Secretion and Sensing

Question 1

What are intracellular compartments that many proteins are transported to after translation (targeting)?

Answer 1

Nucleus, mitochondrion, lysosome and chloroplast.

Question 2

What are extracellular compartments that many proteins are transported to after translation (secretion)?

Answer 2

Periplasm, cell wall and bloodstream.

Question 3

What are the 3 main reasons we need to secrete proteins?

Answer 3

1. Construction of cell wall in microbes and plants.
2. Enzyme-mediated extracellular degradation of nutrient sources (ligases, celluloses, phosphates and lipase).
3. Cell communication - Sex pheromones (microbes) and hormones (animals and plants).

Question 4

What is the mammalian growth hormone and what does it do?

Answer 4

A hormone synthesised and secreted from the anterior pituitary. Acts on many tissues to promote growth, stimulates cells to take up amino acids, and stimulates liver to produce insulin-like growth factors that stimulate bone and cartilage growth.

Question 5

What does overproduction or underproduction of GH cause?

Answer 5

Overproduction of GH in children causes gigantism, and underproduction causes pituitary dwarfism.

Question 6

What is the plant growth hormone and what does it do?

Answer 6

The plant embryo secretes gibberellins, a class of plant hormone. They then trigger changes in the aleurone layer inside the seed coat that causes it to synthesis and secrete enzymes that digest proteins and starch stored in the endosperm. Developing seeds also produce gibberellins which diffuse out into the immature fruit tissues. Gibberellins are not proteins but they are a large family of closely related organic compounds.

Question 7

What does gibberellin A1 do?

Answer 7

Controls plant stem elongation and we can treat dwarf plants with this hormone to restore natural growth.

Question 8

What do sex pheromones allow and promote? What are 2 examples?

Answer 8

They allow fungi to recognize cells of the opposite mating type and promote mating between cells of the opposite mating type. Examples are yeast a-factor and yeast alpha-factor.

Question 9

What is yeast alpha-factor and what does it do?

Answer 9

A short peptide that is processed by cleavage from a longer peptide. Secreted from yeast alpha cells and detected by yeast a cells which promotes mating between a and alpha cells.

Question 10

What is the secretory pathway?

Answer 10

Endoplasmic reticulum –> Golgi apparatus –> Vesicles –> Secretion (or lysosomes).

Question 11

What are the mechanisms of secretion (12 steps)?

Answer 11

1. Polypeptide sequences contain signals which indicate where in the cell the polypeptide belongs.
2. Secreted proteins contain a circa 25 amino acid, N-terminal Signal Sequence.
3. During translation the newly synthesised signal sequence binds to the SRP.
4. Translation then stalls temporarily.
5. The complex docks at a specific receptor on the surface of the ER.
6. A channel opens in the membrane.
7. The SRP disassociates from the complex.
8. Translation restarts and the protein is co-translationally translocated into the lumen of the ER.
9. The N-terminus signal sequence is cleaved from the nascent polypeptide chain.
10. Co-translational translocation continues into the ER.
11. Chaperonins refold the protein in the ER.
12. Translation terminates and the completed polypeptide is released into the lumen of the ER.

Question 12

What do specific retention signals do in post-translational events?

Answer 12

Allow the protein to stay in the ER.

Question 13

What happens when sugars are added to the polypeptide as a post-translation event?

Answer 13

Happens at the Golgi to form glycoproteins which then go to the lysosomes or plasma membrane.

Question 14

What are the 3 main types of post-translational modifications?

Answer 14

1. Proteolysis (cleaving).
2. Glycosylation (adding sugars).
3. Phosphorylation (adding phosphate groups.

Question 15

What does the basic biological circuit require?

Answer 15

A receptor –> signal transduction pathway –> response.

Question 16

How do cells receive signals from other cells?

Answer 16

Signals secreted from one cell are detected by other cells that express the corresponding receptor and cells that don’t express the receptor do not receive the signal.

Question 17

What are the 3 well-studied types of transmembrane receptors in complex eukaryotes?

Answer 17

1. Protein kinases
2. Ion channel receptors
3. G protein-linked receptors.

Question 18

What is the process of a signal attaching to the cytoplasmic receptor?

Answer 18

1. The steroid hormone (ligand) enters the cell.
2. A complex is formed between the hormone and the receptor.
3. The receptor dissociates from the chaperone.
4. The receptor enters the nucleus where it acts as a transcription factor.

Question 19

What is direct signal transduction?

Answer 19

Results from the action of the receptor itself on effector proteins. There is a protein kinase cascade which results in the response being amplified.

Question 20

What is a second messenger and what are some examples?

Answer 20

Something used by indirect transduction to amplify the interaction between receptor binding and cellular reaction. Examples are cAMP and cGMP and lipids, calcium ions and nitric oxide.

Question 21

What are examples of responses to sensing?

Answer 21

1. Fertilization of an egg which triggers a wave of calcium ions to stimulate the initiation of development.
2. Serotonin activates a response in nerve cells via cAMP signalling pathway.
3. Glycogen is metabolised and glucose is released in response to adrenaline and this is mediated by a cAMP-protein kinase signal transduction pathway.
4. Sense of smell is dependent upon G-protein-coupled receptors and cAMP-gated ion channels.