BIOB10H3 Lecture Notes - Lecture 6: Lipoprotein, Endosome, Conformational Change
May 24, 2018
Lecture 6
Cellular Uptake of Particles and Macromolecules
• materials too large to pass through membrane
o e.g. uptake of extracellular materials (ligands)
o e.g. uptake of transporters and signaling receptors no longer required
• uptake occurs into cytoplasmic vesicles derived from invaginations of cell membrane
• Two separate categories: 1. Endocytosis 2. Phagocytosis
Endocytosis
• uptake of dissolved solutes, macromolecules, signaling receptors, etc.
• Receptor-mediated endocytosis (RME): uptake of specific extracellular
macromolecules (ligands) that bind to receptors on
external cell membrane surface
• Take up hormones, growth factors, enzymes,
plasma proteins by binding to receptors
Endocytic Pathway (receptor-mediated) red=receptor;
blue=ligand
• receptors bind ligand – undergo conformational
change, clathrin binds on cytosolic side
• PM pinches in (via clathrin) to form early
endosome (EE)
• Clathrin goes off; early endosome matures into
sorting compartment (SC)
• sorting compartment lumen fluid acidified due to
action of membrane H+ pumps
• in low pH, ligands are released from cell
membrane receptors
• receptors are recycled back to cell membrane in
vesicles
• endosome carrier vesicles (ECV’s) leave SC and
fuse with late endosomes (LE) => mature into
lysosomes where contents are digested (via acid
hydrolases)
Phagocytosis (cell eating)
• uptake of relatively large particulate matter (>0.5
µm in diameter)
• for food, eg. Unicellular heterotrophic protists
• for protection, eg. macrophage, dendritic cells and
neutrophil immune cells
• because these cells have receptors on cell binds
particles (bacteria, dead cells, etc.)
• macrophage have Fc-receptor that binds to antibodies (IgG) that coat (called “opsonize –
has a layer of antibodies on it”) pathogens, etc.
Phagocytosis Pathway
• receptors(FcR) cluster into lipid rafts