Lecture 22: Cell Signaling Flashcards
What is the importance of signaling
Living organisms constantly receive and interpret signals from their
environment.
Signal transduction
is the process of converting external signals into a cellular response through (often) transmembrane receptors
What does signal transduction allow for?
allows for the alteration of gene expression and protein activity in response to environmental signals. • It allows for communication between the different cells of an organism • It allows for cells to adapt to their environment
• Cells of multi-cellular organisms….
receive signals from other cells,
including signals for cell division and differentiation. Most cells in our
bodies must constantly receive signals that keep them alive and
functioning.
Protein kinases
transfer a phosphate from ATP to a protein.
This usually activates the protein
can act as cascades, where
one type of kinase activates the
next ‘step in the cascade.
Protein phosphatases
remove the phosphate, reversing the ‘switch’.
- inactivating
Approx_________of all proteins are regulated by ___________!
Approx 50% of all proteins are regulated by phosphorylation!
Phosphorylase
enzyme adds a P to a substrate using inorganic phosphate (not ATP)
These can target ion channels, transcription factors and other regulatory proteins. In the case of signal transduction cascades, the substrate for the phosphorylases and kinases are often also phosphorylases and kinases. This causes a chain reaction that ultimately leads to a cellular response.
Give examples of post-translational
modification (PMT) of proteins
phosphorylation, ubituitinylation, and acylation as
covalent attachments to proteins. also cleaving off proteins
- attaching some molecule covalently
Phosphorylation can affect proteins in different ways:
– Activate or inactivate an enzyme (or other protein function)
– Target protein for degradation (via initiating ubiquitinylization)
– Allow movement from one cellular compartment to another
– Increase or decrease protein-protein interactions
Phosphoproteomics
new type of proteomics that quantifies not only all the proteins in a cell,
but which proteins are phosphorylated, and at what amino acid
autocrine
the cell has receptors on its surface
that respond to an extracellular messenger it releases.
cell releases molecules and it is bound back to the same cell
paracrine signaling
the extracellular messenger travel
short distances to nearby cells through the extracellular
space (example: nerve cell releasing acetylcholine to
trigger muscle contraction).
endocrine
extracellular messengers (i.e. hormones) can travel long distances through the bloodstream and target distal cells.
Explain the general idea of cell signalling
- A ligand binds to a receptor on the PM. This
causes a conformational change in the receptor
on the cytosolic side (a process called signal
transduction).
receptor - This triggers a cascade of effects in the
cytosol. These cascades amplify the signal
inside the cell. - The final ‘layer’ of the proteins in the
cascade trigger effects in the cell. These
can include alteration of transcription of
genes, or changes in the activity/function
of proteins.
Gene transcription
Protein activity changes
(enzymes, cytoskeletal,
Ion channels, etc.)
Why and how are signals amplified?
Amplification of the signal permits - the initial signaling molecule (hormone) to be in limited concentrations and still be effective – one hormone to activate numerous enzymes • e.g. each protein kinase (a second messenger) can activate several MORE kinase molecules. – coordination of several different pathways simultaneously, as all are induced by a single signa
Describe the two main ways signaling (in general) works
A:
Extracellular signaling molecule (1st messenger)
The activation of the receptor activates an effector protein (4).
The effector makes a soluble second
messenger (5) which diffuses into the cell.
The second messenger triggers the signal
cascade leading to cell effects.
Our example: glucagon
signaling
B: The activation of the receptor forms a
‘recruiting station’ that in turn recruits other
proteins.
These proteins trigger the signal cascade.
Our example: MAP
kinase cascade
Ligand
any molecule that binds to a receptor that triggers signaling
Hydrophobic ligands
• Often made from cholesterol
• can cross the cell membrane
• Thus, the receptors are inside the cell
• We`ve already seen an example of this
(PEPCK activation by glucocorticoid in
the transcription factor lecture)• Often made from cholesterol
• can cross the cell membrane
Hydrophilic ligands
Many types (proteins, peptides, amino
acids, small molecules,
• Can`t cross the cell membrane
• Bind to integral membrane receptors
Cyclic AMP (cAMP)
Second messenger
Made by the effector adenylyl cyclase
cAMP can readily diffuse into the
cytosol and trigger downstream
effects
adenylyl cyclase
effector makes cAMP
is an integral membrane
protein
Inositol phosphates and diacylglycerol (DAG)
Derived from phosphatidylinositol
• The inositol portion can be phosphorylated by kinases
• Seven different possible P patterns
PIP2 is the substrate for the effector
• The main lipid second messengers are produced by the
effector phosphatidylinositol-specific phospholipase C.
– abbreviated PI-PLC
– produces two signal molecules: • Diacylglycerol (DAG) Inositol triphosphate (IP3),
phosphatidylinositol-specific phospholipase C.
effector
produces two signal molecules: Diacylglycerol (DAG), Inositol triphosphate (IP3),
cuts the PIP2 in 1/2
• Diacylglycerol (DAG),
2nd messanger
which stays within membrane
Inositol triphosphate (IP3)
highly soluble, enters
cytoplasm
G protein
G-proteins are inactive when bound to GDP. The GEF enzyme swaps the GDP for a GTP, activating the G-protein.
What are the 2 ways s G-proteins switch ‘off’
They have a slow intrinsic
GTPase activity and will self-hydrolyze the GTP to GDP.
If the proper GAP is present, the GAP will greatly speed up the process. So, G-proteins are more
like ‘timers’ than ‘switches’.
Why are G proteins more
like ‘timers’ than ‘switches’
B/c eventually they will all shut off on their own b/c of GTPase activity; can be sped up by GAPS
Describe a G-Protein Couple Receptor’s structure and function
A family of integral proteins,
all have seven transmembrane a-helix segments
• All work in the same way
– through the heterotrimeric Gproteins with α, β, γ subunits
* Ga and Gy are lipid anchored
– Which then turn on an effector molecule which makes the second messenger • e.g. epinephrine and glucagon turn on adenylyl cyclase to make cAMP (2nd messenger). • Other ligands (e.g. acetylcholine also activates GPCRs) use phosphoinositol and DAG second messengers • others (e.g. photoreceptors) use cyclic GMP
Describe how GPCRs induce G-protein and second messengers
- When receptor combines with
ligand, receptor changes shape and
binds the a subunit of the G protein
2. Activation of the G protein: a subunit then exchanges a GDP for a GTP, entering activated state - the receptor/ligand can activate several G proteins, as long as ligand is bound
3. Relay: the a subunit dissociates from b,g and associates with effector, producing second message - b,g stay together - second message is made for duration of binding
- Activated effector produces second
messenger (eg. adenylyl cyclase
makes cyclic AMP = cAMP) - a subunit hydrolyzes GTP into GDP,
thereby deactivating itself - a subunit binds other two subunits -
now inactive
How does GPCR-mediated signaling stop?
To prevent overstimulation, activated receptors can be blocked from interacting with G-proteins 7. G-protein-coupled receptor kinase (GRK) phosphorylates receptor
- Arrestin protein binds to
phosphorylated receptor to
prevent G-proteins from binding – “desensitization”
– Gsα
stimulates adenylyl cyclase
– Gqa
activates phospholipase C
Gia
inactivates adenylyl cyclase
Provide an example of a disease state brought about by G-protein mis-regulation:
Cholera toxin specifically binds Gsα in intestinal epithelial cells
Locks it into the active state by adding an ADP-ribose to the G-protein
Adenylyl cyclase remains active all the time and makes gobs of cAMP
cAMP induces chloride channels to always remain open, and water follows by
osmosis. Severe dehydration can be fatal
Utilization of glucose:
– primary energy source (of course)
– stored as the polymer glycogen in liver and
muscle
– glycogen conversion to glucose is promoted by
hormones:
glucagon
(released from pancreas), boosts
blood glucose when blood glucose drops
epinephrine
(adrenal gland), boosts blood
glucose during stress
To get more glucose into the bloodstream:
Promote breakdown of glycogen to glucose-1- phosphate (first step in catabolism). This
glucose is either catabolized or sent to
bloodstream for delivery to other places.
Inhibit glycogen synthase: this enzyme makes
glycogen, so it has to be turned off in order for
the cells to release or burn glucose.
Promote gluconeogenesis to make glucose
from smaller molecules
In liver cells, glucagon and epinephrine (adrenaline)…..
In liver cells, glucagon and epinephrine (adrenaline)
bind to different GPCRs, but the GPCRs then both
activate Gsα which activates adenylyl cyclase
Outline the signaling pathway following glucagon reception that results in glucose
production
- Hormone binds to receptor
which binds to G-protein - Activation of effector:
Adenylyl cyclase, formation of
cAMP, diffuses into cytoplasm
3. cAMP then binds to Protein kinase A (PKA) & activates it
- PKA phosphorylates glycogen
synthase, inactivating it.
Glycogen no longer produced - At same time, PKA phosphorylates
the enzyme Phosphorylase kinase,
activating it - the phosphorylase kinase then
phosphorylates its target enzyme:
Glycogen phosphorylase,
activating it,’ - phosphorylase catalyzes glycogen
break-down, glucose-1-phosphate
is released
8. also at same time, in the nucleus, PKA phosphorylates transcription factor: cyclic AMP response element binding protein (CREB)
9. phosphorylated CREB dimerizes and binds to cAMP response element (CRE), turning on PEPCK gene, gluconeogenesis increases to produce even more glucose
Outline how the cell terminates all components of glucagon-induced signaling
- cAMP is broken down by phosphodiesterase
2. Phosphatases reverse the phosphorylation of the three proteins: 1. phosphorylase kinase 2. glycogen synthase and 3. phosphorylase
- Adenylyl cyclase is deactivated when the α subunit of
the G-protein hydrolyzes the GTP back to GDP (α
subunit re-associates with γ and β) – this hydrolysis is
a timed event - aided by another protein: RSG
(regulator of G protein signaling)… a bit like GAP - inactivation of the receptor, a 2-step process of
desensitization. This means that the cell stops
responding, even when ligand is still present around
the cell
a. Phosphorylation of the receptor by G-protein
receptor kinase (GPRK), inactivates the receptor
b. The phosphorylated receptor binds another
protein called arrestin, which acts as adaptor for
clathrin, allowing receptors to be internalized by
endocytosis, thus further desensitizing the cell
Which is a faster way to yield glucose?
. The cytoplasmic conversion of glycogen to glucose
cAMP then binds to Protein
kinase A (PKA) & activates it
– how?
PKA is a tetramer with 2 regulatory subunits and 2 catalytic subunits; cAMP removes the inhibitory regulatory subunits
Describe how GPCRs are involved in light perception
Rhodopsin is one of a few GPCR
in rod cells that make up your
retina
Retinal is a small molecule
cofactor (made from vitamin A)
that is bound to the receptor
An incoming photon is absorbed by the retinal, which causes the retinal to go from cis to trans conformation, which then causes the GPCR to change shape and start signal transduction inside of the cell. The signals initiate nerve signaling by opening ion channels, and these signals are (somehow!) interpreted by your brain as an image
