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Classical adaptors: e.g. the AP2 adaptor complex
There are 4 different subunits (as illustrated in the model above based on the crystal structure) and thus they are sometimes called polymeric or hetrotetrameric adaptors. The trunk domain binds to membranes and to cargo via the mu and beta subunits. The appendage domains (sometimes called Ears) bind to accessory proteins. Structures of a number of appendage domains have been solved and binding motifs and partners characterised (see appendages). The hinge domains bind to clathrin terminal domains. AP1, AP3 and AP4 adaptors have similar subunit compositions and bind to different cargo and accessory proteins (see Adaptor table 1).
For further information on cargo binding motifs, see Adaptor table 1.
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The AP2 adaptor complex works on the plasma membrane to internalise cargo (see CCV assembly). In the above picture the beta-append
age is bound to the clathrin cage while the alpha-appendage is free to recruit accessory proteins. The AP2 complex is important in synaptic vesicle recycling, and thus brain homogenates are a common source of material for biochemical experiments. |
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Other AP complexes
There is significant homology between subunits of similar colours. Note that the gamma appendage domain in the AP1 complex has only one subdomain like the GGA appendage domain in the next panel.
AP1 complex: cargo selection from the TGN and endosomes
AP2 complex: cargo selection from plasma membrane
AP3 complex: cargo selection to lysosomes
AP4 complex: ??
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GGAs: an Alternative Clathrin Cargo-adaptor
There are now a number of different 'alternative' cargo adaptors. These include the GGAs, Hrs, arrestins and epsins. These proteins are discussed in the coming pages.
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