Week 10(Cellular Compartments II Endo- and Exocytosis) Flashcards
(54 cards)
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
