Functional domain mapping of the clathrin-associated adaptor medium chains mu 1 and mu 2

The clathrin-associated adaptors AP-1 and AP-2 are heterotetrameric complexes involved in the recognition of sorting signals present within the cytosolic domain of integral membrane proteins. The medium chains of these complexes, mu 1 and mu 2, have been implicated in two types of interaction: assembly with the beta 1 and beta 2 chains of the corresponding complexes and recognition of tyrosine-based sorting signals. In this study, we report the results of a structure-function analysis of the mu 1 and mu 2 chains aimed at identifying regions of the molecules that, ape responsible for each of the two interactions. Analyses using the yeast two-hybrid system and proteolytic digestion experiments suggest that mu 1 and mu 2 have a bipartite structure, with the amino-terminal one-third (residues 1-145 of mu 1 and mu 2) being involved in assembly with the beta chains and the carboxyl-terminal two-thirds (residues 147-423 of mu 1 and 164-435 of mu 2) binding tyrosine-based sorting signals, These observations support a model in which the amino-terminal one-third of mu 2 is embedded within the core of the AP-2 complex, while the carboxyl-terminal two-thirds of the protein are exposed to the medium, placing this region in a position to interact with tyrosine-based sorting signals.