Mod A: a post-translational mutation affecting phosphorylated and sulfated glycopeptides in Dictyostelium discoideum.

The Mod A mutation in Dictyostelium discoideum results in a post-translational modification which reduces the activity and electrophoretic mobility of a group of lysosomal glycoproteins. To determine whether this mutation might affect protein bound oligosaccharides, metabolically labeled [2] 3H-mannose glycopeptides were isolated from wild-type (AX3) and mutant cells (M31) of Dictyostelium discoideum. A group of large, negatively charged glycopeptides are significantly depleted in strain M31 compared to AX3. Cells of each strain double labeled with 3H-mannose and 35SO4 or 32PO4 showed that the large, negatively charged glycopeptides of AX3 contain both sulfate and phosphate while those of M31 are depleted in these groups. The kinetics of 35SO4 release from the glycopeptides of each strain suggested that both contained similar sulfated sugar(s), but that M31 glycopeptides contained three-fold less than those of AX3. Acid hydrolysis of 32PO4 containing 3H-mannose glycopeptides showed the presence of 3H-mannose-6-32-P-phosphate in the AX3 hydrolysates while the glycopeptides of M31 contain only 15% as much mannose-6-phosphate as those of AX3.