10.1. Secretion Flashcards
Cells secrete proteins for three main reasons…
Communication (hormones, in animals and plants, and sex pheromones, in microbes).
Enzyme-mediated, extracellular degradation of nutrients (digestion).
Construction of the cell wall (in plants and microbes).
Mammalian hormones are secreted by cells…
(The journey)…
Then they diffuse into the extracellular fluid and are distributed by the circulatory system.
They coordinate anatomical, behavioural and physiological changes in an animal.
Some endocrine cells aggregate into endocrine glands.
Mammalian growth hormone…
Synthesised and secreted by the anterior pituitary gland.
Gigantism: the result of too much.
Pituitary dwarfism: the result of too little.
GH can also stimulate cells to take up amino acids.
GH can also stimulate the liver to produce insulin-like growth factors that stimulate bone and cartilage growth.
Plant growth hormone…
The plant embryo secretes gibberellins, a class of plant hormone that promotes growth.
The gibberellins trigger changes in the aleurone layer inside the seed coat.
This causes it to synthesise and secrete enzymes that digest proteins and starch stored in the endosperm.
Developing seeds produce gibberellins, which diffuse out into the immature fruit tissue.
For example, gibberellin A1 controls plant elongation.
Fungal sex pheromones…
Allow fungi to recognise cells of the opposite mating type.
This promotes mating.
The yeast alpha-factor is a short peptide that is processed by cleavage from a longer polypeptide.
It is secreted from yeast alpha cells and detected by yeast A cells.
After synthesis, many proteins are transported from the cytoplasm to another cellular compartment…
Intracellular compartment (targeting): transported to the nucleus, mitochondrion, lysosome and chloroplasts.
Extracellular compartment (secretion): transported to the periplasm, cell wall, bloodstream (via exocytosis).
Proteins traverse from endoplasmic reticulum –> Golgi apparatus –> vesicles –> secretion / lysosomes.
12-step mechanism of secretion…
- Many polypeptide sequences contain ‘signals’ that indicate where the peptide belongs.
- Secreted proteins contain a ~25 amino acid N-terminus signal sequence.
- During translation, the newly synthesised signal sequence binds to the signal recognition particle.
- Translation stalls temporarily.
- The complex docks at the specific receptors on the surface of the ER.
- A channel opens in the membrane.
- The SRP disassociates from the complex.
- Translation restarts and the protein is co-transitionally translocated into the lumen of the ER.
- The N-terminus signal sequence is cleaved from the nascent polypeptide chain.
- Co-transitional translocation continues in the ER.
- Chaperonins refold the protein in the ER.
- Translation terminates and the completed polypeptide is released into the lumen of the ER.
Post-translational events…
Proteins with no further signals are secreted from the cell, via the Golgi apparatus.
Other signals can direct proteins to remain in the ER. This can assist the formation of glycoproteins, as sugars are added to the Golgi. These go to lysosomes or the plasma membrane.
Most proteins are modified after they are translated, a feature that is often essential to functioning (this happens at the RER).
Three types of protein modification…
Proteolysis: cleaving excess proteins from the protein chain.
Glucosylation: adding sugars to the protein chain.
Phosphorylation: adding phosphate groups to the protein chain, to provide energy.
