Department of Biology
Technical University of Darmstadt, Germany
Abstract
Microbial eukaryotes display a stunning diversity of feeding strategies, ranging from generalist predators to highly specialized parasites. The unicellular "protoplast feeders" represent a fascinating mechanistic intermediate, as they penetrate other eukaryotic cells (algae and fungi) like some parasites but then devour their cell contents by phagocytosis. Besides prey recognition and attachment, this complex behaviour involves the local, pre-phagocytotic dissolution of the prey cell wall, which results in well-defined perforations of species-specific size and structure. Yet the molecular processes that enable protoplast feeders to overcome cell walls of diverse biochemical composition remain unknown. We apply Omics-informed cell biology to the algivorous protoplast feeder Orciraptor agilis (Viridiraptoridae, Rhizaria) to shed some light on proteins which are involved in prey cell recognition, attachment and cell wall dissolution. Several carbohydrate-active enzymes (CAZymes) are highly expressed and upregulated during the attack on the alga; most prominent is a putative glycoside hydrolase of family GH5_5. As demonstrated by enzyme assays, the catalytic domain of this protein acts on cellulose derivatives and appears to be a typical endocellulase. Immunocytochemistry with a polyclonal antibody reveals that the endocellulase localises to the contact zone of Orciraptor and the algal cell wall, and to intracellular granules that are enriched during attack. Furthermore, the anti-GH5_5 antibody applied to live cells significantly reduces the feeding success of Orciraptor and results in incomplete perforations of the algal walls. Our experimental data from comparative transcriptomics, enzyme assays, immunocytochemistry and inhibition experiments strongly suggest a key role of the GH5_5 endocellulase in the cell wall dissolution by Orciraptor agilis. With that, we provide the first evidence that the well-defined perforations produced by protoplast feeders are caused by the extracellular activity of carbohydrate-active enzymes. In addition, we discovered an unexpected chitin metabolism in Orciraptor, which poses new cell biological questions and is a resource of novel enzymes with potential relevance in biotechnology applications.
