Striated fibre assemblins localise in the feeding groove of the ‘typical excavate’ Paratrimastix pyriformis

Spatial arrangement of the cytoskeleton in the cells of protists has been used for decades for taxonomy and phylogenetic inference at various levels. In contrast, the protein composition of non-microtubular structures is mostly unknown. Exceptions are system I fibres in algae, which are built of striated fiber assemblins (SFA). Interestingly, SFAs are also components of a range of other, dissimilar structures, playing a role in the cortex of ciliates, cells division in apicomplexans, and adhesion of the parasite Giardia to the intestine. In a broad bioinformatic survey, we show the presence of three ancestral eukaryotic paralogues of SFA, and note that they are present in all ‗typical excavates‘ – simple flagellates bearing a ventral feeding groove. In one representative, Paratrimastix pyriformis, we localised one of the SFA paralogues using specific antibodies and expansion microscopy. We show that it co-localises specifically with structures attached to the basal body of the posterior flagellum, namely the right microtubular root, composite fibre, and B-fibre. As the morphology of the ‗typical excavates‘ may be ancestral to eukaryotes, we speculate that the role in the development or function of this feeding apparatus may be ancestral to the protein.