| Abstrakt: |
A single point mutation in the lysosomal proenzyme receptor-inhibiting sequence near the N terminus of mouse procathepsin L can result in glycosylation of a normally cryptic site near its C terminus. When alanine replaced His36, Arg38, or Tyr40, the nascent chain of the mutant protein cotranslationally acquired a high mannose oligosaccharide chain at Asn268. In contrast, when alanine replaced Ser34, Arg37, or Leu39, this second carbohydrate chain was not added. This alternating pattern of abnormal glycosylation suggested that propeptide residues 36-40 normally assume an extended conformation having the side chains of residues 36, 38, and 40 facing in the same direction. When tyrosine conservatively replaced His36or lysine replaced Arg38, Asn268was not glycosylated. But the procathepsin L mutant having phenylalanine in place of Tyr40was glycosylated at Asn268, which indicates that the hydrogen bond between the hydroxyl group of Tyr40and the carboxylate group of Asp82is necessary for normal folding of the nascent proenzyme chain. |
