Lecture 16 Flashcards
CompartmentaLization
Only in eukaryotes
Why is it imp
Makes the function much more eefficient
What is not a part of EM and why ?
Not physically linked through vesicles or interconnected / fused
Don’t perform synthesis and modification of protein
Peroxisome - origin not known and is not part of the endomembrane systems
What is a lysosome
A digestive membrane
What is similar to a lysosome
Peroxisome - because it contains a peroxidase enzyme
What is the overall function of EM
Synthesis, sorting and secretion pf proteins and small molecules to their appropriate location in the cell
Golgi specifically modifies proteins and lipids
List the structures that are part of EM system
Nuclear envelop ER glogi complex Transport vesicle Endosome Lysosome Vacuole - not double membrane
Name the membrane enclosed organelles
Cytosol Nucleus ER GOlgi apparatus Lysosome Endosomes Mitochondria Chloroplast Peroxisomes
What is the function of cytosol
Contains many metabolic pathways
Eg- Like protein synthesis , the cytoskeleton
What is the function of nucleus
Contains main genome
DNA and RNA synthesis occurs here
Function of endoplasmic reticulum
Synthesis of lipids
Synthesis of proteins
Function of Golgi apparatus
Further Modification , sorting, packaging of proteins and lipids
SPECIFICALLY
- sorts proteins for export to other secretory pathway organelles or outside of the cell
- to Make Glycoproteins- add sugar to proteins
- make complex carbohydrates to be exported from the cell ( especially in plants )
Function of lysosome
Contains digestive enzymes that degrades intracellular
Endosomes
Sorting of endocytose material
Mitochondria
ATP synthase by oxidative phosphorylation
Function of chloroplasts
ATP synthase and carbon fixation by photosynthesis
Peroxisome
Oxidative breakdown of toxic molecules
Note , where does ATP synthesis occurs ?
Chloroplast
And
Mitochondria
Two types of ER
Smooth ER
rough ER
Rough ER vs Smooth ER
Rough ER
Flattened sacs (cisternae) + internal space (lumen)
Associated with nuclear membrane
Has ribosome on the SURFACE
Ribosome on ER is the site of protein synthesis
Synthesizes only a small amount of lipid
Smooth ER
very curved and tubular membrane spaces
Extends throughout the cytoplasm not just with nucleus
No ribosomes, and main function is the synthesis of lipids ( eg steroids) for making specially membranes
Ca stored in muscle cells - sarcoplasmic reticulum (SER)
Similarities
Found in both plant and animal cells
Where does protein synthesis occurs ?
Rough ER - on non free ribosome , ( 1/3 of protein synthesis )
These proteins have to go through the endoplasmic membrane
- secreted proteins
- integral membrane proteins
- soluble proteins targeted to ER
-goes to organelles other then chloroplast and mitochondria.
Cytosol- free ribosome. - (2/3 OF protein synthesis )
THESE proteins are used up in the same cell for
- cytoplasmic proteins
- internal plasma membrane peripheral proteins
- nuclear proteins
- soluable mitochondrial and chloroplast proteins
Note - cytosol makes proteins that stay in the cell (cytosol) , however the proteins for:ed by ER can retain in cells or go out
Where do all protein translation begin? ⁉️
Free ribosomes in cytosol
Protein sorting mechanism
Some proteins are retained in the cytosol
The others ho through the sorting mechanism in by 3 pathways
1- cytosolic proteins move to the nucleus VIA NUCLEAR PORES
2- cytosolic proteins move to the ER, mitochondria, or chloroplast VIA MEMBRANE TRANSPORT
3- protein from the ER move to other part of endomembrane system ( or between ER spaces) VIA TRANSPORT VESICLES
Describe the pathway of the soluble and membrane proteins coming from the ER ?
ER contains 2 types of proteins
1- soluble proteins
2- membrane proteins
Soluble proteins sent to
1- some sent to golgi
2- some sent to plasma membrane
Membrane proteins
1- some proteins retained in the ER membrane
2- some sent to OTHER EM ORGANELLES or to the plasma membrane
How is a protein sorted/ placed into its appropriate target organelle?
There is a signal sequence on the N terminal that helps the protein know which compartment to go
When the ribosome translates the signal sequence ( this can be even bfr the completion of translation of proteins) the ribosome guides the protein to its destination( AGAIN this can occur while the protein is synthesized)
A protein without localization signal is retained in the cytosol
Protein synthesis occurs in 2 sites- free ribosomes and non free ribosomes , however, we learnt earlier that protein synthesis behind at the free ribosome. How does the protein synthesis end up occurring at the non - free ribosomes ?
Protein synthesis starts at the cytosol, if the mRNA doesn’t have any ER signal then the protein synthesis will remain ongoing in the cytosol
But if the n terminal of the mRNA hAs a localization signal for ER then it is directed towards the ER and the rest of protein synthesis occur there.
Note- polyribosome synthesis can occur at both sites
Explain the process from the mRNA translated in the cytosol to the mRNA ending in the ER lumen.
As soon as an ER signal is translated the SRP attaches to the mRNA and ribosome
The SRP guides the ribosome to the the ER
Once it reaches ER , the SRP binds to the SRP receptor on the ER membrane
This binding causes the release of SRP
The ribosome is then translocated from SRP receptor to a protein translocator
Protein synthesis resume at original speed and the protein translocator , and positions the protein across the lipid bilayer
The protein translocator binds to the signal sequence and passes the rest of the growing polypeptide into the lumen
At a point SIGNAL PEPTIDASE cuts the polypeptide from the signal sequence, and the signal sequence is ejected in the bilayer and is degraded.
Once the protein synthesis is completed , the TRANSLOCATED POLYPEPTIDE is released as a soluble protein into the ER lumen, and the protein translocator closes
What is the most extensive organelle
ER
Describe vesicles structure
Small blebs of lipid bilayer membrane surrounding an aqueous interior.
What can vesicles carry
- soluble proteins and small molecules in the interior
- integral membrane in the membrane of the vesicle
—— membrane in a membrane is how membrane of other organelles grow
There are two types of proteins are embedded into the membrane list them.
Also describe the function of start and stop signals
1- A single pass transmembrane protein embedded in the lipid bilayer
2- A double pass protein that has an internal ER signal sequence
Start and stop determines the arrangement of a transmembrane protein in the lipid bilayer
How is a protein embedded in the membrane- SINGLE PASS
There are 2 signalling sequence in the in the protein
-The Start sequence at N terminal ER signal
-And a hydrophobic stop transfer sequence
The N sequence is attached to the protein translocator and initiates the TRANSFER .
The protein translocator positions the polypeptide across the bilipid layer until the hydrophobic signal is reached
The N terminal signal is cleaved by signal peptidase and the growing polypeptide is discharged into the LIPID BILAYER
The N terminal signal stays in the lipid bilayer and is degraded
The polypeptide continues to grow on the cytosolic side and continues to discharge into the lipid bilayer
Note| the single pass transmembrane protein is retained in the lipid bilayer and doesn’t fall into the lumen , because it is supposed to be embedded in the lipid bilayer
How is protein embedded in the membrane -DOUBLE PASS
The double pass transmembrane protein has an internal ER signal while the single pass had an N terminal ER sequence
The same process of bringing the Ribosome to ER occurs involving a SRP , however this time it attaches to the internal hydrophobic ER signal sequence ( as there is no N terminal ER signal sequence )
The internal ER sequence acts as 1- start transfer signal and 2- anchors the final protein in the membrane
The protein translocator attaches to the hydrophobic ER signal sequence and positions the rest of the polypeptide across the lipid bilayer until hydrophobic sequence is reached
When a stop sequence is recognized, both sequences , the growing COOH and the N terminal sequence are released into the Lipid bilayer .
No sequence is cleaved off and the entire polypeptide remains anchored to the LIPID BILAYER
Note - both the internal and stop transferase signal are hydrophobic
Describe the Polarity and structure of integral membrane proteins.
Cytosol side of membrane proteins - + charged
ER LUMEN OF PROTEIN- - charged
This polarity is present when being synthesized
The integral membrane is made of alpha helices
The amino acid in the centre of protein is are hydrophobic
And the charged amino acid can interact with the hydrophilic
Types of protein modification that occurs in the ER
They three types of protein modification are
1- disulphides bond formation
2- glycosylation
3- protein folding
What do the modification do?
1- disulphide bridges - stabilizes the structure of secretory proteins
2- glycosylation- short branched ovligosacharides are added from a lipid to protein by an enzyme called GLYCOSYL TRANSFERASE
This helps
- protection from degradation
- cell cell recognition
- facilitate protein folding
3- protein folding - chaperones bind and alter the folding of newly forming proteins
Folding is required before proteins carry out their function
Where do glycosylation occurs ?
Golgi and ER
What happens to modified proteins⁉️
Leave the ER - can be retained within the cell or can be secreted outside
OR
Retain within the ER if misfolded or if used as membrane proteins
How is the exit at ER controlled to ensure protein quality ?
By ensuring
- if a protein has ER retention signal ( recognized by ER membrane receptors and retained)
- IF a secretory protein is folded properly
- If they are misfolded CHAPERONES bind to it and retain them in ER until they are folded properly
If they still dont fold properly they are destroyed
What is important in the folding of proteins and why ?
Correct tertiary state - native state - for functioning properly
Proteins can be misfolded due to
Heat
Chemicals such as urea
What is UPR and when is it activated?
When misfolded protein are retained in theER
They accumulate in ER
The accumulation activiate an unfolding protein - UPR
UPR causes the activation of genes that increase protein folding capacity
What are the 3 main factors involved in UPR
1- IREI
2- PERK
3- ATF6
These 3 factors are required for activation of UPR
Translocation also cones to a haul to avoid accumulation of misfolded proteins .. T F
T
Where does most modification occur
In the ER
