Transcript powerpoint

Signal Transduction
BL 4010
12.07.06
Outline
• Extracellular signals
• Signal-Transducing Receptors
– single membrane spanning receptors
– 7 TMS receptors
• Intracellular Second Messengers
– cAMP
– Calcium
– G-proteins
• Enzyme cascades
Additional resources
• http://www.signaling-gateway.org/
• http://web.indstate.edu/thcme/mwking/signaltransduction.html
• http://stke.sciencemag.org/
Extracellular Signals
• Light
• Small molecules
– hormones
– toxins
– metabolites
• Large molecules
– oligosaccharides
– proteins
Transcriptional activation
Interferon
The phosphorelay system
Phosphorelay system
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Figure 19-1a Classification of hormones.
(a) Endocrine signals are directed at distant
cells through the intermediacy of the
bloodstream.
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Figure 19-1b Classification of hormones.
(b) Paracrine signals are directed at nearby
cells.
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Figure 19-1c Classification of hormones.
(c) Autocrine signals are directed at the cell
that produced them.
Classes of
Hormones
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Figure 19-3a Binding of ligand to receptor.
(a) A hyperbolic plot.
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Figure 19-3b Binding of ligand to receptor.
(b) A Scatchard plot.
Classes of Hormones - Steroid Hormones
• Derived from cholesterol- (e.g. Glucocorticoids,vitamin D,
sex hormones)
– regulate metabolism, salt/water balances,
inflammation, sexual function. May bind to PM receptor
or enter nucleus directly
• May either act at nucleus or at plasma membrane
• Steroids are hydrophobic and cannot diffuse freely to
nucleus
• Receptor proteins carry steroids to the nucleus
• Steroid receptor proteins are all apparently members of a
gene superfamily and have evolved from a common
ancestral precursor
Classes of Hormones - Steroid Hormones
Classes of Hormones - Nonsteroid hormones
• Amino Acid Derived Hormones (e.g. epinephrine)
bind to PM receptors
– regulate smooth muscle , blood pressure, cardiac
rate, lipolysis, glycogenolysis
Effects of epinephrine
Receptor
Alpha1
Alpha2
Beta1
Beta2
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Molecule
Epinephrine, Norepinphrine
Epinephrine, Norepinphrine
Epinephrine, Norepinphrine
Epinephrine
Effect
Increased Ca2+
Decreased cAMP
Increased cAMP
Increased cAMP
Increased rate and force of contraction of heart muscle:
predominantly an effect of epinephrine on beta receptors.
Constriction of blood vessels: increased blood pressure.
Dilation of bronchioles: assists in pulmonary ventilation.
Stimulation of lipolysis in fat cells: provides fatty acids for energy
production and conserves dwindling blood glucose.
Increased metabolic rate
Inhibition of certain "non-essential" processes: e.g. inhibition of
gastrointestinal secretion and motor activity.
Dilation of the pupils: particularly important in situations where you
are surrounded by velociraptors under conditions of low ambient light
Classes of Hormones - Nonsteroid hormones
• Peptide Hormones (e.g. insulin) - bind to PM receptors
– regulate many processes in all tissues - including release of
other hormones
– All secreted polypeptide hormones are synthesized with a
signal sequence (which directs them to secretory granules)
– Usually synthesized as inactive preprohormones ("pre-pro"
implies at least two precessing steps)
– Proteolytic processing produces the prohormone and the
hormone
Insulin is a peptide hormone
Secretion of insulin
Single TMS Receptors
• What is a receptor?
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Three main classes
Extracellular domain to interact with hormone
Single transmembrane segment
Intracellular domain with enzyme activity
Activity is usually tyrosine kinase or guanylyl cyclase
Each of these has a "nonreceptor" counterpart
src gene kinase - pp60v-src was first known
Two posttranslational modifications
Receptor Tyrosine Kinases
Membrane-associated allosteric enzymes
• How do single-TMS receptors transmit the signal from
outside to inside??
• Oligomeric association is the key!
• Extracellular ligand binding
Protein-Tyrosine Phosphatases
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The enzymes that dephosphorylate Tyr
Some PTPases are integral membrane proteins
But there are also lots of soluble PTPases
Cytoplasmic PTPases have N-term. catalytic domains and Cterminal regulatory domains
Membrane PTPases all have cytoplasmic catalytic domain, single
transmembrane segment and an extracellular recognition site
Guanylyl Cyclases
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Soluble or Membrane-Bound
Membrane-bound GCs are the other group of singletransmembrane-segment receptors (besides RTKs)
Peptide hormones activate the membrane-forms
Note speract and resact, from mammalian ova
Activation may involve oligomerization of receptors, as for
RTKs
Soluble Guanylyl Cyclases
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Receptors for Nitric Oxide
NO is a reactive, free-radical that acts either as a neurotransmitter
or as a second messenger
NO relaxes vascular smooth muscle (and is thus involved in
stimulation of penile erection)
NO also stimulates macrophages to kill tumor cells and bacteria
NO binds to heme of GC, stimulating GC activity 50-fold
Read about NO synthesis and also see box on Alfred Nobel
Types of Receptors
• 7-TMS receptors (G protein receptors)
– extracellular site for hormone (ligand)
– intracellular site for GTP-binding protein
• Single-transmembrane segment receptors
– extracellular site for hormone (ligand)
– intracellular catalytic domain - e.g. kinase or
guanylyl cyclase
• Oligomeric ion channels
Second Messengers
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Many and there may be more!
The hormone is the "first messenger"
The second messenger - Ca2+, cAMP or other - is released when
the hormone binds to its (extracellular) receptor
The second messenger then activates (or inhibits) processes in
the cytoplasm or nucleus
Degradation and/or clearance of the second messenger is also
(obviously) important
Steroid Receptor Proteins
• Hydrophobic domain near C-terminus that interacts with steroid
itself
• Central, hydrophilic domain that binds to DNA
• Central DNA-binding domains are homologous to one another,
with 9 conserved Cys residues
• Three pairs of Cys residues are in Cys-X-X-Cys sequences - as in
Zinc-finger domains
• Steroid-receptor complex may bind to DNA or to transcription
factors
• Thyroid hormone receptor proteins are similar
Steroid Receptor Proteins
Steroid Receptor Proteins
cAMP and Glycogen Phosphorylase
Earl Sutherland discovers the first second messenger
• In the early 1960s, Earl Sutherland showed that the stimulation
of glycogen phosphorylase by epinephrine involved cyclic
adenosine-3',5'-monophosphate
• He called cAMP a "second messenger"
• cAMP is synthesized by adenylyl cyclase and degraded by
phosphodiesterase
How are the hormone receptor and AC
coupled?
• Purified AC and purified receptor, when recombined, are not
coupled.
• Rodbell showed that GTP is required for hormonal activation of
AC
• In 1977, Elliott Ross and Alfred Gilman at Univ. of Virginia
discovered a GTP-binding protein which restored hormone
stimulation to AC
• Hormone stimulates receptor, which activates GTP-binding
protein, which activates AC
Heterotrimeric G Proteins
A model for their activity
• Binding of hormone, etc., to receptor
protein in the membrane triggers
dissociation of GDP and binding of GTP to
-subunit of G protein
• G-GTP complex dissociates from G and
migrates to effector sites, activating or
inhibiting
• But it is now clear that G also functions
as a signalling device
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Figure 19-13 Activation/deactivation cycle
for hormonally stimulated AC.
Signalling Roles for G()
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A partial list
Potassium channel proteins
Phospholipase A2
Yeast mating protein kinase Ste20
Adenylyl cyclase
Phospholipase C
Calcium channels
• Receptor kinases
Stimulatory and Inhibitory G
G proteins may either stimulate or inhibit an effector.
• In the case of adenylyl cyclase, the stimulatory G protein is
known as Gs and the inhibitory G protein is known as Gi
• Gi may act either by the Gi subunit binding to AC or by the Gi
complex complexing all the Gi and preventing it from binding to
AC
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Figure 19-16 Mechanism of receptormediated activation/ inhibition of AC.
The ras Gene and p21ras
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An oncogene and its product
a gene first found in rat sarcoma virus
Normal cellular ras protein activates cellular processes when GTP
is bound and is inactive when GTP has been hydrolyzed to GDP
Mutant (oncogenic) forms of ras have severely impaired GTPase
activity, so remain active for long periods, stimulating
excessive growth and metabolic activity - causing tumors to
form
G-protein coupled receptors
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Receptors that interact with G proteins
Seven putative alpha-helical transmembrane
segments
Extracellular domain interacts with hormone
Intracellular domain interacts with G proteins
Adrenergic receptors are typical
Note desensitization by phosphorylationby
protein kinase A