Module 2 - The endomembrane system Flashcards
How are proteins refolded after entering the ER?
Chaperone proteins fold the proteins again
BiP binds to hydrophobic residues
Calnexin binds N-glycosylated proteins
How can proteins be modified after entering the ER?
Signal sequence clevage
Disulfide bridge formation
N-terminal glycosylation
What is the function of most proteins entering the ER?
Some will function in the ER but most will be moved around for use in other subcellular locations or secreted out of the cell
Vesicular transport with the ER: how does it work?
The membrane buds around the cargo and the vesicle moves to the target location before fusing with the target location
The formation that the bud has (ie any proteins sticking out of the vesicle) will remain after fusing with the target (proteins that were facing the cytosol will still face the cytosol after fusing)
The secretory pathway
Allows material moving outwards (ER-Golgi-Plasma membrane-endo/lysosome)
The endocytic pathway
Allows material to move inwards (plasma membranes-endosomes-lysosomes)
What are protein coats?
Coats that are formed by proteins
Adaptin binds to the cargo receptor and clathrin binds to this which forms a protein coat.
This coat helps pull the vesicle off and dynamic (GTPase) pinches the bud off
COPI, COPII, and Clathrin: what movements do they facilitate?
COPII -> ER -> Golgi and rest of cell
COPI -> Golgi to ER
Clathrin -> Outer cell -> inner cell organelles
Rab proteins: what do they do?
Bind to the vesicle and interact with a tethering protein on the target membrane and, along with t-snares which bind with v-snares, they pull the vesicle to the target membrane and fusion occurs
V-snares: what do they do?
Bind to the vesicle and interact with a t-snare on the target membrane and, along with Rab proteins which bind with tethering proteins, they pull the vesicle to the target membrane and fusion occurs
Fusion between vesicles and target membrane: how close do they have to be for fusion to occur, is it energetically favourable, and is another signal needed?
Bilayers must be brought within 1.5nm for lipids to mix (fuse)
Water must be displaced => energetically unfavourable
Fusion may require an additional signal
KDEL sequences: what are they and what do they do?
Sequence (LysAspGluLeu) at the C-terminus of soluble proteins recognised by KDEL receptor in Golgi
Recruited into COPI vesicles that return the protein to the ER
What is the signalling sequence for the Golgi apparatus?
18 amino acids (unusually short)
What is the signalling sequence for the lysosomes?
Addition of Mannose-6-Phosphate to N-linked glycans of some glycoprotein
What is the Golgi apparatus and what does it do?
Sacs of cisternae located near the nucleus
Control the movement of proteins from the ER (cis) into the Golgi body and then out of the Golgi to other locations (trans)
Two models explaining protein movement through the Golgi
Vesicular transport
Cisternal maturation
Vesicular transport: what is the idea behind it, how does the theory work, and what suggests this theory is accurate?
Golgi cisternae viewed as static compartments containing specific enzymes.
Vesicles bud from one cisterna and fuse with the next transporting cargo through each cisterna in turn
Experimental evidence shows cargo molecules present in small transport vesicles that are ≈100 nm in diameter
Cisternal maturation: what is the idea behind it, how does the theory work, and what suggests this theory is accurate?
Each Golgi cisterna matures as it migrates outward through the stack
Golgi resident enzymes carried forward with the cisterna are returned to an earlier compartment in vesicles
Explains the transport of large cargo like procollagen (300 nm long) that are too big for typical vesicles of 100 nm in diameter
Protein modifications in the Golgi apparatus
O-linked oligosaccharides are added to some –OH side chains of Serine (S) and Threonine (T) residues in proteins as they transit the Golgi.
N-linked oligosaccharides added to proteins in the ER can be trimmed and rebuilt at the Golgi
N-linked oligosaccharides on lysosomal proteins modified with Mannose-6-Phosphate
Constitutive secretion of transport vesicles: what is it, what does it do, and how is it initiated?
Involves vesicles being sent to the membrane which supplies it with membrane proteins and lipids for growth
Allows protein secretion – cell surface, extracellular matrix or fluid
No signal required – ‘default’ pathway
Regulated secretion of transport vesicles: what is it, what does it do, and how is it initiated?
Proteins are sorted into vesicles until a signal is received
Occurs with specialised secretory cells
What are the three secretory mutations?
Secretory mutant A - Mutation at rER which prevents proteins from moving to Golgi
Secretory mutant B - Mutation at Glogi which prevents proteins from moving to membrane
Secretory mutant C - Mutation at the membrane which prevents proteins from being secreted
The three forms of endocytosis
Phagocytosis - Using the membrane to form a phagosome to break down engulfed material
Pinocytosis - Non-selective uptake of material mediated by clathrin-coated vesicles
Receptor-mediated - Receptors bind to molecules before forming a vesicle with a clathrin coat which moves to the endosome for intracellular sorting
Endosomes: what are they and what type of molecules are sorted?
Intracellular sorting organelles composed of tubules and vesicles (with a low pH which causes the release of the receptors from the molecules) which are vital in the endocytic pathway
Cholesterol (LDL), Essential metabolites (VB12, iron), Signalling molecules (Epidermal growth factor (EGF)), and Virus particles (Influenza, HIV, and coronavirus)
What happens to the receptors after dissociating from the bound molecules?
- Can be recycled (LDL receptors)
- Can be degraded (EGF receptors)
- Transcytosis (movement to a different domain of the plasma membrane)
Lysosomes: what are they, what is the optimum pH, and how is it maintained?
Contains 40 hydrolytic enzymes (inc proteases, nucleases, glycosidase, etc) which break down macromolecules before pumping the products into the cytosol
Optimum pH = 5
pH maintained by an ATP-dependent proton pump
Sorting enzymes to lysosomes
Lysosomal enzymes made at the ER, moved through the Golgi and modified with Mannose-6-phosphate, and transported to the lysosome after interacting with M-6-P receptors in the trans-Golgi network.
Autophagy: what is it?
Self-consuming within the cell by forming an autophagosome which transports damaged organelles/obsolete parts of the cell to the lysosome
