Lack of proteolytic processing of α‐L‐fucosidase in human skin fibroblasts

Acid hydrolases are synthesized as precursors that undergo several posttranslational modifications including proteolytic processing to a smaller mature enzyme. The amount of proteolytic processing varies for different acid hydrolases, and many details of the intracellular pathways are not known. The processing of α‐L‐fucosidase was distinguished from that of other acid hydrolases reported when studied in systematic pulse‐chase labeling experiments. Only one form of α‐L‐fucosidase, M_r 56,000‐57,000, was demonstrated intra‐ and extracellularly. Under the same conditions, N‐acetyl‐β‐D‐glucosaminidase was shown to be processed with several forms, as previously reported by Hasilik and Neufeld (1980a). To obtain these results, human skin fibroblasts were labeled metabolically with L‐[³H]leucine for periods of 20 min to 8 hr with varying periods of chase from 1 to 96 hr with nonradioactive L‐leucine. α‐L‐Fucosidase was immunoprecipitated by a polyclonal antibody from material extracted from cells and ammonium sulfate precipitated medium and was examined by polyacrylamide gel electrophoresis under denaturing conditions. N‐Acetyl‐β‐D‐glucosaminidase was examined with similar procedures and served as a control for the methods. Tunicamycin treatment of the cells was used to show that glycosylation did not obscure proteolytic processing because, again, only one form of the intra‐ and extracellular enzyme was observed, although of smaller size, M_r 52,000‐53,000. In addition, separation of the cells into prelysosomal and lysosomal fractions showed only one form of the enzyme. It is concluded that α‐L‐fucosidase does not undergo proteolytic processing in human skin fibroblasts in the usual manner described for other acid hydrolases.