Cell signalling

Question 1

INTRACELLULAR SIGNALLING

Answer 1

A signal molecule is bound to a receptor
often located at the cell surface.
* Receptor activation turn on intracellular
signaling pathways.
* The signaling cascade may activate gene
regulatory proteins, metabolic enzymes and
other effector proteins.
* The human genome contain more than 1500
genes that encode receptors proteins.
a signal molecule usually binds to a receptor protein that is embedded in the plasma membrane of the tharget cell. the receptor activates one or more of the intracellular signalling proteins. Finally one or more of the intracellular signalling proteins alters the activity of effector proteins and thereby the behavior of the cell.

Question 2

INTERCELLULAT SIGNALLING

Answer 2

Contact dependent signaling: In some cases, the signaling molecule is bound to the surface of
the signaling cell and affects only cells that contact it.
Development and immune system responses.
Paracrine signaling: Signaling molecules may also act as local mediators affecting only cells
located close to the signaling cell.
Autocrine signaling: the cell respond to its own signaling molecule.
* To act locally the signaling molecule must rapidly be taken up by nearby target cells or immobilized.
* Secreted protein antagonist may bind to the signaling molecule itself or to the target receptor and restrict
its effect range.
To move signals over longer distances two types of signaling are used.
* Synaptic signaling: Upon activation the neuron send electrical impulses along its axon. At the
synapse the cell releases a neurotransmitter acting on its target cell.
Advantages: fast and precise signaling.
* Endocrine signaling: The signaling molecules (hormones) are secreted into the blood stream and
act on target cells within the organism.

Question 3

CELL SURFACE RECEPTORS

Answer 3

Most extracellular signaling molecules bind specific receptors on the cell surface of the
target cell.
* The cell surface receptors convert the extracellular signal into an intracellular signal that
alter the behavior of the cell.
* There are three large classes of cell surface receptor proteins.
▪ Ion-channel coupled receptors
▪ G-protein coupled receptors
▪ Enzyme-coupled receptors

Question 4

G-PROTEIN COUPLED RECEPTORS

Answer 4

G-protein coupled receptors (GCRs): binds to a signal molecule and act indirectly by regulating
the activity of a plasma membrane bound target protein (enzyme or ion channel).
* GCRs are seven-transmembrane domain receptors.
* A heterotrimeric G-protein mediate the interaction between the activated receptor (with bound
ligand) and the target protein.

Question 5

ENZYME COUPLED RECEPTORS

Answer 5

The enzyme-coupled receptors may function directly as enzymes (have catalytic intracellular
domain) or interact with and activate associated enzymes.
* They are normally single-pass transmembrane proteins.
* The majority are protein kinases or associate with protein kinases that phosphorylate target
proteins when activated.

Question 6

MAP KINASE PATHWAYS

Question 7

CALCIUM MEDIATED SIGNALLING

Answer 7

Calmodulin and CaMK

Question 8

CALMODULIN

Question 9

CAMK

Question 10

PI3 KINASE-Akt-mTOR PATHWAYS

Question 11

NOTCH, Wnt, JAK-STAT, TGFbeta, NFkbeta PATHWAYS

Question 12

SH2 AND SH3 DOMAINS

Question 13

ADAPTORS AND SCAFFOLD PROTEINS

Question 14

POSITIVE / NEGATIVE FEEDBACK

Answer 14

Intracellular signaling pathways often incorporate positive and negative feedback loops.
* In positive feedback the output stimulate its own signal, while in negative feedback it inhibit
and reduce the signal.
These feedback loops can operate in
timescales from hours (ie. circadian
oscillations) to milliseconds (action
potentials).
Sometimes they involve secretion of
extracellular signal molecules but, can
also operate exclusively within the target
cell.

Question 15

ADAPTATION AND DESENSITIZATION

Answer 15

Adaptation or desensitization is a reversible process where a prolonged exposure to a stimulus
decreases the cells response to that stimulus.
* Adaptation allows cells to respond to changes in the “concentration” of an extracellular signal
over a wide range of concentrations.
* Negative feedback loops with short time delays are important in
such adaptive responses.
Per Winge, Department of Biology, NTNU
Desensitization can operate on several levels.

Question 16

REGULATION OF cAMP LEVELS

Answer 16

PKA and AKAPs

Question 17

PKA

Question 18

AKAPs

Question 19

TYROSINE KINASE RECEPTORS

Question 20

RAS SIGNALLING

Question 21

MOLECULAR SWITCHES

Answer 21

Many signaling proteins act as molecular switches.
* They switch from an inactive to an active conformation after receiving a signal and return to the inactive
state after another process turns them off.
* Gain and loss off a phosphate group play an important
role in some of these switches.
* A large class of proteins are activated or inactivated by
phosphorylation.
* Protein kinases covalently adds one or more phosphate
groups to a protein, while protein phosphatases remove
the phosphate groups.
About 30%-50% of human proteins are phosphorylated.
(~520 protein kinases and ~150 protein phosphatases).
Many protein kinases are themselves regulated by phosphorylation,
often organized into phosphorylation cascades / kinase cascades.
Another important class of molecular switches are GTP-binding proteins.
* They switch between an active GTP-bound form and an inactive GDP-bound form.
* In the active GTP-form they have an intrinsic
GTPase activity (hydrolyze GTP to GDP) and may
gradually shut themselves off.
There are two main classes of GTP-binding proteins:
* Large trimeric GTP-binding proteins,
(heterotrimeric G-proteins) coupled to G-protein
linked receptors.
* Monomeric GTPases, relay signals from many
classes of cell surface receptors.

Question 22

GTPase

Answer 22

Monomeric GTPases have low intrinsic GTPase activity and are also regulated by other proteins.
* Guanine nucleotide exchange factors (GEFs) activate monomeric GTPases by replacing bound
GDP with GTP. (The cellular concentration of GTP is 10-times higher than the GDP concentration).
* GTPase activating proteins (GAPs) drive the
monomeric GTPases into the off state by increasing
the rate of GTP hydrolysis.
* Heterotrimeric G-proteins are activated by G-protein
coupled receptors (act like a GEF).
* The intrinsic GTPase activity is higher in heterotrimeric
G-proteins than in the monomeric GTPases, but they
also need associated GAP proteins.

Question 23

LARGE SIGNALLING COMPLEX

Answer 23

example based on insulin receptor which is enzyme-coupled receptor. First the activated receptor phosphorylates itself on tyrosine and one of the phosphotyrosines then recruits a docking protein called insulin receptor substrate (IRS) via PTB domain. the PH domains also binds to specific phosphoinositides on the inner surface of the inner membrane. then the activated receptor phosphorylates IRS on tyrosines and one of these phosphotyrosines binds the SH2 domain of the adaptor proteinGRB2. netxt GRB2 uses one of its two SH3 domains to bind to a proline-rich region of a protein called Sos, which relays the signal downsteram by acting as GEF to activate a monomeric GTPase called Ras. Sos also binds to phosphoinositides in the plasma mambrane via its PH domain. Grb2 uses its other SH3 domain to bind several other signalling proteins and the other phosphorylated tyrosines on IRS recruit additional signalling proteins that have SH2 domains

Question 24

POSITIVE FEEDBACK LOOP

Answer 24

Positive feedback loops play important roles during development when cells must decide between
alternate developing pathways and plays a crucial role regulating chemical and physical processes in
cells, (ex. cell memory)
* If the positive feedback is of moderate strength, it steepens the response (a sigmoidal response), if the
positive feedback is strong, it can produce an all-or-none response

Question 25

NEGATIVE FEEDBACK LOOP

Answer 25

In negative feedback loops the level and the duration of the response is reduced.
* A negative feedback with a short time delay act as a change detector. It gives a strong initial signal, but
the negative feedback quickly dampens the response (adaptation).
* A negative feedback with a longer time delay can produce responses that oscillates. The oscillations can
be maintained as long as the stimulus is present.