Week 26 / Protein Sorting and Secretion 1 Flashcards
(21 cards)
Q: What is the process of protein generation? [4]
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]
[what does the protein lack?]
[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, Protein 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 are they generated? 0
what do they target 0
]
[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? [7]
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.
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:
1. 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?
-
Proteins Enter ER
– New proteins go into the ER to fold and get sugar tags (glycosylation). -
Move to Golgi
– Vesicles carry proteins from the ER to the Golgi. -
Check for Mistakes
– Misfolded or ER proteins go back to the ER. -
Golgi Processing
– Proteins are modified as they move through the Golgi. -
Golgi Cleanup
– Golgi-specific proteins are sent back to earlier Golgi sections. -
Sorting at the TGN
– Proteins are sent to:- The cell surface (for secretion), or
- The lysosome (for breakdown).
-
Incoming Vesicles
– Materials from outside the cell enter via vesicles, fuse with lysosomes, and get broken down or recycled.
A:
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.
Sure! Here’s a simplified version of the steps in Protein Processing & Trafficking:
