Diatom Vacuolar 1,6-β-Transglycosylases can Functionally Complement the Respective Yeast Mutants

Diatoms are unicellular photoautotrophic algae, which can be found in any aquatic habitat. The main storage carbohydrate of diatoms is chrysolaminarin, a nonlinear beta-glucan, consisting of a linear 1,3-beta-chain with 1,6-beta-branches, which is stored in cytoplasmic vacuoles. The metabolic pathways of chrysolaminarin synthesis in diatoms are poorly investigated, therefore we studied two potential 1,6-beta-transglycosylases (TGS) of the diatom Phaeodactylum tricornutum which are similar to yeast Kre6 proteins and which potentially are involved in the branching of 1,3-beta-glucan chains by adding D-glucose as 1,6-side chains. We genetically fused the full-length diatom TGS proteins to GFP and expressed these constructs in P. tricornutum, demonstrating that the enzymes are apparently located in the vacuoles, which indicates that branching of chrysolaminarin may occur in these organelles. Furthermore, we demonstrated the functionality of the diatom enzymes by expressing TGS1 and 2 proteins in yeast, which resulted in a partial complementation of growth deficiencies of a transglycosylase-deficient Delta kre6 yeast strain.