Kiichiro Totani

Endoplasmic reticulum-associated degradation facilitates the elimination of misfolded glycoproteins through a sequential process involving deglycosylation by cytosolic PNGase, followed by the removal of GlcNAc residues by ENGase. Genetic mutations in NGLY1, the mammalian ortholog of PNGase, have been implicated in the rare congenital disorder known as NGLY1 deficiency. The pathology of this disorder is suggested to be partially due to N-GlcNAc proteins generated by ENGase activity. Consequently, the development of tools to detect PNGase and ENGase activities is crucial for advancing our understanding of related diseases. In established assay systems for PNGase activity, glycan probes incorporating cyclic peptides with protease resistance have been used. However, the cyclic peptide motif is associated with synthetic complexity owing to multiple reaction steps. In this study, we developed glycopeptide probes, Man9GlcNAc2-Asn(Fmoc)-Leu-Leu-R (R = Bn or OH), specifically designed to monitor PNGase and ENGase activities. The Man9GlcNAc2 moiety, a high-mannose-type oligosaccharide that is efficiently recognized by ENGase, was selected due to its ready availability from hen egg yolk. Additionally, the Asn-Leu-Leu sequence, which is easily synthesized and exhibits high affinity for PNGase, was chosen for its anticipated protease resistance, as it is a common motif in protease inhibitors. Both glycopeptide probes successfully detected the activities of purified PNGase and ENGase. Notably, Man9GlcNAc2-Asn(Fmoc)-Leu-Leu-OH enabled accurate activity detection even in crude rat liver lysates without peptide degradation. Furthermore, in wild-type and Ngly1 (Pngase)- or Engase-knockout cytosol from MEF cells, the probe demonstrated practical utility as a molecular tool in terms of degradation resistance and monitoring of enzymatic activities.