Clathrin-coated vesicle assembly

There are 3 main types of molecules:
Clathrin: the self-polymerising scaffold and organiser of CCV formation
Adaptors: Cargo and accessory protein recruitment molecules
Accessory proteins: Clathrin recruitment, membrane bending and scission molecules etc.
We show the following schemes in a linear sequence yet it is unlikely that they occur in this linear fashion in the cell, where multiple events can occur simultaneously. In fact we believe that clathrin-coated pit formation is initiated when a network of interacting proteins are stabilized by their associations with cargo and PiP2. For synaptic vesicle retrieval by this pathway the network is centred around the AP2 adaptor complex. The network hypothesis is set out in Praefcke et al 2004.

AP180 recruitment of clathrin to membranes via PtdIns(4,5)P2 binding Epsin bends membranes, and clathrin stabilizes this curvature
STEP 1: Clathrin recruitment

Clathrin can be recruited to membranes by a variety of proteins including AP180, epsin, amphiphysin and adaptors. AP180 is illustrated here because this is a very efficient recruitment protein that binds to PiP2 in the plasma membrane and also induces the polymerisation of clathrin into a lattice (see AP180 pages).

STEP 2/3: Clathrin polymerisation and Membrane Bending

The membrane is deformed with the aid of curvature driving molecules like epsin and amphiphysin. As illustrated for epsin, this protein dips into the membrane and displaces lip ids in one leaflet and also binds to clathrin (see Epsin pages).

Network of interactions at clathrin-coated pits Model of how dynamin coulod act by a lengthwise extension of the dynamin helix on GTP hydrolysis
STEP 3/2: Cargo and accessory protein recruitment

Adaptor proteins bind to cargo proteins and sequester these proteins into the forming clathrin bud. There are many different types of adaptor with different specificities (see Adaptors). Many of these adaptors can also bind to other proteins via appendage domains. We call these proteins 'accessory proteins' (see comparison of appendage domains).

STEP 4: Scission of vesicle from parent membrane

The large GTPase dynamin forms a helical collar around the neck of an invaginating clathrin-coated vesicle, where it may regulate, pinch or pop the vesicle from the parent membrane. We have shown that GTP hydrolysis is coupled to vesicle scission and that on GTP hydrolysis dynamin spirals undergo a length-wise extension in vitro- which we believe drives the vesicle away from the membrane causing lipid fission. (see Dynamin pages).

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See a review on endocytosis proteins (.pdf)