AP2 complex

AP2 is a tetramer containing two large subunits alpha-adaptin (938aa) and beta2-adaptin (937aa), a medium chain mu2-adaptin (435aa) and a small subunit sigma2-adaptin (142aa). The molecule consists of two small terminal or "ear" domains of the large subunits (~700-938 alpha, ~707-937 beta2) attached by unstructured linkers or hinges to the core consisting of the main parts of the large subunits (1-~600 alpha, 1-~590 beta2) and the two smaller subunits.

The structures of the C-terminal appendage or "ear" domains were determined separately from the core ("trunk"). The structure of the C-terminal domain of mu2 has also been determined separately in complex with signal peptides: in the core the signal peptide binding site is blocked by the beta2 subunit, so this structure represents the inactive conformation with respect to signal binding. The linkers in this picture are purely schematic. In the crystal, a single inositolhexakisphosphate (IP6) is bound between two molecules, delineating two binding sites on the alpha and mu2 subunits for PtdIns(4,5)P2.

Download Movie of AP2 spinning (Quicktime, 9.3MB)

Assembly of the core from the subunits. Mu2 contains two domains: N-mu2 which is similar to sigma2 and C-mu2 which contains the signal sequence binding site

Binding of YxxPhi signal peptide to the free mu2 C-terminal domain. The peptide forms an additional beta-sheet interaction with the domain.

Phosphoinositide binding modules

CALM (AP180)

The ENTH domain of epsin binds PtdIns(4,5)P₂ headgroups in a cleft formed by helix alpha0 which is only folded when the ligand binds. This exposes a hydrophobic ridge on the outside of the amphipathic helix which may insert into the membrane to aid curvature.

Epsin

The ANTH domain of CALM (a homologue of AP180) has a fold similar to the ENTH domain of epsin (though extended at the C-terminus) but binds PtdIns(4,5)P₂ headgroups on the surface, on the tips of lysine sidechains.

The N-terminal BAR domains of amphiphysin and endophilin are crescent-shaped dimers of coiled-coils, and bend membranes into tubes by binding positively-charged lipid headgroups along their concave surface.

Amphiphysin