BMS1060 - Cell biology Wk6 - Protein Processing Flashcards
What do transmembrane proteins do?
They keep cargo inside the vesicle.
These proteins get delivered to the target membrane.
What do coat proteins do? What happens to them?
They bend the membrane - enabling budding - and indicate where the vesicle needs to go to.
They are later disguarded (and recycled) allowing membranes of vesicle and recipient compartment to fuse.
What are the 3 coat protiens and where do they allow transport to and from?
COPII -> allows transport from ER to Golgi
COPI -> allows transport from Glogi back to ER.
Clathrin -> allows transport between Golgi, Lysosomes and plasma membrane.
Describe the structure of Clathrin coat protein.
Has 2 subunits - 3 large and 3 small polypeptide chains. Together form a 3-legged triskelion.
What else is needed to attach the clathrin coat to the membrane? How does it do this?
Adaptor proteins.
By interacting with transmembrane proteins.
What protein pinches off vesciples from membranes?
Dynamin protein
What do vesicles have to identify them according to origin and cargo?
What are the 2 types and how do they work?
Snare proteins
v-snares (on vesicle) and t-snares (on target membrane)
These are complimentary to each other and when meet wrap around each other, locking the 2 membranes together (DOCKING)
Does exocytosis or endocytosis require ATP?
Exocytosis
Give an example of exocytosis in the body.
At nerve terminals to generate action potentials - vesiscles containing acetylcholine fuse with membrane and release neurotransmitters into synapse.
Why is exocytosis regulated?
It allows a conc of proteins to be secreted.
Allows further processing of cargo proteins.
Allows release in reponse to a trigger (some vesicles dock only when a specific trigger occurs).
The concentration of secretory proteins increase as vesicles ______. Why?
Mature
because:
- retrieval of membrane back into golgi
- increased acidity -> proteins stick together - mroe compact
Why is further processing of cargo inside vesicles beneficial?
Some proteins can be broken down into a more active product.
Processing inside of vesicle protects cell from its own hydrolytic enzymes.
How does endocytosis recover the cell membrane after exocytosis? What is this balance between secretion and internalisation called?
Large increase in SA of plasma membrane from exocytosis reversed by endocytosis.
Endocytotic-Exocytotic cycle.
What are 3 types of endocytosis?
PINOCYTOSIS - (cell drinking) - samples thigns outside of cell.
RECEPTOR-MEDIATED ENDOCYTOTIS - taking in substances from outside of cell through its brinding to receptor proteins in the plasma membrane.
PHAGOCYTOSIS - a way of remoing microbes from the body (“cell eating”)
[More detailed notes in folder]
What are lysosomes? What is their role in the cell?
Vesciles which contain hydrolytic enzymes (acid hydrolases) and have a low pH.
They degrade substances from outside the cell and components of cell membrane.
They also recyle products (e.g. amino/nucleic acids) for use.
Describe Actin Filaments
Made up of globular proteins (actin) which assemble into 2-stranded helical polymers.
These line up to form bundles, 2D networks or 3D gels.
They are dispersed throughout the cell.
Roles in shape and movement of assets.
Describe microtubules
Made up of globular proteins called tubulin.
These come together to form hollow tubes called microtubules.
These are more rigid than actin filaments and are long and straight.
One end of the microtubule is attached to a MTOC (microtubule organising centre). The other end grows and shrinks.
Microtubles have roles in positioning organelles, intracellular transport and mitosis.
Describe Intermediate filaments.
Made of various intermediate filament proteins which are extended as helical regions and wind together into dimers.
These associate with tetrameres - winding together to form rope-like fibres.
Intermediate filaments play roles in mechanical support of cell structures.
e.g. Keratin (hair)
Describe how actin filaments grow and shrink. What about microtubules?
- Actin monomers carry an ATP molecule. Actin-ATP has high affinity for actin polymer and binds.
- ATP -> ADP
- Actin-ADP has lower affinity to polymer so drops off.
Similar process with tubulin but uses GTP instead of ATP.
How do myosins move along actin filaments?
Myosins = motor proteins
- Myosin head bound to actin filament.
- Myosin binds to ATP and releases actin.
- ATP hydrolysis (ATP -> ADP + Pi) and conformation change (movement).
- Reattachment to actin. Phosphate is released.
- Conformation change and ADP realeased.
Myosin loves ATP more than actin, but actin more than ADP - must let go of Pi first though before ADP.
How do muscles contract?
Contractile units (sacromeres) arraned in long repeating chains (myofibrils).
Each sacromere contains a partially overlapping array of thin actin filaments and thick myosin filaments.
The motor action of myosin pulls on the actin filaments, causing them to slide over each other.
Filaments don’t change size, but the sacromere shortens.
Other than myosin, what other motor proteins do we need to know?
What is the difference between them?
Kinesin and Dynein -> microtubular motor proteins.
Like myosins they have globular heads.
Kinesin moves from the - to + end of the microtubule, while dynein moves from the + to - end.
How do Kinesins move along microtubules?
- Leading head (1) bound to tubulinTrailing head (2) bound to ADP.
- Head 1 binds ATP causing……a conformation change which throws head 2 forward.
- Head 2 binds tubulin
- Head 2 releases ADP while head 1 hydrolyses ATP.
i.e. back to the beginning except head 2 is now leading & head 1 trailing. It is rather like swinging arm over arm along a rope.
What is the role of the cytoskeleton in intracellular transport?
Various organelles are positioned along microtubules.
e.g.
The Golgi is retained close to the centrosome (microtubule -ends) by the action of dynein.
The ER is dispersed to the cell periphery by kinesin.
Vesicles can be transported along mcirotubules.
