BIO 421 Exam 2: endocytosis and phagocytosis

Endocytosis

_____: vesicle-mediated transport into the cell​

Phagocytosis

• pseudopods

Endocytic Pathways: _____​

• Mainly found in specialized cells: macrophages, neutrophils,and dendritic cells​

• These cells are sometimes referred to as phagocytes​

• Used to ingest microorganisms and dead or damaged cells ​

• Is a triggered process, requires binding of surface receptors on the phagocyte​

• Signals recognized can include: antibodies, oligosaccharides,phospholipid composition​

• There are also “don’t eat me” signals on normal cells which inhibit phagocytosis​

​

Not every cell has the ability to do this ​

Often in immune cells ​

Used to inject pathogens to get rid of them or to get rid of dead or damaged cells ​

Not a random process... its always triggered (binding of surface recpetors onto the phagocyte) ​

Has + and – signals... regular cells have negative signals so they do not get eaten; cells to be eaten has a positive signal so they can get eaten ​

• Requires polymerization of actin to extend “____” (extensions of the plasma membrane) to engulf relatively large objects​

• We will learn more about this in Lectures 14& 15​

• Is really cool to watch!​

Since its injesting large things... it has a large way of rearrageing ​

Actin extnds pseudopods to engluf the material ​

Pseudopods: Large protusions of the plasma membrane to help engulf  ​

**Very active process! Lots of rearrgagement happening of the cell membrane ​

Gets degraded by lysosome ​

Similar to last lecture with autophagy ​

What changes is where the membrane orginally comes from​

Autophagy: membrane comes from inside the cell ​

Phagocytosis: membrane comes from outside the cell ​

How will the particles that have been phagocytosedget degraded once they are in the cell?​

read and comprehend

Phagosome... forms from the plasma membrane ​

Phagocyte- cell that does phagocytosis (ex. Immune cells) ​

Bubble that froms is the phagosome ​

Once it fuses with the lysosome... it becomes the phagolysosome​

Endocytic pathways: Pinocytosis​

• Cells display continual ingestion of membrane, extracellular fluid and surface molecules. ​

• - some forms are constitutive, some regulated​

• Cells ingest their own plasma membrane at rates ranging from 1%-3% per minute!​

• Several different mechanisms for this

Phagocytosis: cell eating... engulfs large things ​

Pinocytosis: cell drinking... englufs smaller things (mostly water soluble materials) ​

Most cells are doing some type of pinocytosis at all times (own plasma membrane) - a lot is occuring all the time! ​

Some happen all the time (constitutive) and other are regulated ​

3 different mechanisms ​

Answer: the cell would become larger​

Endocytosis/pinocytosis is in the cell; exocytosis is leaving the cell ​

If everytime the cells exocytosis something... it adds the surface area of the plasma membrane a little bit more ​

Conversely, everytime endocytosis occurs.. It loses some of the plasma membrane ​

Therefore, if plasma membrane is being added... its getting larger ​

Blocking either would tip it in either direction

What would happen to the cell if you blocked pinocytosis but allowed exocytosis to occur

• Caveolae

• lipid rafts

• endosome

Endocytosis: caveolae + noncoated vesicles

• These are some of the less well understood endocytic pathways​

• _____ (latin for “little cavities”) are membrane invaginations that form from “lipid rafts”​

• ____ are particular bits of the membrane rich incholesterol, gylcosphingolipids, and caveolin (amultipass integral membrane protein)​

• Once formed, deliver their contents to the ____​

• Other pathways that use noncoated vesicles: not well understood​

​

Caveolae: membrane invaginations (indendtations) that form lipid rafts (bits of phospholipid bilayer rich in cholestrol, etc.) ​

They dot the membrane and form a vesicle not through any typical coat proteins... the lipid rafts are enough itself ​

Not used for regulating endocytosis but for constitutive endocytosis ​

clathrin-coated vesicles

Endocytosis:______​

• The cell membrane is dotted with“clathrin-coated pits”​

• Clathrin is a vesicle coat protein​

• Clathrin coated vesicles are used mainly for receptor-mediated endocytosis​

• - Regulated endocytosis​

Intakes so much stuff that there is a lot of clathrin that just hangs out at the plasma mebrane waiting to do endocytosis ​

Answer: at the trans face of the golgi, forming at golgi and going to endosome, forming at plasma mebrane and coming into the cell, endosome​

NOT find: ER (COPII), or the cis phase of the golgi going back to the ER

Which other vesicles (apart from ones originating in the plasma membrane) also use clathrin coats? ​

• triskelion

Endocytosis: clathrin-coated vesicles​

• Forms through a process similar to intiation, budding, scission process ​

• Composed of protein subunits that assemble into a triskelion “triple spiral, with rotational symmetry” ​

• The ____ assemble to make a basket of composed of hexagons or pentagons

• clathrin

• adaptin2

Receptor-mediated endocytosis​:

• A regulated form of endocytosis​

• Cargo molecules bind to receptors on the plasma membrane​

• Receptors interact with ____ via ___ protein​

Cargo molecules are things in extracellular space what the cell wants to uptake ​

Mediator protein: adaptin 2 – helps bring clathrin to bind ​

• dynamin

• GTPase

Receptor mediated endocytosis: clathrin-coated vesicle formation​

• clathrin coat assembles ​

• A GTPase called “_____”promotes vesicle scission​

COPII – does not know protein that does scission ​

For clathrin, we do – dynamin ​

Dynamin is a ____ (hydrolyzes GTP).. Which tightens until it snaps the neck of the vesicle ​

Wraps around the neck to do scission ​

read and comprehend

Clathrin coated vesicles must uncoat before fusion with target membrane​

• Vesicle is now in cytosol and must uncoat before it fuses with target membrane ​

• To the endosome!​

read and comprehend

The endosome sorts endocytosed molecules for transport, degradation, or recycling​

• Receive vesicle cargos from the plasma membrane as well as the golgi​

• They also recycle cargos back to the plasma membrane and/or golgi via vesicle transport​

• They are a sorting compartment and intermediary on the way to the lysosome​

• LDL

Receptor-mediated endocytosis:Cholesterol Intake

• ____: Low density lipoprotein,known for “carrying” cholesterol.​

• Cholesterol is a vital component of the cell membrane but cannot freely pass through the phospholipid bilayer. ​

• How does cholesterol move from the blood to inside cells? ​

Cholestrol is carried by LDL... its not by itself ​

Cannot pass freely through the lipid bilayer... it gets in through the vesicle process ​

Cholesterol Intake​

• LDL receptors recognize LDL particles​

• LDL particles then endocytosed via clathrin-coated vesicles​

• LDL containing vesicles fuse with the endosome

Uptaken by cells that need cholesterol ​

Recongizes exterior of LDL particles as they float past ​

Vesicle forms, allowing LDL to get out... vesicle uncoats and fuses with endosome ​

They dissociate at lower pH level (around 6)

Remain in endosome until lyososome binds to get rid of shell... releasing the cholestrol to be used by the cell ​

• Endosomes fuse with lysosome​

• LDL particles are hydrolyzed in the lysosome, freeing cholesterol ​

LDL receptors are shipped off back to the plasma mebrane to be used again... they are not degraded by lysosome! ​

Answer: 1 and 2 ​

1. Would prevent uptake of LDL... the vesicle would not form ​

2. Without it, vesicle would not form... prevents uptake ​

If you are not bringing cholestrol to the cell... the blood level of cholestrol is going to be high ​

3. affects levels of maintaining cholestrol ​

Familial hypercholesterolemia is a genetic disorder which leads to high levels of blood cholesterol and very high risk of heart disease. Which of the following mutations could cause this phenotype? ​

1. Mutation in adaptin 2 affecting interaction with LDL receptor ​

2. Mutation in LDL receptor affecting binding activity ​

3. Mutation making endosome less acidic ​