Structural determinants of interaction of tyrosine-based sorting signals with the adaptor medium chains

Many integral membrane proteins contain tyrosine-based signals within their cytoplasmic domains that mediate internalization from the cell surface and targeting to lysosomal compartments. Internalization depends on an interaction of the tyrosine-based signals with the clathrin-associated adaptor complex AP-2 at the plasma membrane, whereas lysosomal targeting involves interaction of the signals with an analogous complex, AP-1, at the trans-Golgi network. Recent studies have identified the medium chains mu2 of AP-2 and mu1 of AP-1 as the recognition molecules for tyrosine-based signals. We have now investigated the structural determinants for interaction of the signals with mu2 and mu1. The position of the signals was found to be an important determinant of interactions with mu2 and mu1; signals were most effective when present at the carboxyl terminus of a polypeptide sequence. Another important determinant of interactions was the identity of residues surrounding the critical tyrosine residue. Mutation of some residues affected interactions with mu2 and mu1 similarly, whereas other mutations had differential effects. These observations suggest that both the position and the exact sequence of tyrosine-based sorting signals are major determinants of selectivity in their interaction with clathrin-associated adaptor complexes.