Signal Transduction Across a Membrane

1
Signal Transduction Across a Membrane
specific channels can let particular things
through the membrane ion channels let positive
and negative charges diffuse through porins
allow water and other things to diffuse
through transporters ie. glucose transporter,
brings food into a cell What if cells only
wanted to convey a signal, not actually use a
molecule? 3 major types of signals given off by
cells endocrine signals (ie. hormones) acting
at a distance from the signal paracrine signals
short range, sometimes even contact,
signals autocrine signals signal produced by
the same cell that responds to it often a
self-reinforcing loop (ie. signal, respond,
increase signal)
2
Signal Transduction Across a Membrane
receptor protein that will bind and respond to a
specific signal many are found on the surface
of cells so that they can sense the
outside ligand compound bound by a
receptor second messenger signal generated by a
receptor after binding its ligand usually a
small molecule that is produced inside the cell
by a membrane receptor on the outside signal
transduction process of sensing a signal on the
outside of a cell and converting and
responding to the signal on the inside of a
cell some ligands, particularly steroids, pass
through the cell membrane and bind to
intracellular receptors, skipping second
messenger formation If a cell does not have a
receptor for a particular signal, the cell
cannot respond to it ie. a blind person
doesn't respond to flashing lights
3
Signal Transduction Across a Membrane
2o
2o
2o
2o
2o
receptors are very specific for what they
activate several different receptors can activate
the same 2o messenger system different times/
locations/ multiple systems activated causes
response
4
Signal Transduction Across a Membrane
FGF-b
FGF-b bound to its receptor dimer
receptors having binding sites very similar to
enzyme active sites must fit exactly and
requires particular chemical bonds (which allows
the specificity of receptors for their ligands)
5
Receptor Binding and Signaling
receptor affinity how tightly will a receptor
bind its ligand described by its dissociation
constant, Kd that is similar to Km
concentration at which half of the receptors are
bound to ligand determines what concentration
of ligand a cell will respond to a cell might
have 2 or more different receptors for the same
ligand one might be a low affinity receptor,
the other might be high affinity the same ligand
could therefore send more than one signal!
6
Receptor Binding and Signaling
desensitization 'silencing' of a receptor that
has been active too much also known as
tolerance ie. same ligand doesn't have as much
effect commonly seen in drug addiction can
occur in several different ways 1) less protein
can be made-- fewer receptors lower signals 2)
receptors can be removed from the cell surface
(endocytosis) 3) blocking the signal transduction
of the receptor ligand still binds, but
second messenger formation doesn't occur
often occurs by phosphorylation of the
receptor phosphorylation addition of a
phosphate group to serine, threonine, or
tyrosine (amino acids with a free hydroxyl
group) kinase enzyme which phosphorylates
another substance phosphatase enzyme which
removes phosphates
7
Receptor Binding and Signaling
agonist synthetic chemical that mimics the
function of a ligand agonists activate receptors
just like ligands but may not be broken down
as easily antagonist chemical which binds to
receptors and block their function similar to
enzyme inhibitors fail to activate second
messengers
Many drugs on the market today are receptor
agonists and antagonists
8
G-protein Coupled Receptors
G-protein coupled receptor 7 pass
transmembrane receptor family which uses
heterotrimeric G proteins for signal
transduction one of 2 main families of cell
surface receptors includes odorant receptors,
growth factor receptors, neurotransmitter
receptors, light receptors, etc. over 1,200
different known G-protein receptors! extracellula
r loops usually bind to ligands on the outer
surface of the cell intracellular loops bind to
the heterotrimeric G protein
9
G-protein Coupled Receptors
heterotrimeric G-proteins composed of 3
different subunits (a, b, and g) each subunit
has multiple isoforms, giving many different
combinations receptor activation separates Ga
from Gbg , allowing Ga to bind GTP Ga stays
active until GTP is broken down to GDP so it can
bind Gbg
messing up G-protein signaling via toxins can
cause diseases like cholera
10
G-protein Coupled Receptors
some Ga proteins activate adenylate cyclase to
make cyclic AMP (cAMP) on Ga can make many
many many cAMP molecules, causing the signal
to be amplified many time over
cAMP binds to protein kinase A, removing an
inhibitory subunit by an allosteric change
and releasing the active kinase to
phosphorylate target proteins cAMP is degraded
to AMP by the enzyme phosphodiesterase
11
G-protein Coupled Receptors
other G proteins activate phospholipase C, an
enzyme which makes inositol triphosphate (IP3)
and diacylglycerol (DAG) DAG activates
protein kinase C protein kinase C can then
phosphorylate many things, including ion
channels, cytoskeleton, and other receptors IP3
causes release of Ca2 into the cytoplasm from
'intracellular stores'
12
Calcium is another common second messenger
normally calcium is pumped out of the cytoplasm
via a calcium ATPase pump in the cell
membrane and the sodium/calcium exchanger Ca2
from stores or channels allow calmodulin
protein to bind Ca2 bound calmodulin changes
shape so that it can activate other enzymes
EF 'hand' structure of calmodulin is a common
calcium binding domain
13
G proteins can also activate Nitric Oxide
acetylcholine activates a specific G-protein
coupled receptor that is linked to
phospholipase C, which goes on to form IP3 IP3
causes release of Ca2 ions to bind
calmodulin calmodulin binds to nitric oxide
synthase, the enzyme which makes NO NO can now
diffuse through the cell membrane and activate
guanylate cyclase (enzyme much like adenylate
cyclase but makes cGMP) cGMP acts as an
important second messenger for controlling blood
flow nitroglycerin for heart patients works by
increasing NO, increasing blood flow Viagra
inhibits the phosphodiesterase that normally
breaks down cGMP
14
Protein Kinase Receptors
instead of activating G proteins, receptor
kinases phosphorylate proteins inside the cell
when they bind their ligands many receptor
kinases undergo autophosphorylation to become
activated autophosphorylation adds phosphate
groups to itself many receptor kinases
function as dimers, where one subunit
phosphorylates the other after ligand is bound
unbound
bound
P
P
P
P
best model basically has the 2 subunits of the
receptor dimer changing shape in relation to
one another after ligand binding after changing
shape, receptor dimer autophosphorylates
activating more
15
Protein Kinase Receptors
many receptor kinases phosphorylate tyrosines, so
are known as tyrosine kinase
receptors tyrosine kinases can either have the
kinase as part of the cytoplasmic side of the
receptor, or it can be a separate protein that
bind the receptor
P
P
P
P
P
P
phosphotyrosines can be bound by a SH2 domain of
a protein different SH2 domains recognize
different proteins, so specific protein
complexes can be made when different receptors
are activated
one receptor tyrosine kinase can activate more
than one SH2 domain containing protein, meaning
several signal transduction pathways can be
activated at once
P
P
P
P
P
P
16
Protein Kinase Receptors
using different proteins, receptor tyrosine
kinases may activate the IP3, cAMP, or Ca2
second messengers may also activate the Ras
pathway, or small monomeric G proteins just
like large heterotrimeric G proteins, the GTP
bound form is active receptor tyrosine kinases
control the ratio of GTPGDP bound forms Ras is
the best studied of a large family of small
monomeric G proteins
GEF
RasGDP
RasGTP
GAP
inactive form
active form
GEF (GTP exchange factor) replaces GDP with GTP,
activating Ras GAP (GTPase activating protein)
hydrolyzes GTP to GDP, inactivating different
Ras family proteins have their own GEFs and GAPs
17
Protein Kinase Receptors
activated Ras binds to mitogen activated protein
kinase (MAPK) MAPK can now phosphorylate
different proteins and transcription factors
18
Effects of Receptor Signaling on Cells
growth factor a type of small, soluble protein
found in body fluids which stimulate one or
more types of cells to divide or
change different growth factors are found at
different times or places in the body FGF
(fibroblast growth factor) family of
developmental growth factors FGFR fibroblast
growth factor receptor family (receptor tyrosine
kinase) FGFs and FGFRs play a major role in the
development of mesoderm, one of the 3 cell
layers of the embryo that gives rise to muscle,
bone, blood, and cartilage a mutated receptor
for FGF that has an inactive kinase domain
blocks formation of the trunk and tail dominant
negative mutation mutation that blocks normal
signaling
19
Effects of Receptor Signaling on Cells
transforming growth factor b a large family of
related growth factors that control
proliferation, specialization, and programmed
cell death TGFb receptors are heterodimers (2
different subunits) that phosphorylate serine
and threonine amino acids of proteins known as
Smads phosphorylated Smad then binds a second
protein and goes to the nucleus where it can
alter gene transcription
P
P
P
P
nucleus (DNA)
P
P
cell membrane
20
Effects of Receptor Signaling on Cells
general model for receptor function 1) Receptor
protein or protein binds its ligand 7 pass
transmembrane proteins change their shape to
activate receptor kinase subunits change their
orientation or cluster together 2) A second
messenger cascade (series of reactions) is
initiated heterotrimeric G proteins get
activated kinase starts phosphorylating
cytoplasmic proteins 3) the initial signal gets
amplified Ga subunits activate adenylate
cyclase or phosphodiesterase receptor kinases
phosphorylate many proteins 4) messengers or
activated proteins change the functioning of the
cell by regulating transcription or the
behavior of other proteins (cytoskeleton)
21
Hormones
hormone chemical messenger secreted by one
tissue that functions in a different tissue
(ie. epinephrine from the adrenal gland
stimulates cardiac muscle to beat faster and
the liver to release more sugars) not every
tissue responds to every hormone-- that tissue
must express a receptor for that hormone for
it to work tissues that make different receptors
can have different responses to the same
hormone both animals and plants use hormones
brassinosteroids regulate leaf growth ethylene
regulates ripening of fruit abscicic acid
causes stomata to close during droughts animals
generate a larger variety of hormones whose
effects can last for seconds (adrenaline) or
days (testosterone and estrogen)
22
Hormones
endocrine hormones function at a distance from
where they are made generally secreted
directly into the bloodstream with a limited
half-life paracrine hormones function only
locally-- only operates within a few cell
diameters some paracrine signals are attach to
cells or matrix usually are broken down or
internalized quickly to keep them local hormones
are characterized by their chemical properties--
4 groups 1) amino acid derived small,
chemically modified. epinephrine is made
from tyrosine cells use many similar chemicals
for different uses melanin pigment
dopamine neurotransmitter
23
Hormones
2) peptides (ie. short chain of amino
acids) oxytocin cys- tyr-ile-gln-asn-cys-pr
o-leu-gly vasopressin cys-tyr-phe-gln-asn-cys-pr
o-arg-gly 3) protein (ie. FGF or insulin or
TGFb) 4) lipid-like hormones (steroids or
prostaglandins)
24
Hormones
adrenal hormones are synthesized by the adrenal
gland, a fatty-like organ that sits on top of
the kidney synthesizes several hormones,
including norepinephrine and epinephrine
(adrenaline) note the many blood vessels so the
hormones can be secreted into the blood
directly bind to a family of G protein coupled
receptors, the adrenergic receptors 2 receptors
a and b adrenergic receptors, link to different G
proteins a activates phospholipase C (IP3), b
activates adenylate cyclase (cAMP)
25
Hormones
cAMP regulates the enzyme glycogen phosphorylase
which breaks down the storage carbohydrate
glycogen into glucose subunits in the
liver liver should then express more b
adrenergic receptor because that is the
signaling pathway used to activate adenylate
cyclase thus adrenaline increases blood sugar
to provide that quick energy boost
26
Hormones
IP3 signaling via phospholipase C increases Ca2
in smooth muscles surrounding veins, causing
contraction this decreases blood flow to
internal organs, reduces digestion, etc and
thus allows more blood to the voluntary muscles
for fight or flight smooth muscles in digestion
have more a adrenergic receptors smooth muscles
in the lungs, however, have more b adrenergic
receptors cAMP levels here go up, and cAMP
causes blood vessels to relax relaxed blood
vessels allow more blood to flow to the lungs,
increasing the amount of oxygen in the blood
for skeletal muscles to use we will see more
about muscle contraction in chapter 16 Both
receptors work together in order to prepare the
body for action 2 receptors are found in
different tissues to allow different responses
27
Apoptosis
apoptosis programmed cell death-- signaling
pathway that causes cells to essentially
suicide-- needs to be under very accurate
control failure to work uncontrolled growth
(cancer) apoptotic bodies remains of cells
after they suicide necrosis swelling and
rupture of damaged or injured cells
young brain tissue older brain tissue
caspases proteases synthesized as zymogens which
break down key enzymes in the cell when
activated-- essential to cause apoptosis
28
Apoptosis
2 major apoptosis signals are tumor necrosis
factor and CD95/Fas CD95 is a receptor on the
surface of virus infected cells some lymphocytes
have ligands for CD95 on their surface CD95
clusters and autophosphorylate, leading to
caspase activation and eventual apoptosis
29
Apoptosis
in the other pathway, cells die because they lack
a growth factor lack of growth factor signal
causes pro-apoptotic proteins to become
activated (ie. phosphorylation inactivates
Bax) proapoptotic proteins (Bax) insert into
the mitochondrial membrane counteracting
anti-apoptotic proteins such as Bcl-2 if Bax
and/or Bad become too active, mitochondria
release cytochrome C and start a series of
reactions which activate caspases and kill the
cell