Lecture 26 - Clathrin Mediated Endocytosis

clathrin mediated endocytosis CME

process of particle internalization occurring at clathrin-coated pits in PM

ligand/receptor trigger

ligands of CME

usually growth factors

receptors concentrated in patches of membrane

clathrin

3 legged triskeleon protein

made of 3 heavy chains (each with 2 light chains)

forms “cage” soccer ball around vesicle

CME events

2500 per minute per cell

occupy 1-2% of PM surface

takes 1 minute to complete

clathrin coat assembly proteins

FCHo

AP2

Epsin

EPS15

amphiphysin

dynamin/actin

clathrin release proteins

Hsc70

auxilin

FCHo (BAR)

initiate curvature of membrane

bind directly to PM

recruit scaffold proteins

FCHo curving mechanism

1. natural curvature
2. wedges are inserted into PM to curve it

AP-2

adaptor protein 2

4 subunit complex made of adaptins

adaptins

a

o2

B2

u2

a adaptin

large

binds to PIP2 in PM

o2 adaptin

small

binds to transmembrane receptors

B2 adaptin

large

binds and promotes clathrin

u2 adaptin

medium

interacts with tyrosine based sorting signals

epsin

coordinates clathrin coat formation

deforms membrane

binds to EPS15

makes liposomes form tubes

epsin domains

ENTH - recruits clathrin to N terminus

UIM - ubiquitin interacting motifs

Clathrin/AP-2 binding domain

EH - epsin homology domain

EPS15

epidermal growth factor receptor pathway substrate

binds to a adaptin

CME and actin

recruited though Cdc42 and N-Wasp

polymerizes at PM

pushes vesicle into cell

comet tail

actin tail that remains on vesicle thats been endocytosed

gives extra push to vesicle

auxilin and Hsc70

destabilize attachments of clathrin, AP2 + others

uncoat vesicle

tight junctions endocytosis CME

removes claudins, occludin, JAM from epithelial cells

tight junctions endocytosis caveolin

removes claudin-5, occludin and JAM from endothelial cells

tight junction endocytosis

proteins accumulate in early endosomes then:


1. recycled
2. degraded
3. bulk-flow sent to cell surface
4. trans golgi network

CME size limit

roughly 150nm

but actin can shove any size of cell in