Lecture 20: Signal Transduction – Part 1

Cells need to respond how often to changes in external environment?

constantly

What detectd and respond to changes in nutrients, chemicals, light, heat,mechanical forces, etc.

cells

Which organisms must coordinate these responses to different external signals across different cell types

Multicellular

Order of Signal Transduction

1. Receive Signal ---------> 2. Transduce Signal -----------> 3.Respond to Signal

The complex process by which a cell converts a signal from outside (or from inside the cell) to a functional change within the cell

Signal Transduction

Location of Signal Transduction

Signaling Cell ---------> Signal ---------> Receptor ---------> Target Molecule ---------> Response

whereby a protein in the signal transduction pathway (S5) or an effector protein modifies either the receptor or an early protein in the pathway

feedback controls

Ex: hormones (insulin secreted by pancreatic cells travels through blood and acts on target cells in liver, muscles, etc)

Endocrine signaling

Ex: neuron releasing a neuro transmitter, growth factors et

Paracrine signaling

Ex: tumor cells secrete growth factors that act on itself

Autocrine signaling

Ex: notch signaling pathway

Signaling by plasma membrane attached proteins

typically consequences of modifications to specific preexisting enzymes and other proteins that alter their activity or function

Rapid changes

effector proteins are typically transcription factors and results in
changes in gene expression

Slower, long-term changes

Often initiated by covalent modifications such as phosphorylation or ubiquitination or by binding of ions or molecules such as Ca2+ or cAMP

Rapid changes

Induces changes in cell proliferation, cell differentiation, and organismal development

Slower, long-term changes

Such modifications can induce changes in cellular metabolism, secretion of hormones, firing of action potentials in nerve cells etc

Rapid changes

Diffuse through the plasma membrane

Bind to cytosolic receptors

Hydrophobic signaling molecules

Cannot diffuse across the cell membrane

Bind to specific cell-surface receptor proteins

Hydrophilic signaling molecules

Receptor-signal complex moves into the nucleus – binds

promoter regions in DNA to regulate gene expression

e.g., Estrogen – Estrogen Receptor

Hydrophobic signaling molecules

triggers receptor conformational change that activates the receptor

Ligand receptor interaction can be extracellular or intracellular

small molecules [adrenaline, acetylcholine], peptides [yeast

mating factors, glucagon], and proteins [insulin, growth

hormone]

Hydrophilic signaling molecules

Same ligand can induce different cells to respond in a different ways

Effector Specificity of the Receptor – Ligand Complex

small molecules and ions in signaling pathways

Second Messengers

bind to and activate or inhibit specific intracellular proteins

Second Messengers

Plays important role in signal amplification

Second Messengers

a reversible post-translational modification

Protein phosphorylation

covalently attaches phosphate groups to other proteins (substrates) at serine, threonine and tyrosine residues

Protein kinases

Blank activity of protein kinases can be regulated by:
o Binding to other proteins
o Intracellular concentrations of small molecules
o Phosphorylation by other protein kinases

Catalytic

enzymes that removes phosphate groups from substrate proteins

Protein Phosphatases

GTP bound

On Form

GDP bound

Off Form

Made up of alpha (α), beta (β) and gamma (γ) subunits

bind to the cell surface receptors called the G-proteins coupled receptors (GPCR)

Heterotrimeric G-proteins

“low molecular weight G proteins”

Do not directly bind to receptors

Monomeric G Proteins

acts as guanine nucleotide exchange factor (GEF)

G-proteins coupled receptors (GPCR)

binds to G proteins and triggers release of GDP from the Gα subunit

G-proteins coupled receptors (GPCR)

Gα subunit then spontaneously binds GTP inducing conformational change that activates

G-protein

GTPase activity of the Gα subunit hydrolyses GTP to GDP returning the G protein to “which” position

Off

β and γ subunits are closely bound to one another and are referred to as

Gβγ complex

Acts as intermediate proteins in signal transduction pathways

Monomeric G Proteins

Plays important roles in many pathways that modulate cell

division and cell motility

Monomeric G Proteins

Activation of a relatively small number of receptors to
trigger major changes in cell metabolism, movements, or
gene expression

SIGNAL AMPLIFICATION

Effector protein modifies and inhibits an early protein in the pathway, blocking an early step in that pathway
(feedback responses are sometimes referred to as adaptation)

FEEDBACK REPRESSION

How can we quantify the number of receptors on a cell surface
and determine how tightly they bind to a ligand?

Receptor–Ligand Binding Assays

Detect how much ligand is bound

Receptor–Ligand Binding Assays

Proportional to total number of receptors (ligand is in excess)

Maximal amount of binding (Bmax)

the ligand concentration that results in half the receptors being bound at equilibrium

Dissociation constant (Kd)

Lower the K the what the binding

tighter

Binds to receptor, induces conformation change that activates downstream signaling pathway

AGONISTS

Binds to receptor at normal ligand binding site

ANTAGONISTS

Most synthetic analogs bind more tightly to the receptor than natural hormone

AGONISTS

Does not induce conformation change leading to receptor activation
Block binding of ligand or agonists
Inhibits receptor signaling by ligand

ANTAGONISTS

largest superfamily of proteins and are highly conserved

GPCRs

Human genome encodes ~# GPCR

800

G protein–coupled receptors orientation

N-terminus outside, C-terminus in cytosol

All G protein–coupled receptors have # transmembrane α-helices

7

All G protein–coupled receptors have # extracelular segments and # cytosolic segments

4

Blank activates adenylate cyclase via ß2-adrenergic receptor

Epinephrine

Adenylate cyclase activates what via second messenger “cyclic AMP”

Protein Kinase A (PKA)

what activates PKA via GPCR signal transduction pathway

Epinephrine

what induces release of glucose from glycogen

Protein Kinase A (PKA)

releases glucose from
glycogen

Glycogen phosphorylase

incorporates glucose into
glycogen

Glycogen synthase

PKA activation results in what of glycogen phosphorylase

Activation

PKA activation results in what of glycogen synthase

Inhibition

cAMP is converted to what by cAMP phosphodiesterase

AMP

control the cellular levels of the second messenger cAMP and
the rates of their degradation (Repression)

cAMP phosphodiesterase

converts ATP into cAMP

Adenylate cyclase

what is stimulated by different Receptor–Ligand Complexes

Adenylate cyclase

When demand for glucose is high (low blood sugar (hypoglycemia)
or during exercise), what is released by the α cells of the
pancreatic islets

glucagon

polypeptide hormone that
induces glycogen breakdown
in liver and muscles

glucagon

Presence of what pathway in same cell – provides fine-tuned
control of the cAMP level and downstream cellular responses

both activation and inhibition