Lecture 2: The Secretory Pathway in Eukaryotic Cells Flashcards
Molecules must be transported within and into/out of the cell. The molecules are parcelled in …
VESICLES
Lysosome…
Contains digestive enzymes that break down cell parts or substances entering the cell by vesicles
Smooth Endoplasmic reticulum..
Synthesises lipids and have various other functions
Golgi apparatus…basic
modifies lipids and proteins from the ER; sorts and packages them into vesicles
Rough Endoplasmic reticulum…
synthesises proteins and packages them into vesicles. (folding and assembly)
In man, __ of genes that code for proteins that enter the __
1/3 & Endoplasmic reticulum
______ must fold and be modified correctly in the __ before being transported via the ___ ____ to the cell surface or to lysosome/vacuoles
Polypeptides
ER
golgi apparatus
Protein translocation via TRANSLOCAN pore
N-terminal signal peptide is removed while the new polypeptide is emerging into the ER lumen
The ER lumen is specialised for:
folding, assembly, modification, quality control and recycling
how do proteins enhance stability before secretion?
by undergoing processes such as glycosylation & disulphide bond formation
Glycosylation:
process by which sugars are attached to proteins
ER in cells is dynamic, movement is dependant on..
actin/myosin in plants and microtubules in animals
The golgi apparatus…
the distribution, shipping (& manufacturing) department for many of the cells chemical products (proteins, carbohydrates and lipids).
Vesicle traffic..
Transport vesicle with kinesin strand attached. ATP is required for every step, ATP –> ADP + Pi. Move along microtubule towards + end. A protein complex enables vesicles to dock and fuse wit their target membranes precisely.
E.g. of protein type secreted via Golgi in vertebrates (Serum proteins)
E.g. = albumin
cellular site of synthesis: hepatocyte (liver)
E.g. of protein type secreted via Golgi in vertebrates (extracellular matrix proteins)
E.g. Collagen
cellular site of synthesis: fibroblast
E.g. of protein type secreted via Golgi in vertebrates (peptide hormone)
e.g. insulin
cellular site of synthesis: pancreatic B-islet cells
E.g. of protein type secreted via Golgi in vertebrates (digestive enzymes)
E.g. trypsin, amylase
cellular site of synthesis: Pancreatic acini
E.g. of protein type secreted via Golgi in vertebrates (milk proteins)
E.g. Casein, lactalbumin
cellular site of synthesis: mammary gland
E.g. of protein type secreted via Golgi in vertebrates (Glycoprotein)
E.g. Mucus
cellular site of synthesis: widespread
Role of ER and Golgi in MILK SECRETION
- Transport of ions and water
- SMOOTH ER: formation of cytoplasmic lipid vesicles & lipid secretion
- ROUGH ER: synthesis of milk protein (casein) & vesicle delivery via Golgi. Synthesise of lactose from Glc & UDP - Gal occurs in Golgi.
- Vesicular movement (immunoglobulins)
UDP - Gal and Glc meanings
Uridine diphosphate galactose
Glucose
Immunoglobulins..
any of a class of proteins present in the serum and cells of the immune system, which function as antibodies.
Protein bodies:
Proteins stored for growth & development (eg. eggs)
Plants store proteins in protein bodies (_______) to provide ______ for rapid growth &eg
vacuoles
C N & F
eg; Ricin from castor bean
Castor bean seeds contain..
ricin (a lectin) a potent cytotoxin. Anti-herbivory
Ricin stored in
protein bodies
Castor bean: How does ricin work
inhibits protein synthesis by irreversibly inactivating eukaryotic ribosomes. When seed germinates, ricin is rapidly degraded.
Castor bean: How is ricin synthesised and how does the plant avoid poisoning its own ribosomes
as proricin that contains both A and B chains. Proricin modified within the GOLGI, then transported to the PROTEIN BODIES. Ricin not catalytically active until proteolytically cleaved within the protein bodies - hence avoids poisoning
Assassination with Ricin
Pellet from the tip of an umbrella that had some ricin on it was used to assassinate Giorgi Markov in 1978 - v little needed
Understanding sub cellular targeting can help engineer _____ production in plants. E.g.
PROTEIN
Insulin;
-Demand for insulin will potentially double over next ten years.
-Insulin can form 0.24% of total seed protein in this ER-directed version of Arabidopsis.
- 0.4 Kg.ha-1 of insulin in safflower. 6,500 hectares of safflower insulin would meet world demand.
Quality control in the ER
-Proteins that are recognised as being incompletely or incorrectly folded (e.g. mutants) are retained within the ER & targeted for degradation by lysosomes or proteasome.
Chaperons:
are proteins that assist the correct intracellular folding and assembly of polypeptides.
ERAD;
ER-associated protein degradation
CFTR; and what does it do
Cystic Fibrosis transmembrane conductance regulator is a chloride ABC transporter.
ER quality control & cystic fibrosis
- CFTR is a chloride ABC transporter
- Epithelia lining sweat gland ducts fail to take up Cl efficiently from the lumen
- Lack of CFTR allows accumulation of mucus, in the lungs & pancreas, so digestive enzymes cannot get into the intestine
Onemutationcausingcysticfibrosis(70%ofsufferers) results in lack of
Phe508, but does NOT affect channel function (ability to take up Cl) –> SINGLE MUTATION.
-Quality control check in the ER: the mutant protein fails to leave the ER. - believes there is a mistake v. sensitive
what cells are vacuoles and lysosomes present in
Plantandfungalcells,someprotist, animal and bacterial cells.
Autophagy:
destruction of damaged or redundant cellular components in vacuoles
Vacuole and lysosome jobs
Autophagy, lysis and recycling of mis‐ folded proteins
Vacuole and lysosome volume in epidermal cells
99% volume
Vacuole and lysosome volume in mesophyll cells
~70% volume
What do vacuoles and lysosome store & isolate
- Storage of carbohydrates, organic acids, anthocyanins, seed storage proteins.
- Isolation of toxic substances(Na,Al).
Are Vacuoles and lysosomes anti-herbivory?
YES; Tannins are the most abundant secondary metabolites made by plants, from 5‐10% dry weight of tree leaves. Cyanogenic glucosides. Ricin.
Cyanogenic glucosides
Hydrogen cyanide is released from the cyanogenic glycosides when fresh plant material is macerated as in chewing, which allows enzymes and cyanogenic glycosides to come together, releasing hydrogen cyanide.
Vacuoles and lysosomes maintain..
internal hydrostatic pressure (cell turgor)
Cyanogenesis how many species and e,g,
3000 species including white clover, flax, almonds, sorghum, wild lima bean, and cassava. Anti herbivory.
Cassava is a major source of…
calories for man in sub‐Saharan Africa.
Where cassava and lima beans comprise the major item of the diet…
chronic cyanide poisoning exists.
Sweet and bitter amount of HCN and lethal dose
Sweet cassava roots: < 50 mg.Kg‐ 1 FWt HCN
Bitter roots: 400 mg.Kg-1FWt.
Lethal dose is 0.5 - 3.5 mg.Kg-1 of body weight
In Sorghum leaves what the cyanogenic glucoside called
dhurrin
Cyanogenesis and compartmentation; Sorghum leaves
In Sorghum leaves, the cyanogenic glucoside, dhurrin, is isolated in the vacuoles of epidermal cells, whereas the β‐ glucosidase and α‐ hydroxynitrilases are present in the chloroplasts and cytosol, respectively, in the underlying mesophyll cells.
The hydrolysis of dhurrin occurs only after
tissue disruption (by herbivory)
Steps for Sorghum plant: Cyanogenic glucoside
Cyanogenic glucoside combines with Beta-glucosidase –> Alpha-hydroxynitrile. Combines with Alpha-hydroxynitrilises –> to give a KETONE and HCN
Cyanogenesis in animals
• A few species of insects are able to synthesize cyanogenic glucosides.
E.g. of cyanogenesis in animals Larvae
Larvae of the Burnet moth, Zygaena filipendulae, isolate cyanogenic glucosides from their food plants.
•Larvae store them in viscous droplets in cuticular cavities in the integument
integument:
a tough outer protective layer
E.g. of cyanogenesis in animals adults
AdultfemalesreleaseHCNaspheromoneto attract males. Females prefer to mate with males that have greater contents of cyanide. The males give cyanogenic glucosides to the females as a nuptial gift during mating. This extra HCN helps the females protect the eggs.
