Isolation and Identification of Glycosylphosphatidylinositol-Anchored Arabinogalactan Proteins and Novel β-Glucosyl Yariv-Reactive Proteins from Seeds of Rice (Oryza sativa)

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Abstract
Arabinogalactan proteins (AGPs) are highly glycosylated extracellular glycoproteins playing important roles in plant growth and development. We have previously reported the possibility that AGPs are involved in the induction of α-amylase by gibberellin (GA) in barley aleurone layers by using the β-glucosyl Yariv reagent (β-GlcY), which has been presumed to specifically bind AGPs. In this present study, we isolated β-GlcY-reactive proteins from rice bran rich in aleurone cells. The N-terminal sequences of classical AGP and AG peptides were determined from hydrophilic fractions obtained by reversed phase HPLC. Interestingly, a novel non-specific lipid transfer protein-like protein (OsLTPL1) and a novel early nodulin-like protein (OsENODL1) were also identified in the more hydrophobic fractions from HPLC as β-GlcY-reactive proteins. Expression analysis of the genes coding for these proteins was performed. While classical AGP, AG peptides and OsLTPL1 were expressed in various parts of rice, OsENODL1 showed temporally and spatially specific expression in the aleurone layers. This new β-GlcY-reactive protein is a promising candidate for the extracellular signaling factors of GA action in cereal seeds. Furthermore, the possibility that proteins with the AG glycomodule might react with β-GlcY may broaden the definition of AGPs.