CELL SIGNALING

Specificity is achieved by precise molecular complementarity between the

signal and receptor molecules, mediated by the same kinds of weak

(noncovalent) forces that mediate enzyme-substrate and antigen-antibody

interactions.

Multicellular organisms have an additional level of specificity, because the

receptors for a given signal, or the intracellular targets of a given signal

pathway, are present only in certain cell types.

Three factors account for the extraordinary sensitivity of signal

transducers: the high affinity of receptors for signal molecules,

cooperativity (often but not always) in the ligand-receptor interaction,

and amplification of the signal by enzyme cascades.

The affinity between signal (ligand) and receptor can be expressed as the

dissociation constant Kd, usually 10^10 M or less—meaning that the

receptor detects picomolar concentrations of a signal molecule.

Receptor-ligand interactions are quantified by Scatchard analysis, which

yields a quantitative measure of affinity (Kd) and the number of ligand-

binding sites in a receptor sample.

Cooperativity in receptor-ligand interactions results in large changes in

receptor activation with small changes in ligand concentration.

Amplification by enzyme cascades results when an enzyme associated

with a signal receptor is activated and, in turn, catalyzes the activation of

many molecules of a second enzyme, each of which activates many

molecules of a third enzyme, and so on.

When a signal is present continuously, desensitization of the receptor

system results; when the stimulus falls below a certain threshold, the

system again becomes sensitive.

Integration is the ability of the system to receive multiple signals and

produce a unified response appropriate to the needs of the cell or

organism.

All G protein–coupled receptors (GPCRs) contain seven membrane-

spanning regions with their N-terminal segment on the exoplasmic face

and their C-terminal segment on the cytosolic face of the plasma

membrane.

Stimulation of GPCR causes activation of the G protein, which in turn

modulates the activity of an associated effector protein.

GPCR family includes receptors for numerous hormones and

neurotransmitters, light- activated receptors (rhodopsins) in the eye,

literally thousands of odorant receptors in the mammalian nose, and the

receptors that participate in the mating rituals of single-celled yeasts.

The signal-transducing G proteins contain three subunits designated α, β,

and ȣ. During intracellular signaling the β and ȣ subunits remain bound

together and are usually referred to as the Gβȣ subunit.

The Gα subunit is a GTPase switch protein that alternates between an

active (on) state with bound GTP and an inactive (off) state with bound

GDP.

When an extracellular signal molecule binds to a GPCR, the receptor

protein undergoes a conformational change that enables it to activate a G

protein located on the other side of the plasma membrane.

All effector proteins are either membrane-bound ion channels or enzymes

that catalyze formation of second messengers (e.g., cAMP, DAG, and IP3).

The human genome, for example, encodes 27 different Gα, 5 Gβ, and 13

Gȣ subunits. So far as is known, the different Gβȣ subunits function

similarly.

The α-subunit has an intrinsic GTPase activity, and it eventually

hydrolyzes its bound GTP to GDP, returning the whole G protein to its

original, inactive conformation.

Cholera is caused by a bacterium that multiplies in the human intestine,

where it produces a protein called cholera toxin. This protein enters the

cells that line the intestine and modifies the α-subunit of a G protein

called Gs –so named because it stimulates the enzyme adenylyl cyclase.

The modification prevents Gs from hydrolyzing its bound GTP, thus locking the G protein in the active state, in which it continuously stimulates adenylyl cyclase.

In intestinal cells, this stimulation causes a prolonged and excessive outflow of Cl- ions and water into the gut, resulting in catastrophic diarrhea and dehydration.

The condition often leads to death unless urgent steps are taken to

replace the lost water and ions.

In whooping cough (pertussis), the disease-causing bacterium colonizes

the lung, where it produces a protein called pertussis toxin. This protein

alters the α-subunit of a different type of G protein, called Gi, because

it inhibits adenylyl cyclase. In this case, however, modification by the

toxin disables the G protein by locking it into its inactive GDP-bound

state. Inhibiting Gi, like activating Gs, results in the prolonged and

inappropriate activation of adenylyl cyclase, which, in this case,

stimulates coughing.

Cyclic AMP exerts most of its effects by activating the enzyme cyclic-

AMP-dependent protein kinase (PKA). This enzyme is normally held

inactive in a complex with a regulatory protein. The binding of cyclic

AMP to the regulatory protein forces a conformational change that

releases the inhibition and unleashes the active kinase. Activated PKA

then catalyzes the phosphorylation of particular serines or threonines

on specific intracellular proteins, thus altering the activity of these

target proteins.

Guanylyl cyclases are another type of receptor enzyme. When

activated, a guanylyl cyclase produces guanosine 3 ’ ,5 ’ -cyclic

monophosphate (cyclic GMP, cGMP) from GTP.

The insulin receptor is the prototype of receptor enzymes with Tyr

kinase activity. When insulin binds to its receptor, each αβ monomer of

the receptor phosphorylates the β chain of its partner, activating the

receptor’s Tyr kinase activity. The kinase catalyzes the phosphorylation

of Tyr residues on other proteins such as IRS-1.

β- adrenergic receptors of muscle, liver, and adipose tissue. These

receptors mediate changes in fuel metabolism including the increased

break- down of glycogen and fat.

The hormone activates a GPCR, which turns on a G protein (Gs) that

activates adenylyl cyclase to boost the production of cyclic AMP. The

increase in cyclic AMP activates PKA, which phosphorylates and

activates an enzyme called phosphorylase kinase. This kinase activates

glycogen phosphorylase, the enzyme that breaks down glycogen.

Because these reactions do not involve changes in gene transcription or

new protein synthesis, they occur rapidly.

Receptor desensitization varies with the hormone.

In some cases the activated receptor is phosphorylated via a G-protein

Receptor Kinase.

The phosphorylated receptor then may bind to a protein β-arrestin.

β-Arrestin promotes removal of the receptor from the membrane by

clathrin-mediated endocytosis.

β-Arrestin may also bind a cytosolic Phosphodiesterase, bringing this

enzyme close to where cAMP is being produced, contributing to signal

turnoff.