Lecture 12 &13: endocytosis Flashcards
1
Q
what is** endocytosis**?
A
- a cellular process in which **substances are brought into the cell **
- the material to be internalised is then surrounded by an area of the membrane
- it then buds off inside the cell to form a **vesicle ** that contains the ingested material
2
Q
Describe the different types of vesciles
A
- vacuoles - cellular organelles that contain mostly water
- lysosomes - vesicles that contain digestive enzymes
- transport vesicles - vesicles that move molecules between locations
- endosomes- specific vesicles involved in endocytosis
- secretory vesicles - vesicles that contain materials to be secreted / released from the cell eg synaptic vesicles
- **extracellular vesicles ** - eg ectosomes/ microvesicles - play a role in lots of biological pathways
3
Q
Give a summary of the process of endocytosis
A
- extracellular molecules or membrane proteins can be engulfed by the plasma membrane
- they form vesicles which are internalised and delivered to the internal components in the cell called endosomes
4
Q
what are endosomes? what is their function?
A
- membrane bound organelles that are involved in regulating the trafficing of proteins, lipids etc
- endosomes provide an environment for material to be sorted before it reaches the degradative lysosome
5
Q
what are the 3 categories of endocytosis
A
- receptor mediated endocytosis
- phagocytosis
- pinocytosis
6
Q
Describe phagocytosis
what happens from start to finsih & give eg
A
- ‘cell eating’ - whole particles are engulfed and broken down by enzymes
- binding of particle to receptors triggers phagocytosis, it is then taken up by the phagosome, the phagosome fuses with a lysosome
- the fusion of the phagosome and lygosome forms a phagolysosome
example: macrophages ingesting a pathogenic organism
7
Q
Describe pinocytosis
A
- the cell engulfs material that has already been broken down into smaller components
- also known as fluid endocytosis as it involves small particles from the ECF being internalised
8
Q
what is autophagy?
A
autophagy is when cells digest their own cytoplasmic constituents (eg organelles) within lysosomes by a process related to phagocytosis
* ‘self eating’
9
Q
how does autophagy work?
A
- the cytoplasmic constituents are isolated from the rest of the cell within a double membraned vesicle known as an autophagosome
- this autophagosome fuses with the lysosome and the material inside (cargo) is degraded and recycled
10
Q
How do transport vesicles work?
A
- transport vesicles connect one membrane compartment to another
- they carry their cargo from the lumen of the donor compartment to the corresponding lumen of the target compartment
11
Q
what are the 2 important functions of transport vesicles?
A
- specifically **take up **the appropriate cargo
- fuse only with the** correct target membranes**
12
Q
what is located on the cytosolic surfaces of the membrane that allow the cell to segregate membranes into different subdomains ?
A
coat proteins are located on the cytosolic surface of the membranes
13
Q
what are transport vesicles that are derived from **membrane sub-domains ** that contain coat proteins called?
A
coated vesicles
14
Q
what are the** 2 functions** of the coat protein?
A
- the coat protein binds to specific membrane components causing them to cluster together into a specific ‘patch’ - which makes us the vesicle membrane
- the coat proteins then assemble into a curved basket like lettuce that causes the membrane patch to** form its spherical shape**
15
Q
what are the 3 best studied coat proteins?
A
- Clarthin
- COPI
- COPII
16
Q
what is important to note about the different coat proteins?
A
- sometimes the same coat protein is used for more than one subcellular location ( ie membrane sub domain)
- ie one coat protein does not belong to only 1 membrane component
- eg clarthin is present on the plasma membrane, late endosome & early endosome
17
Q
describe the structure of clarthrin
A
- 3 heavy chains
- 3 light chains
- forms a triskelion shape
- arranged in a 3 legged structure
18
Q
how many triskelions form a polyhedral lattice or cage around the cargo to form a vesicle ?
A
36 triskelions
19
Q
What are the N terminals interact with
A
the N terminals (ie the feet of the 3 legged structure) interact with with a inner layer of **adaptor proteins **
20
Q
what happens to the vesicle as it begins to become coated?
A
- as the continues to form, it gradually causes the memrbane to bud (ie a little hill on the membrane) into uniformly sized vesicles
- once the vesicle has detatched from the membrane, it then becomes uncoated
21
Q
what are adaptor proteins?
A
- adaptor proteins form a second layer inside the clarthin cage and link it to the membrane
- they interact with cargo receptors that bind specific soluble cargo molecules to the lumen
22
Q
What are retromers?
A
- heteropentameric protein complex
- they assemble on endosomes to form vesicles
23
Q
what is the function of retromer proteins?
A
- they help recycle transmembrane receptors from the endosome to the golgi network
24
Q
under what 3 conditions does retromer assembly occur?
A
- only occurs if it can** bind to the cargo receptor **
- if it can interact with a **curved phospholipid bilayer **
- and if it can bind to a specific phosphoinositide
25
what are **phosphoinositides**?
* they are a family of lipids
26
how do the phosphoinositides grant **specifity**?
* both retromers and adaptor proteins in clathrin coats bind
* they are used as **molecular markers of compartment identity**
* they help to determine when and where coats will assemble in the cell
27
what is **dynamin**?
* a protein required for vesicle formation during endocytosis
* it assembles around the neck of clathrin coated buds
28
what is the **function** of dynamin?
* it is a GTPase protein
* causes the neck of the clathrin bud to lengthen and tighten
* GTP hydrolysis causes neck to break
29
what are rab proteins?
* they are monomeric GTPases
* G proteins
* GTP bound rab proteins interact with the rab effector proteins on the target membrane
30
what is the **role of rab proteins**?
* they play a vital role in guiding the vesicle to the target membrane
31
what are **snare proteins**?
* one the rab proteins have guided the vesicle to the target membrane
* snare proteins on the membrane dock the vesicles to the target membrane and they catalyse the fusion of the 2 membrane bilayers
32
what are** lipid rafts**?
**cholesterol and sphingolipid enriched microdoomains** of the cell membranes
33
what are vesicles that can be formed from lipid rafts called?
caveosomes
*
34
How do caveosomes form?
cavolae pinch from the plasma membrane using dynamin and deliver their content into endosomal compartments called caveosomes
35
what is** clathrin independent endocytosis**?
used to describe mechanisms of endocytosis that dont involve **clathrin or caveolin **
36
what are other examples of clathrin independent endocytosis pathways?
* phagocytosis and macropinocytosis
37
what is an example of **receptor mediated endocytosis**?
* transport of LDL (low density lipo-protein) that contains cholesterol into cells
38
how does LDL react with it's receptor?
* the cytoplasmic region interacts with adaptin / clathrin via a motif
* a large exoplasmic region with 40 amino acid fold binds to LDL
* once it binds, it is then internalised by clathrin coated vesicle
39
what is the first destination of uncoated vesicles?
endosomes
40
what are the different classes of endosomes?
* early endosomes (just below the plasma membrane)
* late endosomes (close to the golgi apparatus and near nucleus)
41
what are the 3 methods of sorting / receycling that occur in an endosome that contains a receptor?
* **recycling** - the receptors return to the plasma membrane from which they originated eg LDL
* **transcytosis** - specialised form of recycling in which some receptors proceed to a **different domain on the plasma membrane** eg antibodies
*** degradation** - some receptors are delivered along with their ligands to the **lysosome** where they are degraded
42
summary : what are the different pathways of entry into cells (endocytosis)?
* phagocytosis (clathrin independent)
* clathrin dependent endocytosis
* caveolin dependent endocytosis
* clathrin and caveolin independent pathways
