Pathways of protein transport (2) Flashcards
What kind of “check” is done on vesicles?
As they travel from one compartment to another they are being monitored for any misfolding
-> in such case the protein must be degraded
Since degraded proteins seem to be majority the cell clearly favours quality over quantity and economy
Most proteins that enter ER are chemically modified there - how?
For instance, disulfide bonds form between pairs of cysteine chains side chains by the means of ER enzyme’s catalysis
-> helps to stabilize the protein against degradative enzymes and changes in pH outside (+helps with proper folding)
Fill in: Many of the proteins that enter ER lumen and ER membrane are converted to…?
What are their functions?
Glycoproteins
- created by covalent attachement of short brainched oligosaccharide side chains (multiple sugars) = Glycosylation
- NOTE: onlt very few proteins in cytosol go through this process
- Functions:
- Protect the protein againsts degradation
- Hold the protein in ER until correctly folded
- Serve as a transport signal for cargo receptors
- Additionally, when displayed on the surface of the cell -> provides signalling between cells
How does Glycosylation take place?
- Instead of attaching one sugar at a time it binds multitude (14 sugars) to any protein carrying the site of glycosylation
- Initially, oligosacharides are bound to a specialed lipid in ER membrane = Dolichol
-> once protein translocation takes place in its vacinity it can start interacting with the emerging protein
-> binds to aparagine NH2 group
- reaction catalyzed by olygosaccharyl transferase which pertrudes the ER lumen only (doesn’t have access to cytosol -> glycosylation won’t take place there)
NOTE: The process of creating glycoproteins begins in ER (this is just the first step) -> continues in Golgi apparatus
Most proteins leave ER as they are destined to function at different sites -> what happens to the rest?
Some proteins are meant to stay in the ER (or be returned to ER if they escape)
- those are recognized by specialized receptors in ER or Golgi membrane by theit C-terminal sequence of amino acids = ER retention signal
What is meant by the following claim: “Exit from the ER is highly selective”?
Only proteins that are correctly folded will be let out of the ER further
- Proteins that are NOT assembled properly will be retained in ER by binding to Chaperone proteins -> they prevent misfolded proteins from aggregating and steer them towards the correct folding
- In some cases it doesn’t help -> the wrong proteins are discarded into cytosol and degraded
Is there a case in which this selective mechanism could be harmful to the organism?
Yes, for instance if one develops cystic fibrosis
- marked by production of plasma-membrane transport protein that is mutated to the point of slight misfolding -> although the mutation isn’t that severe and the protein could in theory function as a regular Cl- channel if it reached the plasma membrane => however, that never happens as it gets captured by this strict ER process and discarded
=> NOTE: Chloride ions cannot leave the membrane and cannot bring water and salts with them -> mucus on the other side gets dehydrated -> thickens in mucus filled areas e.g. lungs
What happens if there is too much misfolding happening?
The misfolded proteins keep on accumulating in the ER -> if the build up is large enough it can trigger the Unfolded Protein Response (UPR)
-> program promts the cell to develop new ER with more chaperone proteins
Is there a case in which even UPR cannot help?
Yes, it may happen that even within UPR the ER gets overwhelmed
For instance, in adult onset diabetes - tissue becomes gradually less sensitive to insulin -> pancreas tries to compensate this by making its cells produce more insulin -> at one point it too much for their ER to handle => apoptosis
-> that only increases demands on the remaining insulin-secreting cells => further progression of the disease
What is Golgi apparatus?
= organelle located near the cell nucleus, that consists of collection of flattened, membrane-enclosed sacs = cisternae
- Each stack contains about 3-20 cisternae
- number of stacks varies among cells, some have just one larger while other multitude of small
Fill in: “Each Golgi stack has 2….?
Two distinct faces
1. Cis face = where proteins enter
- close to ER
- either move through or are returned to ER if containing the ER retention signal
-> moving ensured by transport vesicles that bud from one cisterna and fuse to the next
2. Trans face = exit
- proteins either head towards the cell surface or are directed to other organelles e.g. lysosomes
- At both faces the outermost cisterna is connected to a network of intraconnected membrane tubes and vesicles
What does Golgi apparatus do with the proteins?
It aids with further modification
- For instance it may execute highly ordered process in which it adds sugars to oligosaccharide chains coming from ER
- enzymes acting on the chain early will be in cys face while later in trans face
What happens within the Constitutive exocytosis pathway?
- Lipids and proteins are transported from the Golgi apparatus to fuse with the plasma membrane via exocytosis
- allows membrane to grow, ad or replace lipids and proteins within
- secretion = releasing the lipids and proteins outside the cell
-> either stays at the surface of the cell
-> incorporate into the extracellular matrix
-> or diffuse into the extracellular fluid.
How is the Regulated secretory pathway different?
As opposed to the previous that occurs continually in all cells, this pathway operated only in cells specialized for secretion e.g. neurotransmitters, hormones, mucus
They utilize secretory vesicles that are send from the trans Golgi face towards the membrane where they accumulate at “active” sites -> upon a specific signal they can initiate fusion
How do secretory vesicles differ from vesicles of the constitutive pathway?
Proteins meant for secreation have special surface protperties that cause them to aggregate with one another under ionic conditions (acidic pH and high concentration of Ca2+) -> these aggregations are packadged into secretory vesicles
The rest of the proteins do NOT aggregate and are just automatically carried in transport vesicles
=> If we aggregate proteins first the vesicles can pack in high concentration of the molecules -> secretory vesicles can release extra large amount of a particular protein
If vesicles tend to fuse with the membrane -> how come the membrane doen’t keep on growing?
Although the membrane can grow via these means it usually controls its size with endocytosis
= removal of lipids and proteins back into vesicular structure and back to the Golgi apparatus (happening almost just as quickly as exocytoses)
What was again the function of Endocytic pathway?
= route by which the cell takes up fluid and molecules from their surroundings via endocytosis
- by budding inward and pinching off endocytic vesicles are created -> carried to endosomes
-> can be recycled to plasma membrane
-> or sent to lysosome for ingestion
What are two main typed of endocytosis? Where do we observe it?
- Pinocytosis (“cellular drinking”) = ingestion of fluids and molecules via small pinocytic vesicles
- performed by all cells continuously (but balance maintained as there is just as much exocytosis happening)
- performed by clathrin-coated vesicles
- Phagocytosis (“cellular eating”) = ingestion of large particles e.g. cell debri, microorganisms
- mainly specialized cells = phagocytic cells
Where has phagocytosis been observed initially -> do we find it in human cells as well?
First observed in protozoa - used as a form of feeding in which the unicellular eukaryot takes up large particles into phagosomes -> fuse with lysosomes for digestion
In animals it doesn’t occure very often e.g. even epithelial absorbant cells in the gut must let the food be broken down by extracellular enzymes before any aborption can be done
If we don’t use phagocytic cells for nutrition = why do we need them?
They also serve defending function: macrophages and neutrophils. which are part of the immune system, can digest cellular debri BUT also microorganisms
- Phagocytic cells tend to bind to antibodies on the surface of the bacterium -> expand sheet-like structure = pseudopods -> engulf the dangerous agent -> fuse tips to create phagosome -> fuses with lysosome and destroyes the microbe
Is there a case in which phacytosis may be compromised?
Yes. The bacterium responsible for tuberculosis (Mycobacterium tuberculosis) -> it can inhibit the fusion of phasome with lysosome -> won’t get ingested and it can multiple within the cell
What is meant by receptor-mediated endocytosis?
Normally, pinocytosis is indiscriminate - it just takes up all molecules that happen to be in the close vacinity
BUT sometimes the cell requires higher concentration of a particular molecule -> in those cases the molecules bind to specific receptors at the surface of the cells -> pinocytosis of that site takes in the receptor-macromolecule complexes in
What well-known macromolecule makes use of the receptor-mediated endocytosis?
Cholesterol = lipid insoluble in water
- transport in bloodstream in a complex with a protein = low density lipoproteins (LDL)
- the whole complex gets secreted by liver -> bind to receptors on cells -> receptor-mediated endocytosis -> delivered to endosomes which has more acidic inside that is the cytosol -> LDL dissociates from the receptor (gets returned via transport vesicle back to the membrane) -> transported to lysosome -> broken down by enzymes -> cholesterol released into the cytosol (ready to be used in membrane sythesis)
What other molecules can be taken up this way?
- Essential metabolites e.g. vitamine B12, iron, which are used for synthesis of hemoglobin
