Slide set 9 Flashcards
Intracellular signaling pathway activated by an extracellular signal molecule
- Signal molecule binds to a receptor protein in plasma membrane of target cell (on cell surface)
- Receptor activates 1 or more intracellular signaling pathways
- involves a series of signaling proteins
- 1 or more of the intracellular signaling proteins alters activity of effector proteins
- this alters the behavior of the cell
types of effector proteins
transcription regulators, ion channels, components of metabolic pathways, or parts of the cytoskeleton
contact-dependent signaling
requires cells in membrane-membrane contact
paracrine signaling
local mediators (molecules secreted into extracellular space)
released into extracellular space and act on neighboring cells
synaptic signaling
neurons transmit signals electrically along their axons and release NTs at synapses
synapses are often located far away from neuronal cell body
Endocrine signaling
autocrine signaling
signaling and target cells are same cell type
EX: cancer
cell-surface receptors
- most signal molecs are hydrophilic (Can’t cross target cell’s plasma membrane)
- bind to signal molecule extracellularly
- binding changes target cell in some way
intracellular receptors
- small signal molecs diffuse across plasma membrane
- bind to receptor proteins in cytosol OR nucleus
- signal molecs are hydrophobic and poorly soluble in aqueous solns
- molecs are transported into blood and extracellular fluids while bound to carrier proteins
- dissociate from carried proteins before entering target cell
how do cells decide on the appropriate cellular response
integrate MULTIPLE SIGNALS
if a cell doesn’t receive the required survival signals, its undergoes apoptosis (cellular suicide)
some extracellular signals are inhibitory
cell surviving vs growing and dividing
depends on PI-3-Kinase/Akt/mTOR pathway
Survival pathway
PI-3-Kinase-Akt signaling pathway: survival, growth, and division
IGF’s
insulin-like growth factor (IGF) signal proteins
stimulate animal cells to survive and grow
bind to specific RTKs that active PI-3-Kinase to produce PI(3,4,5)P3
PI(3,4,5)P3 recruits 2 protein kinases to plasma membrane via their PH domains (Akt and PDK1)
Akt is now active and phosphorylates target proteins at plasma membrane, cytosol, and nucleus
target cells now have enhanced cell survival and growth!
PDK1 aka
phosphoinositide-dependent protein kinase
Akt aka
protein kinase B (PKB)
survival pathway
- Extracellular survival signal activates an RTK
- RTK recruits and activates PI 3-kinase
- PI-3-kinase produces PI(3,4,5)P3 (a docking site for 2 serine-threonine kinases with PH domains)
- Akt and PDK1 are brought into proximity at plasma membrane
- Akt is phosphorylated on a serine by a third kinase (mTOR in complex 2)
- this alters conformation of Akt so it can be phosphorylated on a threonine by PDK1
- PDK1 activates Akt
- active Akt dissociates from plasma membrane and phosphorylates target proteins (EX: Bad protein)
- Bad holds one or more apoptosis-inhibitory proteins (of Bcl2 family) in an inactive state
- phosphorylated Bad releases inhibitory proteins
- they now block apoptosis and thus promote cell survival
- phosphorylated Bad is bind to ubiquitous cytosolic protein 14-3-3 which keeps bad out of action
Bad protein
- targeted by Akt for phosphorylation
- Bad holds one or more apoptosis-inhibitory proteins (of Bcl2 family) in an inactive state
- phosphorylated Bad releases inhibitory proteins
- they now block apoptosis and thus promote cell survival
- phosphorylated Bad binds to ubiquitous cytosolic protein 14-3-3 to become inactive
- active Bad = phosphorylated
- inactive Bad = bound to ubiquitous cytosolic protein 14-3-3
survival vs growth pathways
Without extracellular growth factor
- Tsc2 (a Rheb-GAP) keeps Rheb inactive AND mTOR in complex 1 is inactive: there is no cell growth!
With extracellular growth factor
- active Akt phosphorylates and inhibits Tsc2 (promotes activation of Rheb)
- active Rheb (Rheb-GTP) helps activate mTOR in complex 1: stimulates cell growth
A response to a signal depends on…
the specific cell type AND cohort of proteins that the cell expresses for detection and relay of the signal
types of cell-surface receptors
cell-surface receptors: act as signal transducers by converting an extracellular ligand-binding event into intracellular signals that alter behavior of target cell
ion-channel-coupled receptors
G-protein-coupled receptors
enzyme-coupled receptors
ion-channel-coupled receptors
aka transmitter-gated ion channels/ionotropic receptors
rapid synaptic signaling btwn nerve cells and other electrically excitable target cells (nerve and muscle cells)
mediated by small number of NTs that transiently open or close and ion channel formed by the protein to which they bind
the NTs briefly change ion permeability of plasma membrane and change excitability to postsynaptic target
most ion-channel-coupled receptors belong to a large family of homologous, multipass transmembrane proteins!
G-protein-coupled receptors
indirectly regulate activity of a separate plasma-membrane-bound target protein (either an enzyme or an ion channel)
a trimeric GTP-binding protein (G protein) mediates interaction between activated receptor and this target protein
activation of target protein can change concentration of 1 or more small intracellular signaling molecules (if target protein is an enzyme)
OR
it can change ion permeability of the plasma membrane (when target is an ion channel
Enzyme-coupled receptors
either act as enzymes or associate directly with enzymes that they activate
usually single-pass transmembrane proteins with ligand-binding site outside the cell and catalytic or enzyme-binding site inside
majority are protein kinases or associated with protein kinases (phosphorylate specific sets of proteins in the target cell when activated
Intracellular molecular switches
Signaling by phosphorylation:
- a protein kinase covalently adds a phosphate from ATP to signaling protein
- a protein phosphatase removes the phosphate
- *some signaling molecules are activated by dephosphorylation rather than by phosphorylation
Signaling by GTP binding:
- GTP-binding protein is induced to exchange its bound GDP for GTP (this activates the protein)
- protein then inactivates itself by hydrolyzing its bound GTP to GDP
different RTK families
- different RTKs have different extracellular domains
- RTKs all have an intracellular tyrosine kinase domain
Activation of RTKs
dimerization!
- in the absence of extracellular signals, most RTKs are monomers (internal kinase domain is inactive)
- binding of ligand brings 2 monomers together to form a dimer
- the close proximity in the dimer leads the 2 kinase domains to phosphorylate each other (this has 2 effects!)
- 1) phosphorylation at some tyrosines in the kinase domains promotes the complete activation of the domains
- 2) phosphorylation at tyrosines in other parts of the receptors generates docking sites for intracellular signaling proteins (results in formation of large signaling complexes that can then broadcast signals along multiple signaling pathways)
what protein domain recognizes phosphorylated tyrosines
SH2 domain!
it’s a general protein domain
SH2 domains bind to a phosphotyrosine residue and an amino acid side chain
by changing the binding site of the amino acid side chain, the domain can have specificity for different proteins
scaffold proteins
enhance specificity of signal transduction
scaffolds bind to activated receptors and can be bound by multiple other downstream signaling proteins
this limits what proteins can be activated, which increases specificity
this also keeps the signaling molecules in close proximity, so rxn can have faster rate
different types of scaffold complexes
- preformed scaffold complex
- receptor and some intracellular signaling proteins it activates in sequence are preassembled into a signaling complex on the inactive receptor by a large scaffold protein
- scaffold complex only forms after signal arrives
- signaling complex assembles transiently on a receptor only after binding of an extracellular signal molecule has activated the receptor
- activated receptor phosphorylates itself at multiple sites, which then act as docking sites for intracellular signaling proteins
modified lipids as docking sites for complex formation
assembly of signaling complex on phosphoinositide docking sites
- activation of a receptor = increased phosphorylation of specific phospholipids (phosphoinositides) in the adjacent plasma membrane
- these are then docking sites for specific intracellular signaling proteins (they can now interact with one another)
SH2 and PTB domains
bind to phosphorylated tyrosines in a specific peptide sequence
SH3 domains
bind to short, proline-rich amino acid sequences
PH domains
bind to charged head groups of specific phospholipids
signal relay after RTK activation
how an RTK activates Ras
- Grb2 recognizes a specific phosphorylated tyrosine on the activated receptor by means of an SH2 domain
- recruits Sos by means of 2 SH3 domains
- Sos stimulates the inactive Ras protein to replace its bound GDP by GTP
- now Ras is active and signal is relayed downstream
signal relay after RTK activation (MAP)
MAP kinase module activated by RAS
- MAP kinase kinase kinase (Raf): Ras recruits Raf to plasma membrane and helps activate it
- Raf activates MAP kinase kinase (Mek) which activates MAP kinase (Erk)
- Erk phosphorylates a variety of downstream proteins (including other protein kinases)
- EX: transcription regulators in nucleus
- Resulting changes in protein activity and gene expression cause complex changes in cell behavior
response timing
- Slow responses: increased cell growth + division
- slow bc they involve changes of gene expression and synthesis of new proteins
- Fast responses: changes in cell movement, secretion, or metabolism
- fast bc they don’t change transcription
- can involve rapid phosphorylation of effector proteins in cytoplasm
- synaptic responses mediated by changes in membrane potential are even quicker
- some signaling systems have rapid and slow responses
- allows quick response while preparing slower long term response
coincidence detectors
2 signals that are required to be present simultaneously to elicit a response
