Transcript ch15 FA 11 - Cal State LA

CHAPTER 15
Cell Signaling and Signal
Transduction: Communication
Between Cells
Signal transduction
• Extracellular ligand: 1st messenger
– Autocrine: ligand binds to producing cell
– Paracrine: ligand binds to a neighbor cell
– Endocrine: ligand binds to a distant cell after traveling through the circulatory
system
Signal transduction
• Cells respond to a ligand only if
expressing the cognate receptor
– 1st messenger binds to receptor
• Outside cell
• Binds to cell surface
– Effectors generate 2nd messengers
• Inside cell
• Signal amplification
• Some diffuse through cytosol
• Some diffuse within plasma
membrane
• Exert allosteric effects on
various enzymes & proteins
Signal transduction
• Cells respond to a ligand only if
expressing the cognate receptor
– 1st messenger binds to receptor
• Outside cell
• Binds to cell surface
• Alters receptor conformation
– Signaling pathway
• Each protein alters the
conformation of next protein
• Conformation usually altered
by phosphorylation
• Target proteins ultimately
alter cell activity
– E.g. TXN, TLN, Enzymes, etc
• Signal amplification
A Survey of Extracellular Messengers
• Extracellular messengers (the signal) include:
– Small molecules such as amino acids, steroids, lipids and
their derivatives
serotonin
– Gases such as NO and CO
– Various peptides and proteins
Edn1
G protein-coupled receptors (GPCRs)
BINDING
– GPCRs involved in vision, smell, emotion (1000s of genes)
– Respond to a wide variety of ligands
• Proteins, small chemical compounds, metabolites, photons
G protein-coupled receptors (GPCRs)
BINDING
– GPCRs involved in vision, smell, emotion (1000s of genes)
– Respond to a wide variety of ligands
• Proteins, small chemical compounds, metabolites, photons
– Ligands bind to extracellular side of receptor
• Induces a conformational change in intracellular domains
– 7 transmembrane (7TM) domains
• serpentine structure passed through membrane 7 times
N
1 2 3 45 6 7
C
GPCRs
ACTIVATION / SIGNAL TRANSMISSION
– Receptor is ‘coupled’ to a large heterotrimeric G protein “switch”
• Alpha, beta and gamma subunits
• Alpha subunit = GTP hydrolyzing enzyme (GTPase)
• GTP bound form is switched ‘on’
• GDP bound form is switched ‘off’
• The ligand-bound receptor promotes
exchange of GDP for GTP
“off ” GTP
G-GDP
N
GDP
Active
receptor
“on”
G-GTP
1 2 3 45 6 7
C
 

RGS
G-GDP
G-GTP
Pi
GPCRs
ACTIVATION / SIGNAL
TRANSMISSION
– Four classes of G-alpha subunit
– G-alpha-s increases activity of
Adenylate Cyclase (AC)
• An “Effector”
• ATP --> cAMP + PPi
• cAMP is a 2nd messenger
generated within the
cytoplasm
– G-alpha-i decreases activity of
AC
ACTIVATION / SIGNAL TRANSMISSION
– G-alpha-q increases activity of the effector Phospholipase C (PLC)
• PI(4,5)P2 --> DAG + IP3
• DAG and IP3 are both 2nd messengers
– G-alpha-12/13 is not well understood, but is linked to cancer
GPCRs
TERMINATION
• Regulators of G protein
Signaling (RGS)
– Enhance GTPase activity
• GPCR-kinases (GRKs)
– Phosphorylate internal
portions of active receptors
• Arrestins
– Compete with G-alpha for
binding to phosphorylated
GPCR
• Desensitization: loss of response
to a stimulus in spite of the
continued presence of the
stimulus
Signal transduction
• Epinephrine: glucose mobilization
– G-alpha-s
– Increased AC activity
– Increased [cAMP]
– cAMP allosterically activates Protein
Kinase A (PKA)
• PKA inhibits Glycogen Synthase
• PKA activates Glycogen
Phosphorylase
– Leads to glycogen breakdown
and release of glucose
• PKA stimulates TXN factor activity
– Phosphorylates cAMP Response
Element Binding (CREB) protein
– Drives TXN of various target
genes
GPCRs
• G-alpha-q
• Increased Phospholipase C (PLC)-beta activity
– Hydrolyzes specific phospholipids
– Increased [diacylglycerol (DAG)] and [inositol triphosphate (IP3)]
• G-alpha-q
• DAG allosterically activates Protein Kinase C (PKC)
• IP3 allosterically opens a Calcium channel on the smooth Endoplasmic Retriculum
– Increased [Ca2+] in cytoplasm
• G-alpha-q
• Increased [Ca2+] in cytoplasm
– Ca2+ binds many cytoplasmic
proteins
– Calmodulin, regulator of many
proteins
• G-alpha-q
• Increased [Ca2+] in cytoplasm
– Ca2+ can pass through GAP junctions into neighboring cells
– Ca2+ gated Ca2+ channels in neighboring cells open further increasing [Ca2+]
– Propagation of Ca2+ effects through GAP junctions integrates tissue response
(Ca2+ waves)
Video
Signal transduction
• Receptor Tyrosine Kinases (RTKs): over 90 genes
– Extracellular ligand binding domain
• Ligand binding triggers receptor dimerization
Signal transduction
• Receptor Tyrosine Kinases (RTKs): over 90 genes
– Intracellular tyrosine (Y) kinase domain
• Dimerization allows for trans-autophosphorylation of the receptors
Signal transduction
• Receptor Tyrosine Kinases (RTKs): over 90 genes
– Intracellular tyrosine (Y) kinase domain
• Dimerization allows for trans-autophosphorylation of the receptors
• Phospho-Y (PY) sequences are binding sites for additional protein factors
• SH2 domains in proteins such as Src and Grb2 bind to PY-receptor
Signal transduction
• Receptor Tyrosine Kinases (RTKs): over 90 genes
– Intracellular tyrosine (Y) kinase domain
• Dimerization allows for trans-autophosphorylation of the receptors
• Phospho-Y (PY) sequences are binding sites for additional protein factors
• SH2 domains in proteins such as Src and Grb2 bind to PY-receptor
– Drag partner proteins along with them from the cytoplasm to the
membrane
Signal transduction
• Receptor Tyrosine Kinases (RTKs): over 90 genes
– The Ras - Mitogen Activated Protein Kinase (MAPK) pathway
• Ras is a small G protein “switch” (single subunit, no beta or gamma)
• Over 100 Ras family genes
• Lipid anchor to plasma membrane
• Disease: ~30% of cancers have activating Ras mutations (break the GTPase)
“off ” GTP
Ras-GDP
GDP
GEF
GAP
Ras-GDP
Pi
“on”
Ras-GTP
Ras-GTP
Signal transduction
• Receptor Tyrosine Kinases (RTKs): over 90 genes
– The Ras - Mitogen Activated Protein Kinase (MAPK) pathway
• Ras is a small G protein “switch” (single subunit, no beta or gamma)
• GEF = Guanine Nucleotide Exchange Factor (e.g. Sos)
• GAP = GTPase Activating Protein
“off ” GTP
Ras-GDP
GDP
GEF
GAP
Ras-GDP
Pi
“on”
Ras-GTP
Ras-GTP
RTK: Insulin receptor
Signal transduction
• Convergence
– ECM --> Integrin --> Ras
– EGF --> EGFR --> Ras
Signal transduction
• Crosstalk
– cAMP can block growth by
inhibiting Raf