Comparative transcriptomics reveals the molecular toolkit used by an algivorous protist for cell wall perforation

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.1 Besides prey recognition and attachment, this complex behavior involves the local, pre-phagocytotic dissolution of the prey cell wall, which results in well-defined perforations of species-specific size and structure.2 Yet the molecular processes that enable protoplast feeders to overcome cell walls of diverse biochemical composition remain unknown. We used the flagellate Orciraptor agilis (Viridiraptoridae, Rhizaria) as a model protoplast feeder and applied differential gene expression analysis to examine its penetration of green algal cell walls. Besides distinct expression changes that reflect major cellular processes (e.g., locomotion and cell division), we found lytic carbohydrate-active enzymes that are highly expressed and upregulated during the attack on the alga. A putative endocellulase (family GH5_5) with a secretion signal is most prominent, and a potential key factor for cell wall dissolution. Other candidate enzymes (e.g., lytic polysaccharide monooxygenases) belong to families that are largely uncharacterized, emphasizing the potential of non-fungal microeukaryotes for enzyme exploration. Unexpectedly, we discovered various chitin-related factors that point to an unknown chitin metabolism in Orciraptor agilis, potentially also involved in the feeding process. Our findings provide first molecular insights into an important microbial feeding behavior and new directions for cell biology research on non-model eukaryotes.