Module 7 - Cell response to the environment

Question 1

Types of signalling

Answer 1

1) Cell-to-cell contact
2) Ligand diffusion:
Either local (adjacent cells through interstitial space) or distant

Question 2

Cell-to-cell contact

Answer 2

Plasma membrane-bound signal molecules are used to directly signal with adjacent cells (called locally-paracrine signalling when this happens within one tissue, called locally-neuronal when signalling is neurotransmission between neurons)

Question 3

Long distance contact

Answer 3

Hydrophobic signals that can pass the plasma membrane and bind to receptors in the cell (called remote-endocrine signalling when this process involves transport through the blood)

Question 4

Types of signal transmission

Answer 4

1 signal - 1 receptor

1 signal - 2 receptors (two effects, ie ACh - nACh receptor in skeletal muscle and mACh in heart muscles (in response to increased effort demands))

1 signal - 1 receptor but in different cell types (ie ACh - mACh in heart muscles (slower) and mACh in salivary glands (in response to food))

Question 5

Cell-surface receptors

Answer 5

Ion-channel coupled (open/close)
G-protein coupled (G-protein (de)activated)
Enzyme coupled ((de)activate associated enzyme)

Question 6

How are cell responses to signals regulated?

Answer 6

Although one signal may bind to many receptors, the cell often has molecular pathways that control the effect of the interacting signal

These pathways are mostly different in different cell types so it allows for regulation

Question 7

G protein signal transduction

Answer 7

As the ligand binds to the GPCR and the G protein is activated, it activates the effector in the intracellular side

Question 8

Inactive G proteins

Answer 8

When GTPase converts its GTP into GDP, the G protein becomes inactive

An inactive G protein is a trimeric complex located in the plasma membrane composed of an α, β, and γ unit

Question 9

Active G proteins

Answer 9

When GDP dissociates and GTP binds, the G protein becomes active

An active G protein dissociated into an active α unit and an active βγ complex

Question 10

What do G proteins actually do?

Answer 10

Causes an immediate change in cell behaviour - ie opening/closing ion channels, activating secondary messengers

Question 11

Secondary messengers: what are they and what are some examples?

Answer 11

Quickly produced, diffusible signalling molecules that activate effector proteins that can easily be inactivated again

cAMP, DAG, and IP₃

Question 12

Signal amplification in signal transduction

Answer 12

Primary transduction, relaying the information to the secondary messenger, amplification, divergence to multiple targets

This allows strong activation in a short amount of time

Question 13

Signal termination in signal transduction

Answer 13

Negative feedback causes the receptor protein to be recycled and the bound signal molecule to be degraded

Question 14

cAMP: how does its conversion work, and what makes it a good secondary messenger?

Answer 14

Adrenaline binds to the GPCR, activating it and causing it to activate adenylyl cyclase which concerts ATP into cAMP (which activates PKA)

It can be quickly formed and inactivated by phosphodiesterase

Question 15

PKA activation, structure, and function

Answer 15

PKA has cAMP binding sites on each polypeptide chain which allows for it to be activated

PKA is a serine-threonine protein kinase with two catalytic subunits with each one attached to one polypeptide chain with both chains being bonded by disulfide bridges

PKA, once activated, moves to the nucleus where it affects the transcription of certain genes by phosphorylation transcription factors

Question 16

DAG and IP₃ activation, structure and function

Answer 16

GPCR activated - phospholipase C active - cleaves PIP₂ into IP₃ and DAG

Question 17

Phospholipases C

Answer 17

Three main groups:

PLCβ - activated by GPCRs (cleaves into DAG and IP₂₃)

PLCγ - activated by RTKs (cleaves into DAG and IP₂)

PLCδ - in the cytoplasm (cleaves into DAG and IP)

Question 18

DAG structure

Answer 18

1,2-diacylglycerol (DAG) - glycerol with fatty acid tails at the first and second carbons

Question 19

IP₃ structure

Answer 19

IP₃ - Inositol 1,4,5-trisphosphate – glucose molecule but with the O-H bonds replaced with O-PO₃²⁻

Question 20

PKC: what is it, how is it activated, and what does it do?

Answer 20

A serine-threonine protein kinase that is recruited to the membrane from the cytosol and binds with its C2 region to DAG

IP₃ binds to the endoplasmic reticulum and causes calcium ions to be released into the cytosol where it binds with PKC in its C1 region and activates it

Once activated, PKC phosphorylates substrates using its C4 region to bind and the C3 region which contains ATP

Question 21

PKC structure

Answer 21

Regulatory domain:

N-C2 (binds to DAG)-PS (pseudosubstrate site)-C1 (calcium ion binding site)-catalytic domain

Catalytic domain:

Caspase cleaving site-C3 (ATP)-C4 (substrate binding site)-C

Question 22

Different types of PKC: what is the difference?

Answer 22

Some PKCs do not require calcium ions for activation (no C1 present)

Question 23

Phosphorylation: what does it do, what amino acids are targeted, and what is the major part of it?

Answer 23

Acts as a molecular switch

Amino acids with hydroxyl groups (serine (Ser/S), tyrosine (tyr/Y), and threonine (Thr/T))

The reaction is reversible as protein kinases phosphorylate and protein phosphatases dephosphorylate

Question 24

Phosphorylation cascades: what is involved and what is an example?

Answer 24

Often involve multiple protein kinases

mitogen-activated protein kinases (MAPK) cascade

Question 25

RTKs: what are they, why are they used, what do they do, and how does inactivation occur?

Answer 25

Receptor tyrosine kinases

Used to continue a signal that cannot pass through the cell membrane

The intracellular domain is used as an enzyme after dimerisation occurs and the RTK becomes active and it trans-phosphorylates (phosphorylates its own tyrosines on opposite sides) which act as binding sites for further signalling

The receptor moves by endocytosis into lysosomes for breakdown and the phosphates are removed by tyrosine phosphatases

Question 26

RAS: what does it do and what types of RAS are there?

Answer 26

Acts as a molecular switch downstream from RTKs

• RAS - acting as a molecular switch in signalling
• RAS-GEF (guanine nucleotide exchange factor) - bound to ATP and swaps it with an ADP bound to the RAS, activating it
• RAS-GAP (GTPase activating factor) - removes the ATP bound to Ras, inactivating it

Question 27

Adaptor proteins between RTKs and RAS-GEFs: what are the important binding points and what do they do?

Answer 27

SH2 binds to phospho-tyrosine residues (Y-P) in the RTKs

SH3 domain interacts with proline-rich sequences (found in the RAS-GEFs)

Question 28

MAPK cascade: what is it and what does it do?

Answer 28

Mitogen-activated protein kinases (MAPK) cascade

Variety of functions:
* Enzyme phosphorylation - metabolism control
* Cytoskeleton phosphorylation - cell shape control
* Gene regulatory protein phosphorylation - gene expression control

Question 28

Phosphatidylinositol kinases: what are they and what do they do?

Answer 28

Heterodimeric lipid kinases which phosphorylate as kinases