What are the three parts of the Golgi Apparatus?
Cis, medial and trans
the cis is closest to the ER
What are the two types of secretory pathways and what is the difference between them?
Constitutive – it is unregulated and acts as a shuttle vesicle to the membrane
Regulatory – it is regulated. Vesicles in this pathway must receive a signal (e.g. a hormone or neurotransmitter) before the material in the vesicles can be exocytosed. Found in excitatory cells.
Describe the passage of lysosomal enzymes in the secretory pathway.
- Lysosomal hydrolase precursors move from the ER to the cis Golgi
- The mannose on the lysosomal hydrolase is phosphorylated by phosphotransferase
- The phosphorylated sugar acts as a tag
- It moves through the Golgi apparatus and binds to mannose-6-phosphate receptors in the trans Golgi
- Once bound, the vesicles move to the late endosome
- The late endosome has a proton pump which pumps protons into the late endosome thus making it acidic
- The acidity allows the dissociation of the M6P receptor
- Phosphohydrolases remove the phosphate from the lysosomal hydrolase –meaning it can’t return to the Golgi
- Lysosomal hydrolases accumulate in the late endosome and it matures into a lysosome
What are the three fates of endocytosed material?
Degradation, transcytosis and recycling
Give an examples of something that uses the degradation pathway.
LDL binds to LDL receptors and form clathrin-coated vesicles These vesicles move to the early endosome (clathrin coat is removed along the way)
From there, the LDL receptors are recycled back to the cell surface and LDL is transported to the lysosome where it is degraded to form free cholesterol
Give an example of a disease of endocytosis
Familial Hypercholesterolaemia – caused by mutation in the LDL receptor
what are the types of nuclear transport?
- gated transport (nuclear import)
- trans membrane transport
( import of newly synthesised proteins into ER) - vesicular transport (inter organellar transport)
how does endocytosis work?
- material is recognised at the plasma membrane
- the first pathways is the early endosome - here the material can go into the plasma membrane
- or the second pathway is the late endosome where material destined for destruction is taken to the lysosome to be broken down
an overview of the exocytic pathway?
- the overall movement is from the rER through the golgi apparatus to the plasma membrane
- ribosomes make proteins
- The proteins are incorporated into vesicles transported to the Cis Golgi Apparatus.
- Proteins then pass through the Golgi apparatus and undergoes post-translational modification
- proteins eventually reach far side of golgi apparatus called the trans golgi apparatus
- Here the proteins are sorted into specific transport vesicles, which then go to different destinations
- proteins going to the cell surface have 2 types of pathway
- regulated or constitutive
how do newly synthesised proteins join the ER?
- new proteins will bind to the SRP
- this binds to a SRP receptor on the ER before translation continues
- A common pool of ribosomes is used to synthesise both the proteins that stay in the cytosol and those that are transported into the ER
- The ER signal peptide on a newly formed polypeptide directs the engaged ribosome to the ER membrane.
- At the end of each round of protein synthesis, the ribosomal subunits are released and rejoin the common pool in the cytosol.
what modifications might happen to the protein?
- folding
- forming of disulphide bonds
- glycosylation
- specific proteolytic cleavages
- Assembly of multimeric proteins
what will happen if something goes wrong in the modification stage?
• If something goes wrong in the modification - unassembled and misfolded proteins are retained in the ER and exported back into the cytosol where they are degraded.
what is the CFTR?
• ABC transporter-class chloride channel in epithelial cell plasma membranes.
what is the mutation that causes cystic fibrosis?
(ΔF508)
DELETION of three nucleotides, which causes the loss of phenylalanine (Phe)
therefore the CFTR does not fold properly and is degraded in the ER
-
what happens as the protein passes form the ER to the golgi apparatus?
- as proteins move through the golgi apparatus the carbohydrate structure is modified
- some sugars are added and some removed
- The enzymes that carry out these modifications are packaged very specifically in different stacks
- In the trans Golgi network the proteins are sorted into vesicles, which carry them to the different destinations.
what are the functions of glycosylation?
- folding
- protection
- receptors
- recognition
what is compartment identity?
All the different membrane-bound intracellular compartments have a distinct protein and lipid content. Proteins make sure that they get to the correct compartment using different signals
- targeting signals
- retention signals
- retrieval signals
what is an important example of how carbohydrates are important in certain types of sorting events?
Lysosomal Enzymes.
what types of endocytosis are there?
- Receptor-mediated endocytosis - substances bind to specific receptors and begins to form a vesicle which has a protein coat around it.
- pinocytosis (fluid drinking)
- Macropinocytosis/Phagocytosis
can take up large particles such as bacteria.
what might happen to endocytosed material?
- first goes to early endosome
- Recycling - the material could be recycled and sent back to the plasma membrane
- Degradation
- Transcytosis - material can be carried to the basolateral membrane where it can be moved across the epithelial monolayer.
how does degradation take place?
what is a common example LDLs
- LDLs carry lipids and cholesterol to cells
- The protein component of LDLs are recognised by specific receptors on the cell surface, these accumulate in pits covered by the protein coat clathrin
- The protein pits invaginate and pinch off to form clathrin-coated vesicles
- The Clathrin coating then falls off to form an uncoated vesicle which can then fuse with the early endosome
- The LDL from the early endosome is then transferred to the lysosome
- here it is broken down to produce free cholestrol
what is a disease of endocytosis?
- Caused by mutations in LDL receptor
Familial Hypercholesterolaemia
