Comprehensive analysis of glycosyltransferases in eukaryotic genomes for structural and functional characterization of glycans

Glycosyltransferases comprise highly divergent groups of enzymes, which play a central role in the synthesis of complex glycans. Because the repertoire of glycosyltransferases in the genome determines the range of synthesizable glycans, and because the increasing amount of genome sequence data is now available, it is essential to examine these enzymes across organisms to explore possible structures and functions of the glycoconjugates. In this study, we systematically investigated 36 eukaryotic genomes and obtained 3426 glycosyltransferase homologs for biosynthesis of major glycans, classified into 53 families based on sequence similarity. The families were further grouped into six functional categories based on the biosynthetic pathways, which revealed characteristic patterns among organism groups in the degree of conservation and in the number of paralogs. The results also revealed a strong correlation between the number of glycosyltransferases and the number of coding genes in each genome. We then predicted the ability to synthesize major glycan structures including N-glycan precursors and GPI-anchors in each organism from the combination of the glycosyltransferase families. This indicates that not only parasitic protists but also some algae are likely to synthesize smaller structures than the structures known to be conserved among a wide range of eukaryotes. Finally we discuss the functions of two large families, sialyltransferases and β4-glycosyltransferases, by performing finer classifications into subfamilies. Our findings suggest that universality and diversity of glycans originate from two types of evolution of glycosyltransferase families, namely conserved families with few paralogs and diverged families with many paralogs.