![]() Immunocytochemistry with a polyclonal antibody raised against the GH5_5 domain revealed that the native endocellulase localises to the contact zone of Orciraptor and the algal cell wall (= perforation zone) and to intracellular granules, which were enriched during attack. Crystalline cellulose was not digested by the enzyme, which suggests a typical endocellulase activity. The GH5_5 catalytic domain from Orciraptor showed pronounced activity on soluble cellulose derivatives and mixed-linkage glucans, with reaction optima comparable to known GH5_5 representatives. To assess the importance of this carbohydrate-active enzyme in the feeding act of Orciraptor, we recombinantly produced its catalytic domain and studied the enzymatic activity, cellular localisation and function. Differential expression analyses of the algivorous flagellate Orciraptor agilis (Viridiraptoridae, Cercozoa, Rhizaria) suggested the involvement of a highly expressed putative glycoside hydrolase of family GH5_5. Although their fascinating feeding behaviour has been observed for the last 150 years, it is still unknown how protoplast feeders produce the well-defined and species-specific perforations in biochemically diverse cell walls. These phagotrophic “protoplast feeders” represent an interesting mechanistic intermediate between predators and parasites and pose a number of cell biological questions. Several protists have evolved the ability to perforate the cell walls of algae and fungi to specifically feed on their cell contents.
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