Endoplasmic Reticulum (Lec. 12) Flashcards
Describe the ER as a whole.
Network of membrane-enclosed tubes and flattened sacks called cisternae; largest organelle in most eukaryotic cells. Has two contiguous domains (smooth and rough) that perform different functions.
Describe the smooth ER and its functions
Has no ribosomes; major site of phospholipid and cholesterol synthesis (abundant in cells with active lipid metabolism).
Describe the synthesis of lipids in the smooth ER.
Most lipids are synthesized in the ER membrane (cannot be synthesized in an aqueous environment because they’re lipids); they’re made in the cytosolic portion of the ER bilayer
What are flipases?
enzymes that help phospholipids “flip” from the cytosolic layer to the lumenal layer
Describe the structure and function of the rough ER.
Covered by ribosomes on the outer surface; synthesizes proteins for membrane-bound + lumenal proteins, or proteins that need to leave the cell; folds proteins and ensures incorrectly folded ones are removed; packages exported proteins into vesicles; adds glycosylation and glycolipid anchors
What does glycosylation do?
Carries information, like a label on a package
What is the difference between the cytosol and the lumen?
The cytosol is contiguous with the inside of the cell (nucleus and cytosol). The lumen is contiguous with the outside of the cell (ER, Golgi, inside of vesicles, outside cell membrane).
How does protein targeting and cell topology work?
Proteins are made in one location depending on where they will end up. Domains of membrane proteins that are exposed on the cell surface correspond to regions of polypeptide chains that are exposed to the ER and Golgi lumen.
What are the two different methods of protein translation?
1) Translation on free ribosomes: proteins are free to move to nucleus + cytosol (sometimes also mitochondria, chloroplasts, and peroxisomes).
2) Translation on ER-bound ribosomes: proteins will end up inside the ER, so it can only go where it’s continuous with the ER
How do proteins get in the ER?
1) “Cotranslation Translocation” - ribosomes bind to ER while they’re translating the protein, so it actively “dumps” the protein into the ER lumen.
2) “Post-Translation Relocation” - free ribosomes translate proteins, which are then transported to the ER lumen afterwards.
How does a free ribosome know when to associate with the ER membrane to complete cotranslation relocation?
It’s in the mRNA they’re translating; as the amino acid polypeptide chain is being translated, it contains a segment (the signaling sequence) that signals that this protein needs to be translated with the ER, so the ribosome must relocate.
How does cotranslation work?
1) A signal recognition particle (SRP) binds to the signal sequence on the polypeptide chain.
2) The SRP protein binds to an SRP receptor that is attached to a translocon.
3) The ribosome binds to the translocon (causes translocon to open), which causes the SRP protein to unbind to the SRP receptor.
4) Translation resumes. Signal peptidase cleaves the signal sequence off the growing polypeptide chain.
5) Continued translation causes the growing polypeptide chain to grow into the lumen of the ER.
6) The finished product is “spit out” into the lumen of the ER.
What is BiP and what does it do?
It’s a chaperone protein in the lumen of the ER that binds to the newly translated polypeptide chain and aids in the folding process. Chaperone proteins do not unbind until the protein is properly folded. BiP stays in the ER lumen.
What is protein glycosylation?
Adding sugars to proteins. Helps to prevent protein aggregation, promotes protein folding, and helps with protein sorting.
What are lipid anchors and how are they added?
The protein is held in the ER membrane by a C-terminal hydrophobic sequence. Anchors are assembled in the ER, and the C-terminal sequence is cleaved and exchanged for a lipid anchor.
What does a chaperone protein do with a misfolded protein? What is this process called?
It sends the protein to the ER membrane. The protein then binds to a ubiquitin-ligase complex, gets tagged with ubiquitin, and is then sent off into the cytosol to be broken down by a proteasome. This process is called ERAD (ER-Associated Degradation).
What happens if too many unfolded proteins accumulate in the ER? What happens if cells can’t do the unfolded protein response?
They’ll bind to receptors on the ER membrane, which initiate pathways that lead to increased production of chaperones + proteasomes, and limit the number of new unfolded proteins entering the ER. If cells can’t do this, they’ll undergo apoptosis.
What if we’re trying to make an integral protein that needs to get inserted into the membrane?
We know that integral proteins have 20-25 consecutive hydrophobic amino acids, which serve as the transmembrane domain. When a ribosome translates a polypeptide chain of 20-25 consecutive hydrophobic amino acids, known as the transmembrane sequence, the translocon reads the sequence and inserts that segment into the membrane.
What, for an integral protein, is a functional alternative to the signal sequence to start translation?
The transmembrane sequence can act as the signal sequence.
What happens when a protein has multiple transmembrane domains?
All transmembrane sequences will be read by the translocon and inserted into the membrane.
How is post-translation relocation different?
The signal sequence is at the end, not the beginning.
What is the general order of protein movement after synthesis in the ER?
ER to ER-Golgi Intermediate Compartment (ERGIC) to Golgi to exocytosis, membrane insertion, or lumenal aspects of lysosomes.
