Lecture 12 &13: endocytosis Flashcards
what is** endocytosis**?
- 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
Describe the different types of vesciles
- 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
Give a summary of the process of endocytosis
- 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
what are endosomes? what is their function?
- 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
what are the 3 categories of endocytosis
- receptor mediated endocytosis
- phagocytosis
- pinocytosis
Describe phagocytosis
what happens from start to finsih & give eg
- ‘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
Describe pinocytosis
- 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
what is autophagy?
autophagy is when cells digest their own cytoplasmic constituents (eg organelles) within lysosomes by a process related to phagocytosis
* ‘self eating’
how does autophagy work?
- 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
How do transport vesicles work?
- 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
what are the 2 important functions of transport vesicles?
- specifically **take up **the appropriate cargo
- fuse only with the** correct target membranes**
what is located on the cytosolic surfaces of the membrane that allow the cell to segregate membranes into different subdomains ?
coat proteins are located on the cytosolic surface of the membranes
what are transport vesicles that are derived from **membrane sub-domains ** that contain coat proteins called?
coated vesicles
what are the** 2 functions** of the coat protein?
- 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**
what are the 3 best studied coat proteins?
- Clarthin
- COPI
- COPII
what is important to note about the different coat proteins?
- 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
describe the structure of clarthrin
- 3 heavy chains
- 3 light chains
- forms a triskelion shape
- arranged in a 3 legged structure
how many triskelions form a polyhedral lattice or cage around the cargo to form a vesicle ?
36 triskelions
What are the N terminals interact with
the N terminals (ie the feet of the 3 legged structure) interact with with a inner layer of **adaptor proteins **
what happens to the vesicle as it begins to become coated?
- 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
what are adaptor proteins?
- 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
What are retromers?
- heteropentameric protein complex
- they assemble on endosomes to form vesicles
what is the function of retromer proteins?
- they help recycle transmembrane receptors from the endosome to the golgi network
under what 3 conditions does retromer assembly occur?
- 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
what are phosphoinositides?
- they are a family of lipids
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
what is dynamin?
- a protein required for vesicle formation during endocytosis
- it assembles around the neck of clathrin coated buds
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
what are rab proteins?
- they are monomeric GTPases
- G proteins
- GTP bound rab proteins interact with the rab effector proteins on the target membrane
what is the role of rab proteins?
- they play a vital role in guiding the vesicle to the target membrane
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
what are** lipid rafts**?
cholesterol and sphingolipid enriched microdoomains of the cell membranes
what are vesicles that can be formed from lipid rafts called?
caveosomes
*
How do caveosomes form?
cavolae pinch from the plasma membrane using dynamin and deliver their content into endosomal compartments called caveosomes
what is** clathrin independent endocytosis**?
used to describe mechanisms of endocytosis that dont involve **clathrin or caveolin **
what are other examples of clathrin independent endocytosis pathways?
- phagocytosis and macropinocytosis
what is an example of receptor mediated endocytosis?
- transport of LDL (low density lipo-protein) that contains cholesterol into cells
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
what is the first destination of uncoated vesicles?
endosomes
what are the different classes of endosomes?
- early endosomes (just below the plasma membrane)
- late endosomes (close to the golgi apparatus and near nucleus)
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
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
