Localization of TGN38 to the trans-Golgi network: involvement of a cytoplasmic tyrosine-containing sequence

The sequence YQRL has the structural features of an internalization signal (Trowbridge, 1991) and, as demonstrated by our experiments, can cause endocytosis of surface molecules. Moreover, the critical role of the Y and L residues in TGN localization is in line with the known sequence requirements for internalization signals (Trowbridge, 1991) . Some of our results, however, argue that the function of YQRL in TGN localization is not limited to its ability to cause endocytosis from the cell surface. For instance, the sequence YQDL was considerably less efficient for TGN localization, even though it was almost as effective as YQRL for internalization. This agrees with previous observations that YQDL is the internalization signal of the asiaioglycoprotein receptor H1 subunit (Spiess, 1990) , and that replacement of the transferrin receptor internalization signal by YQDL results in a mutant transferrin receptor with an internalization rate virtually identical to that of the wild-type receptor (Collawn et al., 1991) .