Membrane Trafficking Flashcards
What are signal sequences:
- short peptides within a particular protein with characteristic amino acid sequences so tell cells that a protein must go to particular compartment
- Without cannot enter intracellular organs and remains in cytosol
- Always at amino terminus
How does gated transport work?
- Nuclear import by import receptors that act as address labels
- Nuclear envelope perforated by pored that form gates through which molecules enter/leave
Define Compartment identity:
- all different membrane bound intracellular components have distinct protein and lipid content
How do proteins get into correct compartment?
- Targeting/sorting signals (lysosomes)
- Sorting at trans-golgi (exocytosis) (lysosomal enzymes) degrade proteins and lipids and substances destined for lysosome
How does vesicle transport occur?
inter organellar transport - move along cytoskeleton using motor proteins
- Invagination: coated vesicles bud off from membrane - have distinctive clathrin protein coat
- Basket like network shapes membrane into vesicle
- After formation pinches off membrane using dynamin - forms ring around neck of pit causing ring to constrict - After budding coat is shed so can interact with membrane they will fuse with
- Vesicle movement along cytoskeleton until finds correct acceptor compartment
- Vesicle tethering proteins tether onto membrane of acceptor compartment and fusion
What is the difference between constitutive and regulated secretion?
- All cells have constitutive secretory pathway that keeps shuttling back and forth and constantly going towards plasma (fairly unregulated)
- Specific cells excitatory cells) contain regulated secretory pathway - stored in vesicle in cytoplasm until release triggered by signal
- When signal molecule binds to cell surface receptor it triggers intracellular signal pathways causing release
Summarise the exocytotic pathway:
ER–>Cis Golgi–>Medial Golgi–>Trans Golgi–>Cell surface via vesicle
How does translocation to ER occur?
bind to SRP–>binds to SRP receptor in ER
Give some ER modifications:
- folding, disulphide bonds, glycosylation, proteolytic cleavages, multimeric protein assembly
- Move via vesicles budding off ER to next exocytosis stage
What are the functions of glycosylation?
- Folding
- Protection (eg. Glycoalyx)
- Receptors
- Recognition (e.g blood group)
How is quality control ensured?
If protein not assembled correctly, cell gets rid of it exported back to cytosol and degraded
Give the three types of endocytosis:
- Receptor mediated: binds to receptor and forms vesicle with protein coat around
- Pinocytosis: fluid drinking cell extends membrane and takes in EC fluid
- Phagocytosis/macrophagocytosis: takes up large particles
Where is the first compartment of endocytosis?
Endocytosed material to early endosome- sorting compartment.
What are the 3 paths?
a. Recycling: sent back to plasma membrane e.g transferring receptor take up iron
b. Transcytosis: carried to basolateral membrane and moved across epithelial monolayer
c. Degradation e.g. LDL:
How is LDL tranported in via endocytosis?
- Protein component of LDL recognized by cell receptor surface and accumulates in pits coated by protein coat clathrin
- Pits pinch off forming clathrin coated vesicles –> coating falls off from uncoated vesicle –> fuses to endosome
- LDL transferred to lysosome to produce free cholesterol
- LDL receptor recycled back to plasma membrane for another round
What are the types of intracellular transport?
- Vesicular transport
- Transport across membrane
- Gated transport
What happens to the signal sequence?
may remain permanently bound to the ER as part of a polyribosome
What happens to the ribosomes?
recycled
- at the end of each round of protein synthesis, the ribosome subunits are released and rejoin the common pool in the cytoplasm.
What are the destination pathways from the Golgi apparatus?
- lysosome
- secretory vesicle
- plasma membrane
What are the steps in vesicle transport?
- vesicle formation
- movement along cytoskeleton
- vesicle tethering
- vesicle fusion
