cell signalling

Question 1

why do cells signal

Answer 1

• communicate
• respond to intracellular changes
• response to extracellular environment
• regulate cell behaviour

Question 2

list the 4 types of cell signalling

Answer 2

1. contact dependant
2. autocrine/paracrine
3. synaptic
4. endocrine

Question 3

define contact dependent cell signalling

Answer 3

= requires direct contact (through gap junctions or signal molecule is bound on the membrane surface)

Question 4

define autocrine cell signalling

Answer 4

release of signals that can travel short distance to target cell

= signal made by cell affecting the same cell

Question 5

define synaptic cell signalling

Answer 5

release of neurotransmitters across synapse to target cell

= exclusively to neurons

Question 6

define endocrine cell signalling

Answer 6

release of hormones into circulatory system to target distant cells

Question 7

define paracrine cell signalling

Answer 7

release of signals that can travel short distance to target cell

= signal made by the cell and affecting another nearby cell

Question 8

list the three main steps of cell signalling

Answer 8

1. reception = signal molecule binds to receptor
2. transduction = intracellular signalling proteins
3. response = effector proteins initiate a response to the signal

Question 9

effector protein examples

Answer 9

• transcription factors
• metabolic proteins
• cytoskeletal proteins

Question 10

define reception (cell signalling pathway)

Answer 10

= a signal molecule (ligand) binds to a receptor of a target cell

• sometimes receptor change conformation to allow tighter binding
• if ligands are similar receptor can recognise multiple ligands

Question 11

4 classes of receptors

Answer 11

1. ligand gated ion channels
2. G protein-coupled receptors
3. enzyme-linked receptors
4. intracellular receptors

Question 12

define ion channel coupled receptors

Answer 12

usually in rapid synaptic signalling

Question 13

define G protein coupled receptors

Answer 13

associate with trimeric G protein

Question 14

define enzyme coupled receptors

Answer 14

have enzyme activity or associate with an enzyme

Question 15

define transduction (cell signalling pathway)

Answer 15

= signal relayed from receptor to effector proteins

• involves intracellular signalling molecules
• regulated by molecular switches (switch from inactive to active state via phosphorylation or GTP binding)
• important for signal amplification, regulation & divergence

Question 16

what are intracellular signalling molecules

Answer 16

= secondary messengers such as cAMP, Ca2+, IP3, DAD

Question 17

3 times of responses in a pathway

Answer 17

1. timing & persistence: response can be quick or slow & vary in duration
2. sensitivity & range: response can be triggered by low or high concentration of ligands & may require a threshold
3. integration & coordination: response may need combination of signal activation & inhibition

Question 18

how is cell signalling regulated

Answer 18

1. by molecular switches that control on/off state of proteins

• phosphorylation
• GTP binding

2. location
3. regulated by affinity & specificity of interaction = specific docking sites in intracellular proteins
4. regulated by positive & negative feedback loops
5. regulated by adaption to response

Question 19

define phosphorylation & reverse

Answer 19

1. protein kinase will phosphorylate a protein by adding a phosphate group through the exchange of ATP to ADP = activating the protein
2. reverse = a protein phosphatase can remove phosphate group deactivating the protein

Question 20

GTP binding & reverse

Answer 20

1. GEFs (guanine exchange factor) will activate the protein by adding GTP = protein switched on

GTPase activating protein can deactivate the protein by removing GTP and exchanging it for GDP = protein switches off

Question 21

how does location regulate cell signalling

Answer 21

= cell signalling can only be switched on if all the signalling molecules are assembled at one location

Question 22

what proteins allow intracellular signalling proteins to bind and assemble

Answer 22

scaffold proteins which have interaction domains

Question 23

importance of positive feedback loops

Answer 23

for prolonging the duration of response

Question 24

importance of negative feedback loop

Answer 24

for limiting the intensity of response

Question 25

what does it mean for a cell to adapt to a response

Answer 25

prolonged exposure to signal can decrease the response adaption allows the cells to become more sensitive to small changes in concentration of signals e.g. removing receptor

Question 26

role of phosphorylated tyrosines

Answer 26

act as docking sites for signalling proteins

Question 27

similarities between G protein-coupled receptors

Answer 27

- structure of the receptor | - associate with trimeric GTP binding protein

Question 28

similarities between enzyme-coupled receptors

Answer 28

- structure | - has enzyme activity or associate with enzyme

Question 29

structure of GPCR

Answer 29

- 7 transmembrane | - heterotrimeric (3 subunits)

Question 30

what is bound to G protein when inactive

Answer 30

GDP

Question 31

what happens when a ligand binds to Galpha protein

Answer 31

Galpha protein exchanges GDP for GTP, then dissociates from complex

Question 32

what are the steps in the cAMP pathway (GCPR)

Answer 32

1. Active Ga protein binds and activates adenylyl cyclase enzyme, which in turn catalyze conversion of ATP to cyclic AMP (cAMP). 2. cAMP (secondary messenger) activates other proteins – protein kinase A (PKA) 3. Activated PKA translocate to the nucleus to activate (phosphorylate) cAMP response element-binding (CREB) transcription factor. 4. CREB binds to cAMP response element (CRE) to promote gene transcription – neuropeptides, hormones, neurotrophic factors

Question 33

what are the steps in the PI (phosphatidylinositol) pathway (GCPR)

Answer 33

1. Active G protein activates plasma membrane bound phospholipase C (PLC) 2. PLC (enzyme) converts phosphatidylinositol 4,5 -biphosphate (PIP2) into inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG) 3. DAG activates protein kinase C (PKC) to regulate activity of other effector proteins (catalyse/phosphorylate). 4. IP3 binds IP3 receptors in the ER to regulate Ca2+ concentration – regulate activity of Ca -dependent enzymes (eg. PKC) = postive feedback

Question 34

is the cAMP a slow or fast response

Answer 34

slow

Question 35

is the IP a slow or fast response

Answer 35

fast

Question 36

what does binding of ligands to receptor tyrosine kinase cause (enzyme linked receptor) & the following MAPK pathway

Answer 36

1. dimerisation 2. which activates the kinase domains --> phosphorylates tyrosine residues on the receptor 3. phosphorylation sets off a chain reaction to trigger additional phosphorylation events 4. phosphorylated sites act as docking sites for other adaptor proteins MAPK pathway: 5. Adaptor protein (GRB2) binds receptor phosphotyrosine residues at SH2 domain. (GRB2 contains SH3 domains that allow Sos to bind) 6. Sos recruits Ras to the complex. 7. Sos promotes GTP exchange for GDP on Ras. 8. Ras is a family (HRAS, NRAS and KRAS) of GTPase proteins. 9. Activated Ras complex dissociates from Sos, but remains bound to the cytoplasmic membrane 10. Active Ras phosphorylates (activates) Raf proteins (ARAF, BRAF, CRAF,MAPKKK) 11. Active Raf phosphorylates MEK kinase (MAPKK) - exchange of ATP to ADP 12. Active MEK phosphorylates ERK (MAPK) 13. Activate ERK can phosphorylate other proteins to change protein activity or initiate gene transcription (by inducing transcription pathway)

Question 37

define dimerisation

Answer 37

conformational change to bring kinases in close proximity - brings the two monomers close proximity allowing phosphyrlation

Question 38

what is bound to Ras when active compared to inactive

Answer 38

Ras functions as molecular switch – active = GTP bound, inactive = GDP bound.

Question 39

what activates the PI3K pathway

Answer 39

binding of ligand to RTK

Question 40

role of PI3K pathway

Answer 40

cell survive and growth

Question 41

role of phosphatidylinositols (PIs) in PI3K

Answer 41

= secondary messenger or docking site for other signalling proteins

Question 42

what happens in PI3K pathway

Answer 42

1. PI3K binds to RTKs and is activated. Activated PI3K phosphorylate PIP2 to PIP3 2. PIP3 recruits protein kinases AKT and PDK1, bind via the PH domains (allow close proximity) 3. AKT is phosphorylated by PDK1 and mTOR complex. ``` 4. Activated AKT further phosphorylates other target proteins (Bad). ``` 5. Phosphorylation of Bad results in release of apoptosis inhibitory protein, causing inhibition of apoptosis

Question 43

what does Wnt/B-catenin signalling regulate

Answer 43

cell fate, differentiation & proliferation ``` In the absence of Wnt ligand, the Frizzled and LRP receptors at the cell membrane remain inactive 2. The Dishevelled protein in the cytoplasm is inactive 3. The destruction complex (Axin, APC, GSK3, CK1) IS active 4. The destruction complex targets Bcatenin for degradation – ubiquitinylate, then send to the proteasome to be broken down ```

Question 44

steps in Wnt/B-catenin signalling

Answer 44

1. Wnt ligand binds to the Frizzled and LRP receptors at the membrane 2. Frizzled is activated, recruits and activates Dishelleved at the membrane 3. LRP recruits and is phosphorylated by CK1 and GSK3 4. Axin binds to phosphorylated LRP and destruction complex is disrupted 5. B-catenin is released, translocate to the nucleus and activate transcription by binding LEF1/TCF transcription factors.

Question 45

the steps in the MAPK signalling pathway

Answer 45

1. RTK gets phosphorylated 2. GRB2 binds RTK 3. Sos binds GRB2 4. Sos recruits RAS 5. RAS dissociates from Sos 6. RAF gets phosphorylated 7. MEK gets phosphorylated 8. ERK gets phosphorylated 9. ERK translocate to the nucleus

Question 46

Which protein activates GTPase by stimulating the release of GDP?

Answer 46

guanine nucleotide exchange factor