Cell and Molec UCD chapter 10 phagocytosis

what is endocytosis

a pathway for macromolecules in the cell.

what does endocytosis do

it helps regulate the composition of the plasma membrane in response to extracellular components. and it creates a link to the secretory pathway.

What is the role of phosphoinositides

they help define different membranes in the cell, where each phospholipid head is different, allowing different proteins to bind.

what is phagocytosis

a specialized form of endocytosis where components in the cell are engulfed and travel through a vesicle. this process is triggered through receptors.

what is a phagosome

Vesicles containing ingested particles.

what reconstructs the plasma membrane for phagocytosis

actin cytoskeleton, members of the Rho family of small GTPases and specific phosphoinositide lipids.

how are particiles recognized in phagocytosis

the a,b receptors that are in the pseudopod membrane

how are the Rho family GTPases (Rac, Cdc42) activated

(b,c) receptor activation

What do Rho family GTPases (Rac, Cdc42) do?

stimulate actin nucleation and polymerization (formation and growth of actin)

why is actin depolymerized at the base of the phagocytic cup

because the local lipids PIP2 change to PIP3 which is driven by PI3 kinase activity.

what does PIP3 do

hires myosin motor proteins, which help close off the phagosome

what happens to the phagosome after it enters the cell

it fuses with the lysosomes forming a new compartment named the phagolysosomes

what happens in phagolysosomes

degradative enzymes and free oxygen radicals degrade the engulfed particle

where is the phagolysosome partially derived from

the endoplasmic reticulum

why is the ER's partial contribution important to the phagosome

it helps with the speed of membrane recruitment when the phagosome is formed

what do caveolae look like

deeply invaginated flask-shaped (pouch looking structures)

what are caveolae formed from?

plasma membrane lipid rafts as well as cholesterol, glycosphingolipids, and GPI-anchored proteins

what is the major structural protein of caveolins

unusual important membrane proteins that add a hydrophobic loop into the cytoplasmic leaflet of the plasma membrane

what does dynamin do

large GTPase dynamin is needed for the caveola to pinch off the membrane

where is Caveolin 1 made, and how does it transport in the cell

synthesized in the ER and is transported to the Golgi and then leaves by stimulation of cholesterol levels it leaves

what proteins does caveolin 1 associate with at the cell surface

EHD proteins, cavins, and pacsins

What do caveolae do?

form endocytic carriers (stuff that carries material from the plasma membrane to the cell) that ship material to early endosomes (sorting organelles)

what happens when a caveola faces high membrane tension

it can flatten and release cavins (protein) into the cytoplasm, where they can be degraded if not needed.

what happens to caveolin proteins such as cavins when they are not needed?

they can be endocytosed and delivered to ILVs and degraded.

what happens to the caveolae after they pinch off

they deliver cargo to the caveosome, which is similar to an endosome

what is different about caveolin than other coat proteins

they remain associated with the membrane

what is the best internalized pathway

clathrin-dependent pathway

where is the clathrin coat found

Trans Golgi network, endosomes, and plasma membrane

what are clathrin accumulation clumps called and where are they found

clathrin-coated pits and they are found at the plasma membrane

what do clathrin-coated pits do

they invaginate cargo (fold inward) and form clathrin-coated vesicles

what do CCV's look like

pentagons and hexagons

can CCV's assemble without the presence of membranes and cargo

yes, they can assemble outside the organism (in vitro)

what is each clathrin subunit made of

three large and small protein chains that make the "triskelion"

what does a clathrin-coated pit need to do to become a CCV

it needs the large GTPase dynamin to bind to the neck of the forming vesicle

what does dynamin do after it binds to the neck

recruits more proteins that destabilize the lipid bilayer until they fuse which causes the vesicle to pinch off

what happens if dynamin function is blocked

it doesnt allow the CCV to pinch off and be released

what do adaptor proteins do in clathrin-dependent endocytosis

provide a link between the cargo and the clathrin triskelions

which adaptor protein is used at the plasma membrane

AP-2 adaptor complex

how is cargo selected in clathrin-dependent endocytosis

by binding to the specific cell surface cargo receptors.

what role do the cargo receptors play in Clathrin dependent endocytosis

they provide a link between lumenal cargo and the cytoplasmic coat components.

what happens when cargo receptors bind to AP-2 adaptor proteins

a cluster can form, making it easier for the vesicle to take shape and start bending the membrane

what do adaptor proteins do

recruit clathrin from the cytoplasm to build the clathrin coat

when does clathrin recruitment cycle stop

when the membrane is deformed into a vesicle shape and covered with clathrin

what happens after the CCV buds off the membrane

The coat is lost quickly and the naked vesicle fuses with an early endosome (sorting molecule)

what lipid does the AP-2 adaptor protein interact with

PIP2 for a very short time.

what happens when PIP2 and AP-2 adaptor protein interact

an electrostatic reaction with the μ (mu) subunit of AP-2 which changes the structure of the AP-2 making it "active"

what happens after the AP-2 adaptor protein changes structures

AP-2 can now interact with cargo receptors and stabilize the complex at the membrane

What effect does AP-2 binding to cargo receptors have on clathrin assembly?

it assembles clathrin triskelions, which helps increase the amount of time the AP-2 spends at the plasma membrane. if there are no cargo and cargo receptors, no clathrin assembly happens

what does the LDL (lipid-protein particles) receptor do

captures LDL particles at the plasma membrane and groups them together

what do the clustered receptors/groups do

they recruit AP-2 and clathrin and bring in (endocytose) the LDL particle and its receptor

what do mutations in the cytoplasmic tail of the LDL receptor do

can result in failure to bind AP-2 and endocytosis fails so you cant bring in anything to the cell and LDL remains in blood

where is most endocytic cargo delivered

early endosomes

what happens to molecules that arent specifically called from early endosomes

go to late endosomes and eventually lysosomes where they are degraded

what are early endosomes

complex sorting stations where materials can be reused in the plasma membrane or sent elsewhere (late endosome, TGN, lysosome)

what family plays a key role in early endosome sorting

Rab family of small GTPases (Rab 4,5 and 21)