Examination of an area in beta-hexosaminidase B homologous to that in cathepsin D linked to lysosomal targeting.

A lysine-rich area in the beta subunit of beta-hexosaminidase (beta-N-acetylhexosaminidase, EC 3.2.1.52) homologous to residues 189-203 in Cathepsin D, previously proposed as being critical for efficient lysosomal targeting, was identified. In vitro mutagenesis of the Lys residues was followed by COS-1 cell expression of enzymatic activity. The intracellular mutant beta-hexosaminidase B activity had a T1/2 at 60 degrees C similar to that of the wild type enzyme, indicating that this region is likely on the surface of the folded enzyme, as is the targeting domain of Cathepsin D. However, in the case of beta-hexosaminidase B, mutation of the Lys residues did not affect lysosomal compartmentalization. These data suggest that the hunt for the common protein signal that results in proper intracellular transport of lysosomal enzymes will not be straightforward and that Lys residues may not be an absolute requirement of the signal.