Mod2 - The Endomembrane System Flashcards
Name two examples of molecular chaperones that assist proteins with correct folding once inside the ER (and state what each one does)
BiP is an ATPase which binds exposed hydrophobic residues
Calnexin binds N-glycosylated proteins
Name the three common types of protein modification in the ER
- Signal sequence cleavage
- Disulphide bond formation (oxidation)
- Glycosylation
How are disulphide bonds formed chemically (and which enzyme catalyses this)?
Oxidation of cysteine side chains (protein disulphide isomerase)
Where can disulphide bonds be formed in the cell?
ONLY in ER Lumen (NOT in cytosol - needs oxidising environment)
What occurs in N-linked glycosylation (and which enzyme catalyses this)?
Oligosaccharide added to amino group of Asparagine (Oligosaccharyl Transferase (OST)
4 functions of N-linked Glycosylation?
- Assists protein folding (e.g., allows to bind to chaperones)
- Can be modified for Mannose-6-phosphate (lysosome signal)
- Acts as a ligand for cell-cell recognition
- Plays a role in glycocalyx formation
What is the role of the Scramblase enzyme?
Transfers phospholipids between the two leaflets of the membrane non-selectively (to ensure equal growth of each)
What is the role of the Flippase enzyme?
Flips phospholipids from the outer to the inner leaflet (in the Golgi specifically - asymmetric membrane)
What is the role of coats in vesicle budding?
Deform the membrane into a bud, and capture cargo to be included in the vesicle
What is the role of Dynamin?
Helps the vesicle bud to pinch off from the donor compartment (forms as a ring around the neck of the bud; it is a GTPase and hydrolysis causes a conformational change allowing it to pinch off the vesicle)
Where do each of the three types of vesicle coat proteins mediate transport?
Clathrin = endocytic pathway (plasma membrane -> Golgi)
COPI = Various stages of Golgi trafficking
COPII = Form at ER, deliver to Golgi
What is the role of Adaptins?
Adaptins help Clathrin to attach to the membrane, forming a clathrin-coated pit on the surface (also bind cargo receptors, which recognise sorting signals, and recruit them into the vesicle)
What two proteins interact (initially) in vesicle-membrane recognition?
Rab (on the vesicle) and tethers (on the membrane) -> specific pairs
Which proteins interact to dock the vesicle to the membrane and catalyse fusion?
SNAREs (vSNAREs with complementary tSNAREs)
Describe the structure of the ER retention signal sequence
KDEL (LysAspGluLeu) - at the C-terminus; recruited into COPI-coated vesicles which return the protein to the ER
What is the role of the Mannose-6-Phosphate sorting signal?
Addition of M6P to N-linked glycans of some glycoproteins results in sorting to LYSOSOME
What is the role of an unusually short transmembrane domain in terms of sorting?
Proteins are retained in the Golgi
State the main difference between the Vesicular Transport model and the Cisternal Maturation model of Golgi function
Vesicular Transport: Cisternae viewed as static compartments containing specific enzymes; vesicles bud from one to the next
Cisternal Maturation: Each cisterna matures as it migrates outward through the stack; Golgi resident enzymes are transported back to less mature cisternae via vesicles
What is the difference between constitutive and regulated secretion?
Constitutive: constant stream of transport vesicles bud from the trans Golgi and fuse with the plasma membrane
Regulated: proteins are sorted into secretory vessels and stored until a particular signal is received (only in specialised secretory cells)
Where are glycolipids found in the plasma membrane and why?
ONLY in the outer leaflet, because their orientation is fixed once they are packaged in vesicles, and there are no flippases which can flip them to the inner leaflet
What does the integral region of an integral protein usually consist of?
Around 20 hydrophobic amino acids which form an alpha-helix that crosses the bilayer
Why does the plasma membrane surface area remain constant despite the fusion of vesicles?
Endocytosis is balanced by exocytosis
Describe the process of phagocytosis
The cell extends pseudopods to engulf the microbe, forming a phagosome, which fuses with a lysosome to destroy the microbe
Describe pinocytosis
Non-selective uptake of fluid and macromolecules, mediated by Clathrin-coated vesicles - small areas of plasma membrane and extracellular fluid are internalised
What is the benefit of receptor-mediated endocytosis?
The use of cell-surface receptors to capture cargo increases uptake efficiency up to 1000x
Why do most cargo (e.g., LDL) dissociate from their receptor once it reaches the endosome?
The endosome has an acidic pH
What is the main role of endosomes?
They are the main sorting station in the endocytic pathway (equivalent to the trans Golgi network in the secretory pathway)
How is the acidic pH maintained in lysosomes?
An ATP-dependent proton pump
How does autophagy occur?
Damned organelles are engulfed by a Double Membrane that forms in the cytosol, forms an autophagosome, and fuses with a lysosome
