Intracellular and plant receptors

Question 1

What does ligand binding to a nuclear receptor cause?

Answer 1

Inhibitory complex dissociates from it

Question 2

How do steroid receptors bind to DNA?

Answer 2

1. Steroid receptors bind a ligand in the cytoplasm, displacing HSP70 and moving to the nucleus
2. Interacts as a homodimer with 6bp inverted repeat DNA elements separated by 3bp

Question 3

How do thyroxine and retinol acid receptors activate DNA?

Answer 3

Bind a ligand in the nucleus
Interact as a heterodimer with RXR with direct repeat DNA elements

Question 4

What features does calcium have?

Answer 4

Signal
Second messenger
Switch-like behaviour to prevent random signal cascades
Isn’t broken down
Stored in ER/SR

Question 5

What are P-type Ca2+ ATPases involved in

Answer 5

Nerve impulses, muscle relaxation and secretion/absorption in kidney

Question 6

What types of Ca2+ cotransport take place?

Answer 6

1 Ca2+ out, 3 Na+ in
1 Ca2+ 1K+ out, 4 Na+ in

Question 7

Describe IP3 gated channels

Answer 7

In ER membrane
Ca2+ released activates PLC which converts PIP2 to IP3 and DAG

Question 8

How are Ca2+ signals spread across muscle fibres?

Answer 8

Voltage gated Ca2+ channels along the T-tubule release Ca2+.
This opens ryanodine receptors on the SR, releasing more Ca2+

Question 9

How does Calmodulin activate MLCK?

Answer 9

Ca2+ binds calmoduln at 4 EF hands, changing its shape. This makes the surface more hydrophobic so it binds helices in MLCK

Question 10

What other calcium binding proteins are there

Answer 10

troponin
Aequorin in Aquorea victoria has 3 EF. When all occupied, it emits light

Question 11

What does ethylene do in plants?

Answer 11

Promotes fruit ripening
Leaf Abscission
Plant senscence
Stress signal in wounding, infection and flooding

Question 12

Describe ethylene receptors

Answer 12

On ER
Dimeric
Multipass TM proteins
Copper-containing ethylene binding domain
Domain that interacts with CTR1 closely related to RAF MAPKK

Question 13

What happens when ethylene receptors are activated?

Answer 13

1. Ethylene binds receptor
2. CTR1 is inactivated, so it can no longer stimulate ubiquitylation and degradation of EIN3
3. EIN3 stimulates ethylene response genes

Question 14

What is abscisic acid (ABA) involved in?

Answer 14

Seed and bud dormancy
Shuts off stomata during winter, slowing plant growth and protecting dormant buds

Question 15

What happens when ABA is not present in a plant cell?

Answer 15

SnRK2 (kinase) is sequestered by PP2C (phosphatase)

Question 16

What happens when ABA is in a plant cell

Answer 16

It activates a PYR/PYL/RCAR complex which inhibits PP2C.
This allows SnRK2 to autophophorylate and phosphorylate transcription factors for ABA response genes

Question 17

How do giberellins elicit their response?

Answer 17

1. GA binds and induces a conformation change in its receptor GID1
2. Transcriptional repressor DELLA binds
3. F-box protein GID2 sends this to the proteasome, inducing growth responses

Question 18

What does auxin do?

Answer 18

Induces cell growth and division regulation at shoot tip
Auxin agonists used as weed killers

Question 19

Outline the auxin signalling pathway

Answer 19

1. Auxin inhibits AuxRE genes.
2. F-box protein TIR1 marks Auxin for degradation
3. Auxin Response Factors (ARFs) are released and activate AuxRE
4. Auxin is expressed and sequesters ARFs, completing the negative feedback loop