8. Membrane trafficking Flashcards
Exocytosis
Cellular secretion/ excretion
Substances contained in vesicles are discharged from cell by fusion of vesicular membrane with outer cell membrane
Endocytosis
Cellular ingestion
Plasma membrane folds inward to bring substances into cell
Constitutive secretion
secretion proteins are secreted from a cell continuously, regardless of external factors or signals.
Regulated secretion
proteins are secreted from a cell in large amounts when a specific signal is detected by the cell
How are proteins directed to the correct compartment?
Signal sequences are like address labels and direct the protein to the correct compartment
Types of intracellular transport
- Gated transport
- Transport across membranes
- Vesicular transport (e.g. inter-organellar transport)
Example of gated transport
Import of proteins into the nucleus
Nuclear envelope is perforated by nuclear pores
These form gates through which molecules enter or leave.
Some pore components form mesh-like structures that prevent passage of large molecules but allow small, water soluble molecules to pass freely.
Nuclear proteins are imported by
Import receptors that recognise nuclear localisation signals as the ‘address label’
Example of TRANSPORT ACROSS MEMBRANES
Translocation of newly synthesized proteins into the ER
2 populations of ribosomes in cells
Free in cytosol OR Bound to ER membrane
Both structurally and functionally identical, differ only in the type of protein they are making at any given time.
Free ribosomes
Unattached to any membrane
Make all other proteins encoded in nuclear DNA
Ribosomes bound to the ER membrane
Make all proteins that are being translocated into ER
Proteins destined for other organelles are 1st imported into…
The ER
Unassembled/ misfolded proteins are retained in ER then…
exported back into cytosol where they are degraded
ER: Post-translational modifications
- Folding
- Formation of disulphide bonds
- Initial glycosylation (addition of sugars)
- Specific proteolytic cleavages
- Assembly of multimeric proteins
Name a disease resulting from blocked ER exit due to misfolding
Cystic fibrosis
SMED-SL
Mutations of CFTR gene affect functioning of chloride channels in the membrane
Leading to CF
CF:
Most common mutation (ΔF508) results from deletion (Δ) of 3 nucleotides which causes
Loss of the phenylalanine (F or Phe) at the 508th position on the protein
As a result, CFTR does not fold normally and is degraded
Discoidin Domain Receptor 2 mutations cause
SMED-SL
SMED-SL characteristics
short limbs and abnormal calcifications
Transport from ER to Golgi and from Golgi to other compartments occurs via
budding and fusion of transport vesicles
Golgi stack has 2 distinct faces
Entry face= Cis face, next to ER
Exit face= Trans face, points towards plasma membrane
Proteins are transported through Golgi from
Cis to trans and are sorted further at trans Golgi network
Golgi apparatus: further protein modifications
Many sugar chains added in ER undergo further modifications in Golgi.
On some proteins, more complex oligosaccharide side chains are added and removed by a series of enzymes that reside in sequence as the protein passes through Golgi stack.
The EXOCYTIC (secretory) pathway
- ER
- Golgi
- Trans-Golgi network
- Plasma membrane
TGN activity:
All cells
Steady stream of vesicles bud from trans-Golgi network and fuse with plasma membrane
TGN activity:
Excitable cells
Products are concentrated and stored in secretory vesicles until an extracellular signal stimulates their secretion
Secretory proteins are released from the cell by
Regulated exocytosis
e.g. Secretory vesicles store insulin in a pancreatic β cell. Insulin is released into extracellular space in response to increased blood glucose.
Coated vesicles
Vesicles that bud off from membranes have a distinctive protein coat on their cytosolic surface
After budding, coated vesicles shed their coat…
Allowing the vesicle to interact with the membrane to which it will fuse
Clathrin-coated vesicles have
Outer coat made up of the protein clathrin
Clathrin-coated vesicles bud from
Trans-Golgi network on outward secretory pathway
and
plasma membrane on inward secretory pathway
Steps in vesicular transport
- Cargo sorting and vesicle formation
- Motor Proteins move vesicles along the cytoskeleton
- Vesicle Tethering/Docking proteins bring vesicle and target membrane together
- Vesicle fusion
3 forms of Endocytosis
- Receptor-mediated endocytosis
- Pinocytosis (fluid phase)
- Phagocytosis (fluid / particles e.g. microbes)
Cholesterol isn’t soluble in water
Transported in blood in Low Density Lipoprotein (LDL) particles
If cell needs cholesterol, it will import LDLs via
Receptor-mediated endocytosis
Steps in Receptor-mediated endocytosis
- LDL particles bind to a specific LDLR
- LDLR gets concentrated in clathrin coated pits
- Vesicle forms- Outer coat of clathrin, inner core of LDL particle, bound to LDLR
- Uncoating of vesicle
- Naked vesicle fuses with endosome and delivers content
- LDL particle and receptor dissociate as in endosome, pH is lower
- LDL particles delivered via vesicles to lysosomes, degraded by hydrolytic enzymes in lysosomes, frees cholesterol
- LDLR packaged into recycling vesicle which brings receptor back to plasma membrane to mediate another round of endocytosis
Disease of endocytosis: Familial Hypercholesterolaemia
Caused by mutations in LDLR: either not produced or not functional
Body unable to remove LDL (bad) cholesterol from blood.
-> high level of LDL in blood
