Proteolytic processing of the proβ chain of β-hexosaminidase occurs at basic residues contained within an exposed disulfide loop structure

Publisher Website
Google Scholar
Abstract
Lysosomal β-hexosaminidase (EC 3.2.1.52) occurs as two major isozymes, Hex A (αβ) and Hex B (ββ). The α and β subunits are encoded by the HEXA and HEXB genes, respectively. Extensive homology in both the gene structures and deduced primary sequences demonstrate their common evolutionary origin. While undergoing similar proteolytic modifications in the lysosome, the proβ polypeptide is additionally cleaved internally to produce the mature 24–30 kilodalton βb and βa chains. Previous data have suggested that this processing event occurs somewhere between residues Ser311 and Lys315. In this report we demonstrate that this area is located in a hydrophilic disulfide-loop structure (between Cys309 and Cys360). The cleavage event is prevented by the deletion through in vitro mutagenesis of the Arg312-Gln-Asn-Lys tetrapeptide or by its substitution with the aligned α residues (Gly-Ser-Glu-Pro). Reintroduction of either Arg312 or Lys315 reinstates the processing. Furthermore, we show that this area is not involved in lysosomal targeting of pro-Hex B, or in the increased stability or the variation in substrate specificity of the β as compared with the a subunit. Our data suggest the presence of a novel lysosomal endoprotease. Like other endoproteases it is specific for basic amino acids; however, it cleaves on the amino-terminal side rather than the conventional carboxy-terminal side of such residues and then only if they are fully exposed to the lysosomal environment. These data are consistent with the idea that the partially degraded enzyme resulting from exposure to the harsh environment of the lysosome is the resistant, but still functional product of the protein's adaptation to its lysosomal role. However, the residues that are removed appear to have been necessary for the initial folding and disulfide bond formation of the propolypeptides in the endoplasmic reticulum.Key words: lysosomal, glycosidase, endoprotease, maturation, transport.

This publication has 0 references indexed in Scilit: