THE MEDIUM SUBUNITS OF ADAPTER COMPLEXES RECOGNIZE DISTINCT BUT OVERLAPPING SETS OF TYROSINE-BASED SORTING SIGNALS

Tyrosine-based sorting signals conforming to the motif YXXO (Y is tyro sine, X is any amino acid, and O is an amino acid with a bulky hydroph obic side chain (leucine, isoleucine, phenylalanine, methionine, valin e)) interact with the medium (mu) subunits of clathrin adaptor (AP) co mplexes. We have analyzed the selectivity of interaction between YXXO signals and the mu 1, mu 2, and mu 3 (A or B) subunits of the AP-1, AP -2, and AP-3 complexes, respectively, by screening a combinatorial XYX YXXO library using the yeast two-hybrid system. All the medium subunit s were found to prefer proline at position Y+2, suggesting that YXXO s ignals are stabilized by a bend in the polypeptide backbone. Other tha n for this common preference, each medium subunit favored specific set s of residues at the X and O positions; these preferences were consist ent with the proposed roles of the different adaptor complexes in rapi d endocytosis and lysosomal targeting. A considerable specificity over lap was also revealed by these analyses, suggesting that additional fa ctors, such as the context of the signals, must be important determina nts of recognition.