Discovery of human Golgi β-galactosidase with no identified glycosidase using a QMC substrate design platform for exo-glycosidase.

Post-translational modifications (PTMs) of proteins play important roles in the physiology of eukaryotes. In the PTMs, non-reversible glycosylations are classified as N-glycosylations and O-glycosylations, and are catalyzed by various glycosidases and glycosyltransferases. However, β-glycosidases are not known to play a role in N- and O-glycan processing, although both glycans provide partial structures as substrates for β-galactosidase and β-N-acetylglucosaminidase in the Golgi apparatus of human cells. We explored human Golgi β-galactosidase using fluorescent substrates based on a quinone methide cleavage (QMC) substrate design platform that was previously developed to image exo-type glycosidases in living cells. As a result, we discovered a novel Golgi β-galactosidase in human cells. It is possible to predict a novel and important function in glycan processing of this β-galactosidase, because various β-galactosyl linkages in N- and O-glycans exist in Golgi apparatus. In addition, these results show that the QMC platform is excellent for imaging exo-type glycosidases.