Glycosyltransferases of the GT8 Family

The higher plant genomes sequenced to date include numerous genes encoding proteins classified as belonging to CAZy family GT8. The large number and diversity of GT8 proteins in higher plants, which currently constitute more than 65% of the identified eukaryotic GT8 genes, highlight the importance of these proteins in plants. Here we summarize a detailed phylogenetic study of GT8 proteins from three monocot and four dicot plant genomes that clearly divides higher plant GT8 proteins into two distantly related sets of clades, many of which are further divided into statistically well - supported subclades. One set, the GAUT1 (GAlacturonosylTransferase1)-related family, includes the GAUT and GAUT-Like (GATL) proteins, comprising one proven galacturonsyltransferase and multiple additional members strongly implicated in the synthesis of pectins and xylan, two major types of polysaccharides present in plant cell walls. The second set, which includes Plant Glycogenin-like Starch Initiation Proteins (PGSIPs) and Galactinol Synthases (GolSs), appears not to be directly involved in plant cell wall synthesis. The PGSIPs have been suggested to play a role in priming starch biosynthesis, while the GolSs are key enzymes in the synthesis of the raffinose family of oligosaccharides that play important roles in plant responses to environmental stress. This chapter also summarizes data from biochemical, transcriptional, and mutational studies that provide additional insights into possible functions of higher plant GT8 proteins. However, much work is needed to fully define the roles of GT8 proteins in plants, particularly with respect to their enzymatic function in plant cell wall biosynthesis.