B: ER, Trafficking, Co-translational Import (19) Flashcards
What is the ER
Endoplasmic reticulum
What is the structure of the ER
Mosaic structure with many cell types inside
What is found in the rough ER
Associated with ribosomes
Many proteins, including those destined for secretion are synthesized by ribosomes associated with the rough ER
What is found in the smooth ER
Lacks ribosomes
Primary site of lipid synthesis
Does the RER or SER have ribosomes
Rough ER
What are the functions of the smooth ER (4)
- Lipid synthesis
- Production steroid hormones
- Detoxification
- Sequestration
What steroid hormones are produced in the SER
Ex. glucocorticoids, androgens and estrogens - endocrine cells
How does detoxification occur in the SER
Liver cells contain enzymes that modify foreign compounds
What sequestration occurrs in the SER
Storage of Ca
Why does the cell need to put energy into controlling Calcium levels
Ca2+ does not bind water very well, which will precipitate phosphates and make proteins insoluble
How does the cell control Ca levels
Calcium in the cytosol is either bond by a range of Ca2+ binding proteins, forced out through pumps and transporters or sequestered into specific organelles
Often referred to as Ca2+ sinks or pools
What are the 4 functions of the rough ER
- Synthesis of membrane phospholipids
- Glycosylation of proteins
- Protein folding - quality control
- Protein synthesis, modification and transport
Explain protein folding within the RER
Involve activity of molecular chaperones, specific types of protein that assist in folding process
Where are proteins targeted to in the RER (3)
Proteins targeted to ER
Proteins targeted to other endomembrane compartments (soluble and transmembrane)
Proteins targeted to plasma membrane (secreted and transmembrane)
What do ribosomes synthesize in the cytoplasm?
In the cytoplasm, ribosomes synthesize polypeptides from mRNA
What are the 4 components of protein synthesis
- Searching for start codon
- Beginning of elongation
- Elongation
- Termination
Explain the first step of protein synthesis:
Searching for start codon
Initiation factors recruit the small ribosomal subunit and tRNA and scan the mRNA for a start codon
Explain the second step of protein synthesis: Beginning of elongation
Once the complex reaches AUG, the large ribosomal subunit joins
The initiation factor is released
A tRNA complementary to the next codon binds to the A site
Explain the third step of protein synthesis:
Elongation
A reaction transfers the Met to the AA on the tRNA in the A site, forming a peptide bond
The ribosome moves down 1 codon, which puts the AA carrying the polypeptide into the P site and the now-uncharged tRNA into the E site where it is ejected
A new tRNA complementary to the next codon binds to the A site
What are free ribosomes
Ribosomes not associated with the ER
Where does all protein translation begin
All protein translation begins on free ribosomes
Where is protein translation completed
Translation is completed with free or ER bound ribosomes
What type of proteins are translated with free ribosomes
Cytosolic proteins
Peripheral membrane proteins
Proteins targeted to nucleus, mitochondria, peroxisomes, chloroplasts
What type of proteins are translated with ER bound ribosomes
Secreted proteins
Integral membrane proteins
Soluble proteins associated with inside (lumen) of endomembrane system
What is a signal sequence
Signal sequence is:
- Located in amino-terminus (N-terminus)
- Contains several consecutive hydrophobic AAs
- Directs synthesis to the ER compartment
What does a signal sequence do
Moves ribosomes to the ER to target protien
What happens to ribosomes once they are in the ER
Ribosomes form protein
Protein moves through channel into ER for cotranslational import
What is cotranslational protein import
The transport of a polypeptide chain occurs while it is being synthesized on a ribosome bound to the channel
What are the 4 steps of cotranslational protein import
- After translation of signal sequence, a signal recognition particle (SRP) binds to signal sequence and stops the translation process
- SRP binds to SRP receptor to target the whole translation complex to ER
- SRP is released and the ribosome binds to translocon. Once this is done, protein synthesis resumes
- Polypeptide enters the ER (through the translocon) as it is translated. In the end, the signal peptide is cleaved off and the chaperone folds the protein
How are proteins targeted to the mitochondria
For mitochondria, the TOM complex is the equivalent of SRP complex and translocon
What are the 2 endocytic pathways of protein sorting
- It is retained in the ER lumen, if that is where it functions
- It is transported from the ER to the golgi complex for further meditation and delivered to distal paths of biosynthetic/secretory pathways.
