戸谷 希一郎

Oligosaccharide parts of glycoproteins play important roles in a variety of biological events. In particular, the functions of asparagine (Asn)-linked oligosaccharides in ER glycoprotein quality control are attracting recent attention (Figure 1). Natural glycoproteins usually exist as a mixture of various glycoforms that differ in the structures of oligosaccharides. In order to understand oligosaccharide functions in ER glycoprotein quality control system precisely, molecular probes and glycoprotein probes having homogeneous and structurally defined oligosaccharide are desired. We wish to report the synthesis of novel ER-related N-glycan-methotrexate (MTX) conjugates (1)-(5) as functional oligosaccharide probes (Figure 2). MTX is known to be a strong inhibitor of dihydrofolate reductase (DHFR) (K_D<1nM) and its specific absorption at 304 nm may be useful as detectable tag. The tight binding between N-glycan-MTX and DHFR could be applicable to create N-glycan-grafted DHFR as an artificial glycoprotein molecular probe. Moreover, this approach was applied to create first bifunctional artificial glycoprotein (9a-c) by connecting G1M9-DHFR (8) and N-glycan-MTX (2)-(4) (Figure 3). To correlate the functions of glycan chains with their structures in glycoprotein quality control system, interaction analyses between 2-5 and UDP-Glc: glycoprotein glucosyl transferase (UGGT), which play an important role as a folding sensor by specifically glucosylating Man_9GlcNAc_2-glycans in incompletely folded glycoprotein in ER protein quality control system, were also examined. The unique substrate specificity of UGGT (Table 1), which accepts neither glycan chain (Man_9GlcNAc_2) nor corresponding short glycopeptides as substrates, have hampered the creation of small molecule oligosaccharide probes. We found that M9MTX (2) is first small molecule oligosaccharide probe being recognized by UGGT. Furthermore, the first chemical characterization of glucosylated product of UGGT reaction using HPLC, MALDI-TOF MS and NMR was achieved (Figure 4). Oligosaccharide specificities of UGGT reactions became apparent that M8(B)MTX (3) and M8(C)MTX (4) show 60% UGGT activities compared with M9MTX (2), though M8(B)GN1MTX (5) is completely unreacted.