Proteolytic processing of the alpha-chain of the lysosomal enzyme, beta-hexosaminidase, in normal human fibroblasts

The two subunits of beta-hexosaminidase undergo many post-translational modifications characteristic of lysosomal proteins, including limited proteolysis. To identify proteolytic cleavage sites in the alpha-chain, we have biosynthetically radiolabeled the transient forms, isolated these by immunoprecipitation, gel electrophoresis, and electroelution, and subjected them to automated Edman degradation. The position of the NH2-terminal amino acid was inferred from the elution cycle of the radioactive amino acid and the primary sequence encoded in the alpha-chain cDNA. The amino terminus of the precursor obtained by in vitro translation of SP6 alpha-chain mRNA in the presence of microsomes was leucine 23. The same amino terminus was found in precursor alpha-chain synthesized by normal human fibroblasts (IMR90) in a 1- or 3-h pulse or secreted by these cells in the presence of NH4Cl. The alpha-chain isolated after a 3-h pulse followed by a 5-h chase (intermediate form) included a mixture of molecular species of which the amino terminus was arginine 87 (most abundant), histidine 88, or leucine 90. After a 20-h chase (mature form) the latter species predominated. This mature form of the alpha-chain remained fully reactive with antibody raised against the carboxyl-terminal 15 amino acids, indicating little if any proteolysis at the carboxyl terminus. Thus synthesis and maturation of the alpha-chain of beta-hexosaminidase includes two major proteolytic cleavages: the first, between alanine 22 and leucine 23, removes the signal peptide to generate the precursor form, whereas the second occurs between the dibasic amino acids, lysine 86 and arginine 87. The second cleavage is followed by trimming of 3 additional amino acids to give the mature form of the alpha-chain.