Fig. 3.

Identification of CatSper channels in Aurantiochytrium limacinum. (A) Schematic representation of mammalian spermatozoa. The CatSper channel complex is located at the principle piece that is critical for Ca²⁺-modulated sperm motility. (B) Distribution of CatSpers in select bikont species. Aurantiochytrium limacinum contains CatSper α subunits 1–4, whereas the exact number of CatSper α subunit in Cyanophora paradoxa is unknown due to incomplete genome assembly. (C) Phylogenetic relationship of CatSper α subunits. A maximum likelihood phylogenetic tree (LG + I + G + F model) showing the relationship of protist and animal homologs of CatSper α subunits 1–4 from the thraustochytrid protist A. limacinum (Ali), the amphioxus Branchiostoma floridae (Bfl), Homo sapiens (Hsa), Mus musculus (Mus), and the apusozoan protist Thecamonas trahens (Ttr). NaChBac, a prokaryotic voltage-gated Na⁺ channel isolated from Bacillus halodurans (Ren et al. 2001), was used as an outgroup. Bootstrap values above 60 are shown at the nodes. (D) Sequence alignment of the pore loop regions of NaChBac and CatSper α subunits from A. limacinum (Ali) and H. sapiens (Hsa). The asterisk symbol indicates the location of a key acidic residue important for ion selectivity. CatS, CatSper α subunit.