Molecular Recognition by Clathrin Adaptors The clathrin coat plays a ubiquitous and fundamental role in endocytosis and in endosomal sorting within the eukaryotic cell. Clathrin forms a cage that surrounds cargo-bearing vesicles, but clathrin itself does not directly bind to cargo. Cargo is sorted into clathrin-coated vesicles by adaptor proteins that physically bridge cargo and clathrin. The best-known general purpose adaptors are the heterotetrameric adaptor protein complexes (AP complexes) and the multimodular GGA adaptor proteins. The overall goals of this project are 1) to identify the binding sites for cargo on adaptor proteins and measure their affinities quantitatively;2) to determine the crystal structures of complexes between adaptors and soluble cargo fragments;and 3) to relate structure and function using mutational analysis. Adaptor protein 4 (AP-4) is the most recently discovered and least well-characterized member of the family of heterotetrameric adaptor protein (AP) complexes that mediate sorting of transmembrane cargo in post-Golgi compartments. In the previous FY, we reported the interaction of an YKFFE sequence from the cytosolic tail of the Alzheimer's disease amyloid precursor protein (APP) with the mu4 subunit of AP-4. Biochemical and X-ray crystallographic analyses reveal that the properties of the APP sequence and the location of the binding site on mu4 are distinct from those of other signal-adaptor interactions. Disruption of the APP-AP-4 interaction decreases localization of APP to endosomes and enhances gamma-secretase-catalyzed cleavage of APP to the pathogenic amyloid-beta peptide. These findings demonstrate that APP and AP-4 engage in a distinct type of signal-adaptor interaction that mediates transport of APP from the trans-Golgi network (TGN) to endosomes, thereby reducing amyloidogenic processing of the protein. In this FY we focused on unique recognition mechanisms of AP complexes in polarized cells.