Expression of Tailored α-N-Acetylglucosaminidase in Escherichia coli for Synthesizing Mannose-6-Phosphate on N-Linked Oligosaccharides of Lysosomal Enzymes

Lysosomal enzymes are synthesized as N-glycosylated glycoproteins with mannose-6-phosphate (M6P) moieties, which are responsible for their binding to M6P receptors and transporting to the lysosome. In the M6P biosynthetic pathway, a Man₈GlcNAc₂ glycoform is converted to M6P groups through two consecutive enzymatic reactions, including N-acetylglucosamine (GlcNAc)-1-phosphotransferase (GNPT), transferring GlcNAc-1-phosphate from UDP-GlcNAc to the C6 hydroxyl groups of mannose residues, and then, removal of the covering GlcNAc moiety from the GlcNAc-P-mannose phosphodiester was carried out using an α-N-acetylglucosaminidase (referred to as 'uncovering enzyme', UCE) in the <i>trans</i>-Golgi network (TGN). Here, we expressed differently tailored versions of the UCE, including four truncated variants, in <i>Escherichia coli</i>. The four variants with the signal peptide, transmembrane domain, propiece and cytoplasmic tail truncated, respectively, were purified by affinity chromatography, and their enzymatic activities were assayed using a UDP-Glo kit. By fusing a maltose-binding protein (MBP) in the N-terminus of the UCE variants, the fusion proteins could be soluble when expressed in <i>E. coli</i>. The highest concentration of the purified enzyme was 80.5 mg/L of fermentation broth. Furthermore, the UCE with the core catalytic domain exhibited the highest uncovering activity.