Homology between O-linked GlcNAc transferases and proteins of the glycogenphosphorylase superfamily

The O-linked GlcNAc transferases (OGTs) are a recently characterized group of largely eukaryotic enzymes that add a single beta -N-acetylglucosamine m oiety to specific serine or threonine hydroxyls. In humans, this process ma y be part of a sugar regulation mechanism or cellular signaling pathway tha t is involved in many important diseases, such as diabetes, cancer, and neu rodegeneration. However, no structural information about the human OGT exis ts, except for the identification of tetratricopeptide repeats (TPR) at the N terminus. The locations of substrate binding sites are unknown and the s tructural basis for this enzyme's function is not clear. Here, remote homol ogy is reported between the OGTs and a large group of diverse sugar process ing enzymes, including proteins with known structure such as glycogen phosp horylase, UDP-GlcNAc 2-epimerase, and the glycosyl transferase MurG. This r elationship, in conjunction with an-Lino acid similarity spanning the entir e length of the sequence, implies that the fold of the human OGT consists o f two Rossmann-like domains C-terminal to the TPR region. A conserved motif in the second Rossmann domain points to the UDP-GlcNAc donor binding site. This conclusion is supported by a combination of statistically significant PSI-BLAST hits, consensus secondary structure predictions, and a fold reco gnition hit to MurG. Additionally, iterative PSI-BLAST database searches re veal that proteins homologous to the OGTs form a large and diverse superfam ily that is termed GPGTF (glycogen phosphorylase/glycosyl transferase). Up to one-third of the 51 functional families in the CAZY database, a glycosyl transferase classification scheme based on catalytic residue and sequence homology considerations, can be unified through this common predicted fold. GPGTF homologs constitute a substantial fraction of known proteins: 0.4% o f all non-redundant sequences and about 1% of proteins in the Escherichia c oli genome are found to belong to the GPGTF superfamily. (C) 2001 Academic Press.