Maturation of alpha-mannosidase in Dictyostelium discoideum. Acquisition of endoglycosidase H resistance and sulfate.

alpha-Mannosidase from Dictyostelium discoideum is a heterogenous glycoprotein which is derived from a precursor as a result of proteolytic processing. Its oligosaccharides are phosphorylated and sulfated. We investigated the sulfation of the enzyme by means of pulse-chase labeling and specific immunoprecipitation followed by endoglycosidase H treatment and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The earliest detectable form of the precursor was shown to be glycosylated and sensitive to endoglycosidase H. With time some of its oligosaccharides were sulfated and became partially resistant to endoglycosidase H. In the same time period, the precursor was proteolytically cleaved, yielding four species with different molecular masses (46-58 X 10(3) daltons). When first generated each of these was sensitive to endoglycosidase H but with time the 54,000- and 58,000-dalton forms developed degrees of endoglycosidase H resistance. The fully mature cleaved forms all contained sulfate. Sulfate from pulse-labeled precursor could only be detected in two of the forms implying that sulfation of the others occurs either after precursor cleavage or before cleavage but subsequent to the pulse period. When secretion of precursor was triggered by starvation only the endoglycosidase H-resistant forms were secreted.