A developmentally controlled change in the post-translational modifications on the lysosomal α-mannosidase of the cellular slime mould Dictyostelium discoideum

During the development of the cellular slime mould Dictyostelium discoideum, a second form of a number of lysosomal enzymes begins to accumulate. The second ('late') form of these enzymes differs from the pre-existing ('early') form in post-translational modification. Pulse-chase experiments using [³⁵S]methione show that the late form of α-mannosidase-1 is made by synthesis de novo starting 8 h after the onset of development. These experiments show there is no interconversion between early and late forms in vivo. A one-dimensional peptide map indicated that the early and late forms of α-mannosidase have similar amino acid sequences. The two forms have a similar half-life in vivo when measured during the same period of development. Double-labelling studies were performed with ³⁵SO₄ and [³H]leucine or ³²PO₄ and [³H]leucine, and these studies indicated that the oligosaccharides present on the early form of α-mannosidase contained more sulphate and phosphate than did those on the late form. The early enzyme had a 10-fold higher ³⁵S/³H ratio and a 4-fold higher ³²P/³H ratio. Endocytosis experiments using early and late α-mannoside showed that the early form was efficiently taken up by human fibroblasts, whereas the late form was poorly endocytosed. This suggests that the late form lacks the mannose 6-phosphate residue required for efficient uptake.