Glucan synthase-like 2 is indispensable for paramylon synthesis in Euglena gracilis

The phytoflagellate Euglena gracilis produces a large amount of paramylon (PM), a conglomerate of liner β-1,3-glucan chains, as a storage polysaccharide. PM is synthesized from uridine diphosphate-glucose, but its mechanism of formation is largely unknown. Two enzymes, glucan synthase-like (EgGSL) 1 and EgGSL2 were previously identified as candidates for PM synthesis in a Euglena transcriptome analysis. Here, we performed a reverse genetic analysis on these enzymes. Knockdown of EgGSL2, but not EgGSL1, significantly inhibits PM accumulation in Euglena cells. Additionally, β-1,3-glucan synthesis is detected in a PM-associated membrane fraction extracted from Euglena cells. Our findings indicate that EgGSL2 is the predominant enzyme for PM biosynthesis.