Protein Trafficking from the Golgi Apparatus Flashcards
What is Constitutive secretion ?
Vesicles move directly from the Golgi to the plasma membrane
What is Regulated secretion?
Fusion of vesicles with the plasma membrane depends on a signal
What is an example of Constitutive secretion ?
Exocytosis (process of moving materials from within a cell to the exterior of the cell)
What is an example of Regulated secretion and explain ?
The release of insulin.
- Insulin secretion in beta cells of pancreas
- Triggered by rise in blood glucose
- Uptake of glucose by GLUT2 transporter
- Glycolysis leads to rise in ATP:ADP ratio
- This inactivates the potassium channel that depolarizes the membrane
- …causing the calcium channel to open
- Rise in Ca2+ causes release of insulin from vesicles
- SNAREs controlled by Ca2+ sensor
- neurons: ‘similar’
What is Lysosome and it’s function ?
- membrane-bound sacs of hydrolytic enzymes
- digestion of extracellular material (endocytosis and phagocytosis)
- and ‘worn out’ cellular organelles (autophagy)
Is the pH low or high for lysosome and why ?
- Low pH from an ATP-driven pump of H+ into the lumen
- Collection of hydrolytic enzymes active in acidic conditions
What does the lysosome membrane contain ?
- Membrane contains transporters:
transfer sugars, amino acids, nucleotides etc. to the cytoplasm - All proteins delivered via ER and Golgi
Why are Lysosomes dangerous ?
- Made as an inactive precursor with ‘extra’ peptide
- ‘Extra’ peptide cleaved off in Lysosome
- … and a sorting problem
What are sorting (or signal) patches ?
They are a specific 3D arrangement on the surface of a folded protein similar to active sites in enzymes
Cell surface to lysosomes; Endocytosis ?
Two examples:
- Phagocytosis (=cell eating): ingestion of large particles such as microorganisms
- Receptor-mediated endocytosis: specific cargo taken up by a transmembrane receptor
What are macrophages ?
- macrophages (white blood cell type) defend against infection by ingesting invading microorganisms
- particle ‘swallowed’
- digested by lysosomal enzymes
Explain the ‘Pinching off’: Dynamin and GTP ?
- vesicle formed: Dynamin polymerizes around neck
- hydrolysis of GTP drives conformational change
- vesicle separates from membrane
Explain the first steps of receptor-mediated endocytosis ?
- Specific molecules taken up from extracellular fluid- – Bind to cell-surface receptors
- Receptors incorporated into clathrin-coated vesicles
Receptor Mediated Endocytosis: Adaptins
- Adaptin complexes
- bind to ‘signal’ in cytoplasmic tail of receptor
- ……and to clathrin
- hence selective sorting into clathrin-coated vesicle
What happens in the endosome ?
- first destination for endocytic vesicles
- sorting station for the endocytic pathway
- some molecules sent to lysosome, some recycled
- sorting by pH…….
Where next from endosome depends on ?
Whether the ligand and receptor within the membrane of the vesicle endocytosed stay associated
If receptor bound for recycling to plasma membrane it ?
It will go there
If receptor bound to transfer to other edge of cell then they will ?
Transcytose
If ligand and receptor do not remain bound (due to acidic nature of endosome – then?
It will go to lysosome for degradation
Examples of Receptor-mediated endocytosis ?
- LDL receptor
- Transferrin receptor
- EGF receptor
Briefly explain Cholesterol uptake ?
- Low Density Lipoprotein ( LDL): ‘bad cholesterol’
- Lipoprotein particles : ‘packages’ of lipids and cholesterol bound by Apoprotein B-100
Explain the LDL receptor?
- Transmembrane protein
- Binds to LDL outside cell and adaptins inside cell
- Adaptins bind Clathrin,
- LDL is internalized in coated vesicles
- Delivered to lysosomes
Explain the Transferrin cycle?
Aim: get iron into the cell 3 components: - Fe3+ ions, - transferrin - transferrin receptor
- Iron-dependent binding: Transferrin to receptor - pH-dependent binding: Fe3+ to transferrin, tranferrin to receptor - Endosomes are acidic - Transferrin and receptor recycle to surface - Nothing goes to lysosome
Explain the EGF receptor?
- ‘Epidermal’ growth factor
- EGF binds
- Receptor becomes dimer
- Tyrosine kinase phosphorylates itself
- …and other things
- Cells proliferate
What is the ‘on’ switch and the ‘off’ switch in the EGF receptor ?
- Growth factor = ‘on’ switch for cell proliferation
- Must have an ‘off’ switch
- ‘Off’ switch = endocytosis of EGF and receptor to lysosome
Sorting in Endocytosis ?
- The ‘multi-vesicular body’
- Membranes within membranes
- Budding the ‘wrong’ way:
cytoplasm to interior - ‘Reverse’ coat in cytoplasm
- Escrt complex
- Internal vesicles degraded in
- lysosomes
- Hence can degrade membrane
proteins - i.e. breaks the topology rule
