Showalter Allan M, Basu Debarati
Department of Environmental and Plant Biology, Molecular and Cellular Biology Program, Ohio University , Athens, OH, USA.
Department of Biology, Washington University , St. Louis, MO, USA.
Commun Integr Biol. 2016 May 5;9(3):e1177687. doi: 10.1080/19420889.2016.1177687. eCollection 2016 May-Jun.
Arabinogalactan-proteins (AGPs) are ubiquitous cell wall components present throughout the plant kingdom. They are extensively post translationally modified by conversion of proline to hydroxyproline (Hyp) and by addition of arabinogalactan (AG) polysaccharides to Hyp residues. Two small gene subfamilies within the CAZy GT31 family, referred to as Hyp-galactosyltransferases (Hyp-GALTs and HPGTs), encode enzymes that specifically add galactose to AGP protein backbones as revealed by heterologous expression of the genes coupled with an in vitro enzyme assay and by biochemical characterization of the genetic knock-out mutants. Biochemical analysis of galt2galt5 double and hpgt1hpgt2hpgt3 triple knockout mutants revealed significant reductions in both AGP-specific Hyp-GALT activity and β-Gal-Yariv precipitable AGPs. Further analysis of these mutants demonstrated both overlapping and distinct pleiotropic growth and development phenotypes, indicating the important contributions of the carbohydrate moieties toward AGP function. Current research indicates that all 8 Hyp-GALT/HPGT genes encode enzymes that catalyze the initial step for AGP glycosylation and that AGP glycans play essential roles in plant growth and development.
阿拉伯半乳聚糖蛋白（AGPs）是植物界普遍存在的细胞壁成分。它们在翻译后会通过脯氨酸向羟脯氨酸（Hyp）的转化以及向Hyp残基添加阿拉伯半乳聚糖（AG）多糖而被广泛修饰。在碳水化合物活性酶GT31家族中有两个小基因亚家族，称为Hyp-半乳糖基转移酶（Hyp-GALTs和HPGTs），通过基因的异源表达结合体外酶分析以及基因敲除突变体的生化特性分析表明，它们编码的酶能特异性地将半乳糖添加到AGP蛋白主链上。对galt2galt5双突变体和hpgt1hpgt2hpgt3三突变体的生化分析表明，AGP特异性Hyp-GALT活性和β-Gal-Yariv可沉淀的AGPs均显著降低。对这些突变体的进一步分析显示，它们具有重叠且不同的多效性生长和发育表型，表明碳水化合物部分对AGP功能具有重要贡献。目前的研究表明，所有8个Hyp-GALT/HPGT基因都编码催化AGP糖基化第一步的酶，并且AGP聚糖在植物生长和发育中起着至关重要的作用。
