Week 10(Cellular Compartments II Endo- and Exocytosis) Flashcards
Secretion Overview
Synthesis/ER →Golgi →PM (plasma membrane)/extracellular space.
Secretory Pathway: step 1
Co-translational trans-membrane transport
Most Proteins Are Covalently Modified in the ER
➢Important changes to a protein structure
➢Glycosylation
•sugars added to proteins (glycoproteins)
•major function of ER
•most soluble & membrane-bound proteins are glycoproteins
•cytosolic proteins are simply or not glycosylated
•often essential for function
Glycosylation in ER
A complex of 14 sugars is added en bloc
➢Sugars transferred from a lipid (dolichol)
➢To asparagine residue by protein transferase
➢Not all Asn are glycosylated
•consensus Asn-X (any AA except proline)Ser/Thr
•N-linked glycosylation (Asn = N)
ER Chaperones (proteins) ensure quality
Chaperones bind unfolded proteins- prevents aggregation and ensures that the protein is folded properly before release (speeds up folding process)
➢Binding keeps them in the ER
➢Important for correct folding and assembly of multimeric proteins
Antibodies
•4 polypeptide chains (2 heavy and 2 light)
➢BiP (chaperone) retains Ab in the ER until it is complete
What happens to Improperly folded proteins?
They are exported out of ER and degraded in the cytosol
What happens to misfolded proteins in the ER?
If they start to accumulate in ER, they activate Unfolded Protein Response (UPR)
-Signal to nucleus to reduce translation of proteins
Or
-Signal to increase amount of chaperones
Secretory Pathway: step 2
Vesicular transport to the Golgi for further modification and onwards
Glycosylation in Golgi
➢O-linked glycosylation (not as frequent as N-linked)
➢Poorly understood
➢sugars bind to -OH groups of amino acid side chains (ser/thr)
➢less frequent than N-linked
Secretion pathway- step 3: EXOCYTOSIS
Secretion to the Plasma membrane/extracellular space
Where are Proteins sorted?
- in the trans Golgi network
- and transported in vesicles to their final destinations (fuse with plasma membrane- Transmembrane protein will be inserted in plasma membrane or if the cargo is Soluble it will be released into the extracellular space)after activation
Protein Coats Drive vesicle budding
Vesicles have a distinctive protein coat (cytosolic)
Vesicle Coat Protein function:
- Shapes membrane (cage-like)
- Helps capture cargo molecules for onward transport
- Helps induce membrane bending (coated bud)
- Helps coordinate membrane scission/vesicle release
- Must disassemble to allow vesicle fusion of with target membrane
What are major components of protein coats?
- Clathrin and COP (Coat Proteins) I and II
- Other components mark where the different vesicles function
Different coats in vesicular trafficking
➢Adaptor proteins (based on the compartment the vesicles originated) necessary for clathrin coat formation and cargo sequestering
Vesicle tethering depends on Rab GTPases
➢Rab GTPases ensure the specificity of vesicular transport
➢Functions via Rab effectors to ensure tethering and docking of vesicles to the membrane (=first connection between donor and target compartments)
•Can link 2 membranes that are >200nm apart
Vesicle docking and fusion depends on SNAREs: v-SNARE
on vesicle
Vesicle docking and fusion depends on SNAREs: t-SNARE
on target
Describe the action of SNARES
- Wind together
* Squeeze the vesicle and cell membranes together for fusion
Vesicles move along cytoskeletal fibres:
➢FROM donor compartment (origin)
➢TO target compartment (destination)
Why doesn’t the cell just keep getting bigger?
➢Exocytosis adds membrane to the plasma membrane
ENDOCYTOSIS
Cells are constantly bringing material in
Relies on vesicles
What are the two types of endocytosis?
PINOCYTOSIS and PHAGOCYTOSIS
PHAGOCYTOSIS
(Eating)
Vesicles >250nm
Specialised cells eat large particles
PINOCYTOSIS
(Drinking)
Vesicles <150nm
What does PINOCYTOSIS involve?
➢Cell drinking
➢Uptake of fluid & macromolecules
➢May be indiscriminate
-Captures local fluids as membrane invaginates
properties of PINOCYTOSIS
➢May show specificity
➢RECEPTOR-MEDIATED ENDOCYTOSIS (bind specific cargo molecules)
➢Receptors concentrate desired molecule for uptake with the help of the clatherin coat
Destination of endocytosed cargo
➢PM→Early Endosome→Late Endosome→Lysosome.
Recycling Endosome→recycled back to PM without being degraded
➢Early Endosomes (EE) sort cargo for onwards destination; ligand dissociates from the receptor - has a low pH, receptor can then be recycled back to plasma membrane
➢Late Endosomes (LE) / multivesicular bodies (MVB) prepare cargo for degradation/secretion, it will either mature or fuse with a lysosome
➢Lysosomes degrade cargo
Explain PHAGOCYTOSIS in detail
➢Specialized Phagocytic Cells Ingest Large Particles
➢Phagocytes
•Macrophages
•Neutrophils
➢Large particles (e.g. bacteria) bind receptors
➢Pseudopodia extend around particle (supported by actin cytoskeleton) and fuse together to engulf the particle
➢Tips fuse to create PHAGOSOME
➢Lysosomes fuse and bacterium is degraded
➢Also involved in removal of dead cells
Lysosomes degrade cargo from:
Endosomes
•Phagosomes
•Autophagosomes
Give a Summary of Exocytosis
ER→Golgi →PM ➢Signal sequence dictates path ➢Chaperones check quality ➢Sugar modifications made on journey through ER and Golgi ➢Vesicular transport transfers proteins ➢Vesicles are protein coated ➢Membrane fusion deposits protein cargo
Give a Summary of Endocytosis
ENDOCYTOSIS (reverse mechanism) •PM→EE→LE→Lysosome •PM→EE→RE→PM •Uptake of membrane & other cargo •Pinocytosis •Receptor mediated endocytosis •Phagocytosis (specialised) •Lysosomes degrade ingested material
What are the main options for proteins after they reach the trans Golgi network?
Lysosome
Plasma membrane
Secretory vesicle
COP 1 coats travel to:
Travel to ER
COP 2 coats come from:
On ER membrane
What type of glycosylation occurs in ER?
N
What is autophagy?
A double membrane enclosed a damaged organelle and brought to the lysosome
Cell removes damaged organelles
What is the origin and destination of vesicles with Clathrin + adaptin 1 coats?
Origin: Trans Golgi network
Destination : lysosome
What is the origin and destination of vesicles with Clathrin + adaptin 2 coats?
Origin: Plasma membrane
Destination: endosome
What is the origin and destination of vesicles with COP 1 coats?
origin: Forming at the Golgi
Destination: travelling to ER
Where is the cis Golgi network?
Faces the side closest to the nucleus
Where is the Golgi stack?
in the middle of the cis and trans Golgi network
Where is the trans Golgi network?
facing away from the nucleus (towards plasma membrane)
What are pseudopods?
sheetlike projections of the plasma membrane that surround the invading microorganisms.
supported by actin
What class of enzyme do caspases belong?
protease
What is BAX protein?
Apoptosis regulator BAX, also known as bcl-2-like protein
If the genes encoding Bax and Bak are both inactivated
cells are remarkably resistant to most apoptosis-inducing stimuli, indicating
What are BAX and BAK activated by?
Bax and Bak are themselves activated by other apoptosis-promoting members of the Bcl-2 family such as Bid.
What is the function of the Bcl-2 family?
TheBcl-2 familyof intracellular proteins helps regulate the activation of procaspases
What is the apoptosome?
The apoptosome is a large quaternary protein structure formed in the process of apoptosis. Its formation is triggered by the release of cytochrome c from the mitochondria in response to an internal (intrinsic) or external (extrinsic) cell death stimulus
Which members of the Bcl-2 family inhibit apoptosis?
Some members of this family, likeBcl-2itself orBcl-XL, inhibitapoptosis
is crm-A anti apoptotic?
yes
Is the BAD protein anti apopototic?
No
It is pro apoptotic
-BAD is a member of the BH3-only family, a subfamily of the Bcl-2 family
-It inhibits antiapoptotic BCL-2 and BCL-xL
