BICD 110 Midterm 2

What is phagocytosis

an ancient, actin-dependent pathway that enables cells to engulf particles/cells/pathogens > 500nm (induced mechanism)

What are the steps of detection of opsonized target in phagocytosis?

pathogen that is tagged with Fab/Fc antibody is recognized by Fc region (constant) by Fc receptor on effector cell

What are the steps of the formation of phagocytic cup in phagocytosis?

1. probing: membrane ruffles and extends to interact with tagged pathogen

2. early signaling and cup formation: once enough Fc receptors engage, cup forms and induces downstream signaling

3. pseudopod extension: extends even more to facilitate engulfment

4. phagosome closure: pathogen is fully enclosed

What are the steps of the phagosome maturation in phagocytosis?

1. engulfed bacteria acquires factors: Rab5 and GTPases

2. fuse with early endosome, where proton pump V-ATPase lowers pH to 5.5-6

3. conversion of Rab5-Rab7 + late endosome promotes conversion to late phagosome. pH dec to 4.5

4. fusion with lysosome brings degradative enzyme (now phagolysosome)

5. NADPH oxidase complex recruited to produce reactive oxygen species (ROS) to bleach interior of phagolysosome

6. after degradation, phagolysosome is exocytosed

What is macropinocytosis?

an ancient, actin dependent pathway that enables cell to engulf fluids. macropinosomes range from 200nm-10um. membrane “ruffles” can be induced by growth factors

Is phagocytosis and macropinocytosis induced or constitutive?

phagocytosis: induced
macropinocytosis: constitutive

Steps for clatherin-dependent, receptor-mediated endocytosis

1. specific ligands bind to transmembrane receptors on PM

2. AP2 (locked) recognizes NPXY or LL motifs and binds to cargo receptor tails on PM and PIP2

3. AP2 recruits clatherin triskelion, which curves membrane into CCP (pit)

4. pit buds off into vesicle, AP and clathierin triskelion dissociate

What happens to lysosomal enzymes when the pH is 5 and why?

it becomes active, safeguarding mechanism

How do lysosomes decrease their pH

V-type ATPase are lysosomal membrane pumps, they pump protons (H+) into the lumen of the lysosome

LDL particle structure

the shell is a single apolipoprotein wrapped around and phospholipid monolayer. the core is hydrophobic with neutral lipids. the particle, ApoB, is taken from the bloodstream and transported

Mechanism for receptor-mediated endocytosis of LDL

LDLr: B-propeller domain in PM, with NPXY sorting signal in cytosol. Cys-rich “hook” binds to ApoB (LDL) at pH 7. after endocytosis, pH 5 in late endosome weakens interaction (+ charged), and LDL is released. LDLr is recycled to PM

How does SRE-binding proteins monitor ER cholesterol levels?

when cholesterol levels are low, the ER sends SREBP in COPII vesicles to Golgi, which sends SREBP to nucleus. SREBP activates SRE on DNA and activates cholesterol synthesis pathways

What are mutivesicular bodies (MVB)

they degrade cytosolic portions of membrane proteins

Mechanism for degradation of PM receptor

1. lysosomal enzymes TGN → late endosome

2. endosome carrying PM receptor fuse with late endosome

3. vesicles containing PM receptor bud inward (MVB)

4. MVB fuse with lysosome. lysosomal enzyme activate and degrade PM receptor

Mechanism for MVB formation

1. transmembrane proteins for degradation are ubiquitinated

2. Hrs protein sorts ubiquitinated cargo into inward buds

3. ubiquitin attracts ESCRT machinery and forms inward vesicles

4. ATP is invested into Vps4 to disassemble ESCRT from membrane

Mechanism for autophagy

1. ATG proteins induce formation of cup shaped structure around target in two bilayers

2. Atg8 specifies membrane growth

3. fusion of autophagosome and lysosome

4. degradation of target

What are caveolae?

stable membrane domains that depend on cholesterol (SDS PAGE cannot break apart) and contribute to exocytic and endocytic events

What is transcytosis?

the transfer of macromolecules from the apical to basolateral membrane and vise versa via endo and exocytosis.

What type of junction are apical and basolateral membrane separated by?

tight junction

What are the 5 major endocytic pathways?

phagocytosis, macropinocytosis, clatherin-dependent endocytosis, caveolae, clatherin/caveolin-indepdent

What does chemical and electrical gradients depend on

chemical: concentration of molecules on either side of mem

electrical: ratio of ions (charge) on either side of mem