Signal transmission with cell surface
and intracellular receptors
Prof.Dr.Gönül Kanıgür
Hormones
• Can exert their specific effects in three ways
• By influencing the rate of synthesis of
enzymes and other proteins
• By effecting the rate of enzymatic catalysis
• By altering the permeability of cell
membranes
The steroid hormones.
• Steroid hormones, small and fat soluble
–
–
–
–
made from cholesterol
travel through bloodstream bound to carrier proteins
enter cells by diffusion
include glucocorticoids (including cortisol), estradiol,
testosterone, progesterone.
– cross membranes by diffusion and
enter the cell.
– high-affinity receptors present in cytoplasm
– hormone receptor complex binds to DNA at promoter regions
upstream of certain genes
– Transcriptional enhancement. gene expression increases. relatively
slow, longer term response
expression
they regulate gene
• Steroid hormones ;testosterone,estrogene and progesterone
are the sex steroidsWhich are produced by the gonads
• -Corticosteroids are produced by the adrenal gland
• .glucocorticoids stimulate production of glucose,
• mineralocorticoids regulate salt and water balance
• Thyroid hormone,vitamin D3,retinoic acid are structuraly and
functionaly distinct from steroids
• but they share a common mechanism of action in their target
cells
• steroid hormones,thyroid
hormones,retinoids,and vitamin D are small
hydrophobic molecules.
• They diffuse across the plasma membrane of
the target cell
• And activate intracellular receptor proteins
• They directly regulate the transcription of
specific genes
Protein hormones
• Some terminology
• The ligand is the signaling molecule [first messenger]
• Receptor protein that binds the first messenger [hormone]
• Effector,a hormone responsive enzyme in the cell.
• often they make seccond messengers [enzymes] or import
them into the cell [ion channels]
• There are three main classes of cell surface receptorsof
hydrophilic proteins.peptides.other water soluble molecules.
• -ion channel linked receptors
• G-protein linked receptors
• Enzyme linked receptors
• İon channel linked receptor opens in responce to binding of
its ligand fore exm.when the ligand binds to the, receptor ,it
alters its conformation so as open or close a channel for the
flow of specific types of ions such as Na,K,Ca or Cl across the
membrane
1. membrane receptors
(1) Ligand-gate ion channels type
(cyclic receptor)
ligand→receptor→ion channel open or close
• 2-Enzyme linked receptors
• They are directly linked to intracellular
enzymes
• The largest family of this group is the receptor
protein tyrosine kinases. Which phosphorylate
their substrate proteins on thyrosine residues.
• İncludes the receptors polypeptide growth
factors
• More than 50 receptor protein tyrosin kinases
have been identified,EGF,NGF,PDGF,
İNSULİNand many growth factor
• They are involved in the control of animal cell
growth and differentiation
Receptor tyrosine kinases
• Each receptor consists of an N-terminal
extracellular ligand binding domain,
• transmembrane domain and
• a cytosolic C-terminal domain with tyrosine
kinase activity.
• The EGF receptor consists of one
polypeptide,whereas insulin receptor is a
dimer form of two pairs of polypeptide
chains(alfa and beta)
•
Growth factor binding induces
Reseptör dimerization.
• It leads to autophosphorylation as
the two polypeptide chains
phosphrylate one another. SH2
domains of downstream signaling
molecules bind to phosphotyrosine
part of the activated receptors.
• It leads to their association with
other proteins,promotes their
phosphorylation and stimulates
their enzymatic activities
• 3-G-protein linked receptors.
• The largest family of cell surface receptors
• Transmit signals to intracellular targets via the action
ofG proteins
• Gprotein consist of alfa,beta ,gama subunits
• Ligand binds tothe receptor.this interaction activates
the G-protein,and than it dissociates from the
receptor and carries the signal to an intracellular
target which may be either an enzyme or an ion
channel.
G Protein-Coupled Receptors
(serpentine R)
1) 7-helices transmembrane receptor
2) G protein (Guanylate binding
protein)
• G protein refers to any protein which binds
to GDP or GTP and act as signal transduction.
• G proteins consist of three different subunits
(, , -subunit).
• -subunit carries GTPase activity, binding
and hydrolysis of GTP.
3) Classes of G protein
Gs→ s→AC→cAMP↑
Gi→ i→AC→cAMP↓
Gq→ q →PI-PLC→IP3+DAG
Go→ o→ion channel
Gt→ t →cGMP PDE→cGMP→
Rhodopsin
The molecules of signal transductions
•
•
•
•
•
Ligands
Receptors
G-proteins
Protein kinase and phosphatase
Second mesengers [cAMP,cGMP,cacalmodulin,IP3,DAG]
• Substrats of second mesengers
Pathway of G protein linked receptor
H
R
G protein
Es
secondary messeger
Protein kinase
Phophorylation of Es or functional protein
Biological effect
cAMP
• The cAMP is a second messenger in hormonal
signaling
• cAMP is formed from ATP by the action of
adenylate siklase
• Epinephrine receptor stimulates enzymatic
activity via G protein.thereby increasing the
intracellular concentration of cAMP
• cAMP binds to the regulatory subunits of
protein kinase A.
Protein kinase A
• The inactive form of protein kinase A consists of two catalytic
and two regulatory subunits .Binding of cAMP to the
regulatory subunits induces a conformational change that
leads to dissociation of the catalytic subunits .
• Catalytic subunit active and able to phosphorylate serine
residues on target proteins. For example,it stimulates
glycogen brekdown
• cAMPdependent protein kinase(protein kinase A) also
activates the transcription of specific target genes that contain
cAMP response element [regulatory sequence]
1. cAMP dependent-protein kinase A
pathway
H
R
G protein
AC
cAMP
PKA
Phosphorylation of Es or functional proteins
Biological effects
Phosphorylate specifically Ser/Thr
residues in several proteins
（1）Regulation of metabolism
（2）Regulation of gene
expression
Cholera toxin
Ribosylation of Arg of G
G  -ATP
ATPase
s
G -ADP
s
AC
cAMP
Cl- + H2O
HCO3-
Cavity of intestine
diarrhea
Pertussis toxin
i -ADP-ribosylation
Gi
AC↑
cAMP ↑
allergy of histamine
Phospholipids and calsium
• Many hormones and growth factors stimulate the
hydrolysis of PIP2(phosphatidylinositol4,5
biphosphate) by phospholipase C
• Produces two distinct second
messengers,diacylglycerol,and inositoltriphospate
• DAG activates protein kinase C and mobilize Ca from
intracellular stores.
• Protein kinase C play important role in the control of cell growth an
differentiation
Ca2+ dependent PK pathway
(1) Ca2+ -DAG -dependent PKC pathway
H
R
PIP2
PLC
G protein
IP3
ER
DG
Ca2+ PS
PKC
Phosphorylation of Es or functional proteins
Biological effects
Calmodulin
• The effects of Ca are mediated by the Cabinding protein Calmodulin
• İncreased levels of cytosolic Ca/calmodulin
complex activates a variety of target proteins
,including Ca/calmodulin –dependent protein
kinases.
Ca2＋-CaM dependent protein kinase pathway
H
R
G protein
PLC
IP3
Ca2+
CaM
CaMK
Phosphorylation of Es or functional proteins
Biological effects
IP3 and Protein kinase C have different
effects in different cells
a. IP3
i. Vascular smooth muscle
ii. smooth muscle
iii. skeletal muscle
iv. blood platelet
contraction
contraction
contraction
aggregation of platelets
b. Protein Kinase C. cell growth, differentiation (development into different
tissue types), metabolism.
i. blood platelets
ii. mast cells
iii. smooth muscle
iv. nerve cells
v. adipose tissue
vi. liver cells
serotonin release
histamine release
contractility
neurotransmitter release
fat synthesis
glycogen hydrolysis
Termination of the signal
• adenylyl cyclase remains active while hormones are
present. the cell has a way to stop adenylyl cyclase
activity when it the hormones are removed.
– (see below)
• cAMP is continually broken down by phosphodiesterase
– thus limiting the cell response
• Phosphatase continually reverses the phosphorylation of
the three proteins,
• I.e. phosphorylase kinase, glycogen synthase and
phosphorylase
• this also limits the cell response
4. end the response:
a. a subunit is a GTPase, which hydrolyzes GTP to GDP and
inactivates itself
inactive  subunit reassociates with  and 
this process requires an additional factor: GTPase activating protein
(GAP). enhances GTPase and speeds up the timing step.
b. drop in ligand concentration, resulting in dissociation and
inactivation of the receptor
c. active inactivation of the receptor, a process of
desensitization. This means that the cell stops responding, even when ligand
is still present around the cell
– G-protein receptor kinase (GPRK), phosphorylates the receptor,
– the phosphorylated receptor binds another protein called arrestin, thus
inactivating it (even though hormone is still present and bound to the receptor)
– bound arrestin molecules act as adaptors for Clathrin, allowing receptors to be
internalized, thus further desensitising the cell