Arabinogalactan Glycosyltransferases: Enzyme Assay, Protein-Protein Interaction, Subcellular Localization, and Perspectives for Application

Arabinogalactan proteins (AGPs) are abundant extracellular proteoglycans that are found in most plant species and involved in many cellular processes, such as cell proliferation and survival, pattern formation, and growth, and in plant microbe interaction. AGPs are synthesized by posttranslational O-glycosylation of proteins and attached glycan part often constitutes greater than 90% of the molecule. Subtle altered glycan structures during development have been considered to function as developmental markers on the cell surface, but little is known concerning the molecular mechanisms. My group has been working on glycosylation enzymes (glycosyltransferases) of AGPs to investigate glycan function of the molecule. This review summarizes the recent findings from my group as for AtGalT31A, AtGlcAT14A-C, and AtGalT29A of Arabidopsis thaliana with a specific focus on the (i) biochemical enzyme activities; (ii) subcellular compartments targeted by the glycosyltransferases; and (iii) protein-protein interactions. I also discuss application aspect of glycosyltransferase in improving AGP-based product used in industry, for example, gum arabic. 1. Introduction Arabinogalactan proteins (AGPs, AG proteins) are plant-specific proteoglycans that are found in most of the analyzed plant species. The main component of AGPs is AG glycan, which often constitutes greater than 90% of the molecule. Studies using monoclonal antibodies recognizing different epitopes of AG glycans have demonstrated a temporal and a spatial appearance of certain subpopulations of AG glycans during plant development (for review, see [1]). In animal cells, the structural diversities of cell-surface proteoglycans underlie their various functions, for example, interaction with hormone, extracellular matrix components, and enzymes, and play an important role in signal transduction and/or cell-cell communication and adhesion (for review, see [2]). Analogous mode of action is expected for a specific AG subpopulation during plant development, but little is known regarding the molecular mechanisms. My group has been working on the biosynthetic enzymes of AG glycans to understand the developmentally regulated biosynthesis of AG glycans. AG glycans are synthesized on core proteins by posttranslational modifications (O-glycosylation). The structure of AG glycans is complex and heterogeneous, changes throughout development, and commonly consists of β-1,3-linked galactans as main chains that are substituted with various lengths of β-1,6-galactan side chains, which are further decorated by arabinose and other