Internalization is a very important process. Occurs by small invaginations of PM. The process is so extensive that 50-200% of the PM is internalized every hr in most cells, and the extent is even greater at synapses.
Start w/ some definitions:
- Phagocytosis is the uptake of large particles, mediated by special cells, requires actin
- Endocytosis mediates uptake of fluid & suspended macromolecules. We will consider only one-mechanism: clathrin coated endocytosis, but w/in this there are 2 categories:
- fluid phase uptake (bulk phase)
- receptor mediated; Several categories of receptor mediated:
- constitutive: TfR, LDLR: these receptors recycle: about 10 rounds/hr
- ligand regulated: EGFR
- signal regulated: CD4
- Endocytic Organelles
- (clathrin coated vesicles)
- early endosomes: fused clathrin coated vesicles: recycles receptors
- late endosomes: recieves input from TGN
- lysosomes: degradative organelle, acid hydrolases
- TGN: sends lysosomal enzymes, receives recycling M6PR
Review basic paradigm for sorting (see diagram)
- retention
- retrieval: selective incorporation in reverse direction
- sorting: selective incorporation in any direction
ie sorting complex formation can occur in any direction
- docking: selective pairing prior to fusion (introduced as prelude to next lecture)
How does endosome trafficking work? as w/ biosynthetic: vesicle transport & maturation
- Coated pit formation to clathrin-coated vesicles (CV)
- efficient uptake of dilute ligands: hormones, growth factors, enzymes, plasma proteins
- bound ligands concentrate in coated pits (up to 100 fold conc.)
- Fusion of CV to form early endosome (SORTING station)
- pH reduced due to action of V-type ATP-dep proton pump and Cl- channel for counterion: prevents membrane potential
- this promotes uncoupling of receptor from ligand
- recycling vesicles form, rich in membrane and membrane proteins
- come from tubular compartments of EE
- endosomal carrier vesicle: dissolved material & ligands: central core of EE
- late endosome
- accept input from TGN
- may mature into lysosome
- may fuse w/ preexisting lysosome
- gives off recycling vesicles destined for TGN
- lysosome
- contains 50 different enzymes:
- proteases, nucleases, polysaccharidases, lipid hyrolases etc.
LDL as example
- cholesterol needed for membranes and for steriod hormone syn.
- transported in blood as lipoprotein complex e.g. LDL
- 1500 cholesterol molecules in core esterified to long chain fatty acids
- outer layer of phospholipids containing
- unesterified cholesterol
- single copy of apolipoprotein B-100
- surface LDL receptors (clustered in pits) bind apolipo B-100
- high levels of blood LDL lead to athersclerosis (plaques in vessel wall)
- binds LDLR, internalized, uncoupled in EE, LDLR recycles
TfR as example
- all cells need iron (comes from liver (storage) and intestine (absorbed) via blood
- bound by major blood glycoprotein transferrin (Tf)= ferro-Tf
- bound by receptor on PM, internalized, iron dissociates
- but apo-Tf remains bound to receptor at low pH, so it recycles
- at PM, pH is raised and apo-Tf dissociates.
Phagocytosis (uptake of particles)
- 1-2µm diameter
- functions
- feeding (especially for unicellular organisms)
- PM infolding around substance
- fusion at neck to form phagosome
- fusion of phagosome w/ lysosome for digestion
- protection; phagocytes (macrophages, neutrophils) act on:
- invading organisms
- damaged cells
- aging cells
- debris
- signals
- surface receptors bind particle
- particle may be coated with opsonins
- driving force