Arabinogalactan-like Glycoproteins from Ulva lactuca (Chlorophyta) Show Unique Features Compared to Land Plants AGPs

Arabinogalactan proteins (AGPs) encompass a diverse group of plant cell wall proteoglycans, which play an essential role in plant development, signaling, plant-microbe interactions, and many others. Although they are widely distributed throughout the plant kingdom and extensively studied, they remain largely unexplored in the lower plants, especially in seaweeds. Ulva species have high economic potential since various applications were previously described including bioremediation, biofuel production, and as a source of bioactive compounds. This article presents the first experimental confirmation of AGP-like glycoproteins in Ulva species and provides a simple extraction protocol of Ulva lactuca AGP-like glycoproteins, their partial characterization and unique comparison to scarcely described Solanum lycopersicum AGPs. The reactivity with primary anti-AGP antibodies as well as Yariv reagent showed a great variety between Ulva lactuca and Solanum lycopersicum AGP-like glycoproteins. While the amino acid analysis of the AGP-like glycoproteins purified by the β-d-glucosyl Yariv reagent showed a similarity between algal and land plant AGP-like glycoproteins, neutral saccharide analysis revealed unique glycosylation of the Ulva lactuca AGP-like glycoproteins. Surprisingly, arabinose and galactose were not the most prevalent monosaccharides and the most outstanding was the presence of 3-O-methyl-hexose, which has never been described in the AGPs. The exceptional structure of the Ulva lactuca AGP-like glycoproteins implies a specialized adaptation to the marine environment and might bring new insight into the evolution of the plant cell wall.