Endocytosis

endocytosis

• internalisation of the plasma membrane

• used to internalise soluble molecules

• used to control cell surface proteins (eg receptors) - recognise growth factors, hormones

• rate at which molecules are internalised is limited by amount of corresponding receptor on the cell surface

clathrin-mediated endocytosis

• mediated by clathrin and adaptor proteins

LDL particle internalisation

• dietary lipids and cholesterol transported into LDL particle through blood

• LDL particle binds to LDL receptor - N-terminal exoplasmic segment rich with cysteine repeats which interact with the ApoB adapter protein on the LDL particle

• The LDL receptor undergoes a conformational change so it binds to the adapter protein

• adapter proteins bind to clathrin - clathrin assembles into a cage-like structure with curvature to form a vesicle bud

• the coated vesicle bud is pinched off of the plasma membrane using dynamin

• vesicle is uncoated so that it can fuse with the early endosome using SNAREs

• early endosome fuses with late endosome - low pH so receptor dissociates from cargo

• late endosome fuses with lysosome - lysosome contains lysosomal proteases which hydrolyse surface apolipoproteins, lysosomal cholesterylesterases hydrolyse core cholesteryl esters so that LDL particle is degraded

clathrin cage assembly

• receptor binds to cargo - conformational change so adapter protein binds

• clathrin attaches to the membrane via adapter proteins - adapter proteins fill space between clathrin lattice and membrane, association between globular domain at end of each clathrin heavy chain and one subunit of the AP complex

• clathrin drives membrane formation by assembling into curved cages - accumulation of adaptors and clathrin leads to curvature

clathrin cage structure

• formed from multiple hexomeric complexes called triskelions

• triskelions - can self assemble, consist of 2 light chains and one heavy chain

• light chains stabilise structure - are not essential

• lateral association of legs of triskelions form hexamers and pentamers which make up cage-like structure

• Hsc70-ATP (ATP binding protein - hydrolyses ATP to cause conformational change in clathrin so that they dissociate from vesicle) and auxilin bind to the clathrin cage

dynamin

• cystolic protein

• releases endocytic vesicle

• polymerises around the neck portion of the bud - hydrolyses GTP and energy from GTP hydrolysis allows it to undergo conformational change

• neck of vesicle stretched until it pinches off - membranes pushed together so that they fuse

• hydrophobic environment allows phospholipids in membrane to come together and fuse