Cell Signaling Flashcards
Where do proteins made on free ribosomes go?
Mitochondria, peroxisomes, nucleus, and cytoplasm
Where do proteins made on membrane bound ribosomes go?
ER, Golgi, endosomes, lysosomes, nuclear membrane, plasma membrane and secretion
What determines where a protein goes after synthesis?
The amino acid sequence
What is the amino acid sequence for protein retention in ER
KDEL
What do proteins with a signal get sent?
Where do proteins with no signal get sent?
Appropriate organelles
Cytosol
What are the two types of was to processes translation?
Co-translational processing (translocation during protein synthesis)
Post-translational processing (translocation after synthesis)
What are the two major components that need to happen for Co-translational processing into the ER lumen?
Signal recognition particle (SRP) binds to the ER signal sequence on the polypeptide chain and then once connected to the polypeptide chain the SRP needs to bind with the SRP receptor on the membrane
What is the mechanism for co-translational processing into the ER lumen?
SRP recognizes the ER signal sequence (on polypeptide chain) and then it binds to the SRP receptor. The complex is then passed to ER translocation complex (translocon) and the SRP is released. The Poly peptide chain is then inserted into the membrane channel. Protein synthesis proceeds and it drives the threading of the polypeptide through the membrane channel. Once the protein is fully synthesized it is released into the lumen. TRANSLOCATION IS DRIVEN BY PROTEIN SYNTHESIS.
Post-translational processing into ER lumen
Following synthesis on free ribosomes the polypeptide chain is left in its unfolded state and transported by Hsp70 and Hsp40 chaperones. NO SRP IS REQUIRED. The polypeptide is recognized by sec complex (sec62/63) associated with translocon (channel that spans both sides of the ER membrane). Bip (a chaperone) pulls the polypeptide chain into the ER. TRANSLOCATION IS DRIVEN BY BIP.
How is the ER membrane targeted?
Similar to co-translational processing. The start transfer sequence of the polypeptide chain. 20-25 hydrophobic amino acids and the N-terminal initiates translocation. The stop-transfer sequence is 20-25 hydrophobic amino acids, stops the transfer process into the lumen and the translocation channel opens up and releases the growing polypeptide chain into the ER membrane. IT IS A TRANSMEMBRANE PROTEIN SO HALF (FROM N TERMINUS TO THE STOP-TRANSFER SEQUENCE ARE INSIDE THE ER LUMEN AND THE OTHER HALF (ENDING WITH THE CARBOXY TERMINAL) ARE OUTSIDE IN THE CYTOSOL). The protein is essentially stuck in the ER membrane
How does protein folding in the ER work?`
ER chaperone proteins (Calnexin in the ER lumen and Calreticulin in the ER membrane) direct proper folding, retain proteins in the ER and they prevent aggregating.
What happens when a protein in the ER becomes unfolded?
ERAD: ER associated degradation -> misfolded proteins are identified, the chaperone proteins try to refold them and if that does not work they are distributed to the cytoplasm and degraded by ubiquitin proteasome and the left over material is recycled into the cell.
How does the ER export things
ER exit site (ERES)-> specialized region for vesicle budding that contains ER export signals.
There is a bi directional highway where the cars are vesicles between the ER and the golgi. There is anterograde which is ER -> Golgi via COPII and then retrograde which is Golgi -> ER via COPI
What are the two ER retrevial signals
There is transmembrane proteins that combine with proteins inside the lumen (of the ER) and then bud off at a ER exit site (ERES). There is an intermediate area between the ER and Golgi called ER-Golgi intermediate compartment (ERGIC). COPII vesicles transport things from the ER to the ERGIC and if something needs to come back (retrograde) then the COPI returns it to the ER). From the ERGIC it then goes to the Golgi using the KKXX retrevial signal or if the Golgi wants to send something to the ER it uses the KDEL signal.
What are the compartments of the Golgi apparatus?
Cis- modification/sorting: receives ER proteins, modification of proteins, lipids, and polysaccharides is initiated. N-linked sugars are trimmed or added.
Medial- Additional modification: N-linked oligosaccharides are modified further
Trans- Modification/sorting: vesicles are separated and sorted
What are the types of vesicles and transport?
Transport-move molecules inside of cell
Secretory- move molecules out of the cell
Intracellular transport pathway: movement of cargo between cellular compartment
Secretory pathway: outward movement of cargo (exocytosis) sends molecules out of the cell
Endocytic pathway: inward movement of cargo (endocytosis) ingestion of extra cellular molecules.
How does vesicle budding work?
When vesicles form they use adaptin which binds to cargo receptor which recruits the clathrin coat, CLATHRIN DRIVES BUDDING! Dynamin (a GTP-binding protein) pinches off the clathrin coat. Clathrin eventually uncoats, allowing vesicles to fuse with target.
What are the terms used for sending and delivery
Budding and fusion
What are Rab proteins?
They are like a shipping label for the vesicles, it gives their molecular address.
The vesicle is transported by motor proteins, target is established by rab protiens and the address is recognized by the tethering proteins. SNARE’s provide additional specificity (v- and t- snares)
How does lysosomes form?
Lysosomal acid hydrolases Transported to endosomes from the trans golgi Clathrin coated vesicles fuse with late endosomes Acidic environment (pH~5.5) causes hydrolases to dissociate from Mannose-6-phosphate receptor. As the hydrolases accumulate the endosome matures into lysosome.
How does the golgi transport things to the plasma membrane?
Direct transport: secretion: exocytosis which is incorporated into the membrane of the PM
For hormones the golgi uses immature secretory granules that mature and store the hormone until an enviornmental signal tells them to release
How does endocytosis happen? Phagocytosis and receptor mediated
LDL (cholesterol, hydrophobic) attaches to LDL receptors on the cell surface and then budding occurs, clathrin lines the PM on the cytosol side so when budding happens from the outside the vesicle is surrounded by clathrin coating. As it arrives at the endosome it is uncoated and then it fuses with the endosome, releasing the LDL. The endosome will then deliver LDL to the lysosome were the hydrolyitic enzymes break it down and release free cholesterol.
This process occurs with different things such as metabolites, hormones, proteins and viruses but the process is the same
1. Ligand binds receptor (internalized via clathrin coating)
2.options: recycling (endosomes)or digestion (lysosomes)
how does nuclear import occur?
The nucleus is surrounded by a double membrane with pores in it that act as a gate to selectively mediate import/export.
Selective transport: large molecules & macromolecule complexes, export of RNA molecules and ribosome subunits and import of nascent nuclear proteins.
Nuclear localization signal: imported by nuclear receptors, interact with nuclear pore.
Importin: recognizes NLS on protein and uses that to import protein into the nucleus. Ran-GTP complex binds to importin which displaced protein, ran hydrolysis GTP to make GDP. Ran-GDP then releases importin and importin binds protein with NLS.
How are proteins imported into the mitochondria?
Mitochondria is double membrane. A precurser protein (with a signal sequence at the n terminus) binds to import receptors and begins being fed thru a transmembrane located on the outer membrane of the mitochondria and it is then fed through a transmembrane receptor in the inner membrane of the mitochondria, once fully translocated and inside hsp chaperones refold the protein and the signal peptide is cleaved by pepstidases.
