesses intrinsic tyrosine kinase activity
Once the ligand binds, the receptor can dimerize and it become an active tyrosine kinase
It phosphorylates itself (autophosphorylation), causing:
1. Increase kinase activity2. Increased affinity for other proteins
Once bound, these docking proteins can become phosphorylated
Ligand-dependent autophosphorylation and docking
Ligand-dependent autophosphorylation and docking
Docking of intracellular proteins on phosphotyrosines
Phosphotyrosine domains are binding sites for many different proteins with SH2 (=PTB) domains
These can be enzymes (e.g., PLC, PI3K) or they can act as adaptor molecules to bind other proteins
Linking RTK to Ras and the MAPK cascade
Once an adaptor protein (e.g., Grb2) binds to the RTK, it attracts another protein - Ras GEF (guanine nucleotide exchange factor)
Ras GEF induces Ras to exchange its GDP for GTP (activating Ras).
Active Ras then activates MAPKKK, which phosphorylates and activates MAPKK, which phosphorylates and activates MAPK, which phosphorylates many proteins, including transcription factors.
Activation of Ras
Activation of MAPK cascade
Scaffolding proteins help organize MAPKs
Summary: Enzyme-linked receptors
How do enzyme-linked receptors generate variable cellular responses?
Multiplicity of players (receptors, kinases etc) arise from gene duplication and divergence
Recognize the critical role of phosphorylation/ dephosphorylation control as molecular switches
Adaptor molecules allow construction of protein signaling cascades with variable outputs
G-protein linked receptorsLigand: Diverse ligands, such as epinephrine
Receptor: Integral membrane protein with 7-transmembrane domains
G-protein: Trimeric protein attached to the cell membrane by lipid anchors
Effectors: Target proteins that show altered activity when they interact with activated G-protein subunits (or )
G-protein linked receptors and G-proteins
Receptor
G-protein
Interaction between receptor and G-protein
Once the ligand binds, the activated receptor recruits a G-protein
Nucleotide exchange occurs (GTP replaces GDP) and the trimer dissociates into 2 parts:
- subunit- subunit
Both parts can regulate downstream pathways
G-protein dissociation
GTP hydrolysis ends signaling and induces trimerization
Gs proteins are stimulatory
Upon dissociation, a Gs protein stimulates an effector enzyme, such as adenylate cyclase
Adenylate cyclase converts ATP to cAMP
Elevated cAMP stimulates cAMP-dependent protein kinase (PKA) by inducing the release of inhibitory subunits
PKA activation by cAMP
PKA activates gene expression
Inactivation of PKA pathway
The G-protein -PKA pathway is inactivated by:•Receptor desensitization•GTP hydrolysis in G-protein•cAMP hydrolysis by phosphodiesterase•PKA inhibition•Phosphatase action on PKA targets
•Activation of an antagonistic pathway (Gi)
G-proteins and phospholipases
Some G-proteins activate PLC (phospholipase C), triggering formation of inositol triphosphate (IP3) and diacylglycerol (DAG)
DAG, IP3, Ca2+ and signal transduction
DAG:•substrate for production of eicosanoids, potent signaling molecules including arachadonic acid•activates PKC
IP3:induces release of Ca2+ from ER stores via IP3-sensitive Ca-channels
Ca2+:Elevated Ca2+ can activates PKC and CamK.
Interactions between G-proteins and RTKs
