Week 26 / Protein Sorting and Secretion 1 Flashcards
Q: What is the process of protein generation?
A: The process of protein generation involves the following steps:
Transcription: DNA is transcribed into RNA in the nucleus.
RNA splicing: Introns are removed, and exons are joined together to form mature mRNA.
Export: The mature mRNA is exported from the nucleus to the cytoplasm.
Translation: In the cytoplasm, mRNA is translated into a protein by ribosomes.
Q: What are exons and introns?
A: Exons are coding regions of the gene that remain in the mRNA, while introns are non-coding regions that are removed during RNA splicing.
Q: What are endocytosis and exocytosis?
A: Endocytosis is the process where cells take in substances from the outside (IN), while exocytosis is the process where cells release substances to the outside (OUT).
Q: Where do proteins do their job in the cell?
A: Proteins are used in all parts of the cell, including the nucleus, cytosol, organelles, cell membrane, and even outside the cell.
Q: What is needed to move proteins to their site of action?
A: A very precise and regulated system is needed to move proteins to their correct site of action.
Q: What happens in the non-secretory pathway for protein sorting? [2]
[where is translation completed?]
[what happens if protein has an organelle-specific signaling sequence?]
A: In the non-secretory pathway:
If the protein lacks an ER signal,
translation is completed on free ribosomes in the cytosol (non-secreted cytosolic proteins).
If the protein has an organelle-specific signaling sequence, it is generated in the cytosol and then targeted to its functional site (nucleus, mitochondria, peroxisome).
Q: What happens in the secretory pathway for protein sorting?[4]
[what happens to proteins that are secreted?]
[where is translation completed?]
[where and how do secretory proteins pass ?]
[where are the proteins sorts and where are they sent after?]
A: In the secretory pathway (also used for membrane integration):
Proteins that are secreted are generated on cytosolic ribosomes and then target the ribosome to the ER (rough ER = ribosomes).
Translation is completed on the rough ER.
The secretory proteins pass through the rough ER to the Golgi complex via transport vesicles.
The Golgi complex sorts proteins to either the plasma membrane or lysosomes.
Q: Do all eukaryotic cells use the same secretory pathway?
A: Yes, all eukaryotic cells use essentially the same secretory pathway for synthesizing and sorting secreted proteins, proteins in the luminal space, and membrane proteins.
Q: What are some reasons for protein secretion?
A: Reasons for secretion include digestive enzymes, extracellular matrix, hormones, and cell-to-cell signaling.
Q: What are the three basic steps in protein secretion?
A: The three basic steps in protein secretion are:
Protein synthesis and translocation across the ER membrane.
Protein folding and modification inside the ER lumen.
Protein transport to the Golgi, lysosomes, or cell surface through budding and fusing of vesicles.
Q: What is the function of the N-terminal signal sequence in proteins?
A: The N-terminal signal sequence directs the ribosome to the endoplasmic reticulum (ER) for proteins that need to be sorted into a specific place in the cell, not just the cytosol.
Q: How is the ribosome directed to the ER?
A: The ribosome is directed to the ER by the Signal Recognition Particle (SRP) once it has generated the N-terminal signal sequence.
Q: What are the steps involved in protein secretion and co-translational translocation?
A: The steps involved are as follows:
- The signal sequence on mRNA is translated.
- The signal sequence is bound by the Signal Recognition Particle (SRP).
- The SRP targets the ribosome to the SRP-receptor on the ER
membrane, requiring GTP. - The opening of the translocon allows insertion of the signal sequence and growing peptide chain, with GTP hydrolysis releasing the signal sequence.
- The polypeptide chain passes through the translocon, where the signal sequence is cleaved by a membrane-bound peptidase enzyme and degraded in the ER lumen.
- The polypeptide is elongated from the N-terminal to the C-terminal direction while folding.
- The protein is released into the ER lumen, and the ribosome is released back into the cytosol.
Simplified Steps:
- Start Signal – mRNA is translated and the signal sequence appears.
- SRP Binds – Signal Recognition Particle (SRP) binds to the signal sequence.
- SRP Guides – SRP brings the ribosome to the ER membrane using GTP.
- Translocon Opens – GTP hydrolysis opens the translocon and starts insertion.
- Signal Cleaved – Signal sequence is cut off and degraded inside the ER.
- Chain Grows – Polypeptide elongates N → C terminal and folds.
- Finished – Protein enters ER lumen; ribosome returns to cytosol.
Mnemonic:
“Some Silly Scientists Try Snipping Chains Fast”
(Signal, SRP, SRP guides, Translocon, Signal cleaved, Chain elongates, Finished)
Q: What is the role of the ER in protein sorting?
A: The ER is responsible for sending proteins to the correct location inside or outside the cell.
Q: What are the steps involved in inserting proteins into membranes? [7]
A: The steps involved are as follows:
The ribosome (+ mRNA) attaches to the translocon on the ER membrane.
The protein is translated through the translocon until a hydrophobic stretch is generated and a membrane stop-transfer-anchor sequence is identified.
The hydrophobic stretch is left in the membrane.
The translocon moves laterally and ejects the protein into the membrane.
The rest of the protein is generated, which is external to the membrane.
The complex dissociates.
- Docking – Ribosome with mRNA binds to translocon on the ER membrane.
- Translation begins – Protein starts getting threaded through translocon.
- Hydrophobic stop – A hydrophobic region (stop-transfer) appears.
- Anchor – This hydrophobic stretch stays in the membrane.
- Side exit – Translocon opensideways, inserts protein into membrane.
- Remainder made – Rest of protein is made outside the membrane.
- Finish – Complex breaks apart (ribosome detaches).
Q: How is the orientation of a protein in the membrane established during insertion?
A: The orientation of a protein in the membrane is established when it is first inserted into the membrane, and this orientation persists throughout the entire process. The cytosolic side of the membrane remains on the cytosolic side.
Q: What is the role of the hydrophobic membrane crossing domain in protein insertion?
A: The hydrophobic membrane crossing domain serves as a ‘stop transfer’ signal, leaving the protein inserted in the ER membrane.
Q: What happens when membrane proteins are being translated?
A: As membrane proteins are being translated, they are translocated or transferred into the ER until a hydrophobic membrane crossing domain is encountered.
Q: What is the secretory pathway used for?
A: The secretory pathway is used to send soluble proteins and cell membrane-bound proteins to be delivered to the following locations:
Outside the cell (e.g., hormones, digestive enzymes)
Into the plasma membrane (e.g., receptors, channels)
To lysosomes (e.g., acid-tolerant digestive enzymes, H+ pumps)
Q: What is the key point about the secretory pathway?
A: The key point is that this pathway is used to send proteins out of the cell or to membranes.
Q: What are the steps involved in the transport of proteins through the secretory pathway?
A: The steps are as follows:
Newly generated proteins are translocated into the ER lumen for folding and glycosylation.
Vesicles fuse to form the cis-Golgi, and proteins that are misfolded or ER resident are sent back to the ER via retrograde vesicles.
Transport from the Golgi to the ER is done via retrograde vesicles.
Transport through the Golgi apparatus occurs through a process called cisternal maturation, involving restructuring of the Golgi complex and movement of enzymes.
Golgi-resident proteins are moved back to an earlier portion of the Golgi via retrograde vesicles.
Proteins destined for secretion (plasma membrane or lysosomes) are sent to the trans-Golgi network (TGN).
The TGN sends proteins to the cell membrane via secretory vesicles for regulated secretion.
The TGN also sends proteins to the lysosomes.
Vesicles coming into the cell are sent to the lysosome where their contents will either be degraded or sent back via the TGN to the cell membrane.
Simplified Steps: Protein Processing & Trafficking
- ER Entry – New proteins enter the ER lumen for folding and glycosylation.
- To Golgi – Vesicles carry proteins from ER to the cis-Golgi.
- Quality Control – Misfolded proteins or ER-resident ones are sent back to ER via retrograde vesicles.
- Golgi Processing – Proteins move through Golgi via cisternal maturation (the Golgi reshapes as proteins pass).
- Golgi Cleanup – Golgi-resident proteins are returned to earlier cisternae via retrograde vesicles.
- TGN Sorting – At the trans-Golgi network (TGN), proteins are sorted:
To plasma membrane for secretion
To lysosomes for degradation
- Incoming Vesicles – Vesicles from outside the cell fuse with lysosomes; contents are degraded or recycled back to the membrane via TGN.
