endocytosis 4

phagocytosis

• specialized cells (macrophages, neutrophil, dendritic cells)

• cargo > 500 nm in diameter

• actin dependent process (Rho GTPases) and receptor-dependent pathway

  • antibody on cargo binds to Fc receptor on phagocyte → reorganization of actin cytoskeleton → pseudopod formation

• pseudopods surround cargo in a zipper-like fashion

phagosomal maturation

• phagosomes undergo fusions and protein exchanges first with early endosomes then with late endosomes

• leads to gradual acquisition first of early endosomal proteins, then late endosomal proteins

• once phagosomes have acquired sufficient LAMPs, they begin to fuse with lysosomes

• results in degradative structure called phagolysosome

escape of death by pathogens

• number of strategies used to escape after being endocytosed

• some inhibit phagosome maturation, others convert phagosomes into other structures (autophagosome, ER)

• viruses are small and can still end up in macrophages

  • can be neutralized/killed by antibodies alone

  • antibody-binding to cell receptor can block it

autophagy

• cargo acquired from within cell

• catabolic process for reusing components

• non-specific and specific cargo

  • long-lived proteins, protein aggregates, defective organelles

• used in emergency situations in which cell is starving and needs nutrients

• can destroy bacteria/virus-related proteins that escaped into cytosol

purposes of autophagy

• obtain nutrients under starvation conditions

• turnover of defective organelles

• removal of protein aggregates from cytoplasm

• removal of bacteria/infectious agents

• specialized purposes → liberation of cholesterol from lipid droplets (adipocytes, hepatocytes, immune cells)

chaperone-mediated autophagy

• KFERQ-tagged proteins bind to Hsc70 and are translocated to lysosomes via LAMP2A

• protein directly fed into lysosome (through pore)

• single cytoplasmic proteins

microautophagy

• proteins/Hsc70 adhering to phosphatidylserine on the surface of late endosomes are budded into internal vesicles

• internalization of small regions of cytoplasm

macroautophagy

• large structures (protein aggregates, organelles) are enclosed by double-unit-membrane structure to form autophagosome

• turnover of bulk cytoplasm, damaged or unwanted organelles

• formation of double membrane structure called phagophore / isolation membrane requires ATG proteins

  • C-shaped

• ATG8/LC3 for phagophore expansion and closure

• autophagy receptor (p62) recognizes Ub tagged proteins/structures too large to be degraded in proteasome

LC3 conjugation

1. signaling pathway

2. protein ubiquitination, creation of phagophore membrane

3. LC3-I brought from cytoplasm, cleaved, conjugation to PE

4. LC3-II attached to membrane

5. recognition of p62

6. recognition of targets for autophagy

7. autophagosome formation

LC3-I

• C-terminal tail of LC3 cleaved and binds to ATG8

• soluble

LC3-II

• conjugated to PE (phosphatidylethanolamine) by ATG12, 5, 16L complex

• membrane-bound

• capable of recognizing p62 → recognizing targets for autophagy