Structural basis of EHEP-mediated offense against phlorotannin-induced defense from brown algae to protectakuBGL activity

The defensive-offensive associations between algae and herbivores determine marine ecology. Brown algae utilize phlorotannin as their chemical defense against the predatorAplysia kurodai, which uses β-glucosidase (akuBGL) to digest the laminarin in algae to glucose. Moreover,A. kurodaiemploysEiseniahydrolysis-enhancing protein (EHEP) as an offense to protectakuBGL activity from phlorotannin inhibition by precipitating phlorotannin. To underpin the molecular mechanism of this digestive-defensive-offensive system, we determined the structures of apo and tannic-acid (TNA, a phlorotannin-analog) bound form of EHEP, as well as apoakuBGL. EHEP consisted of three peritrophin-A domains formed in a triangle and bound TNA in the center without significant conformational changes. Structural comparison between EHEP and EHEP–TNA led us to find that EHEP can be resolubilized from phlorotannin-precipitation at an alkaline pH, which reflects a requirement in the digestive tract.akuBGL contained two GH1 domains, only one of which conserved the active site. Combining docking analysis, we propose the mechanisms by which phlorotannin inhibitsakuBGL by occupying the substrate-binding pocket, and EHEP protectsakuBGL against the inhibition by binding with phlorotannin to free theakuBGL pocket.