Basalin: an evolutionary unconstrained protein revealed via a conserved role in basal plate function

Most motile flagella have an axoneme that contains nine outer microtubule doublets and a central pair (CP) of microtubules. The CP is thought to coordinate the flagellar beat and defects in CP projections are associated with loss of motility and human disease. In most cilia, the CP nucleate near a basal plate at the distal end of the transition zone (TZ). Here, we show that the trypanosome TZ protein basalin is essential for building the basal plate, and its loss is associated with inefficient recruitment of CP assembly factors to the TZ, loss of the CP and flagellum paralysis. Guided by synteny, we identified highly divergent basalin orthologs in the genomes of related Leishmania species. Basalins are predicted to be highly unstructured, suggesting that they may act as hubs facilitating many protein-protein interactions. This raises the general concept that proteins involved in cytoskeletal functions and apparently appearing organism-specific, may have highly divergent and cryptic orthologs in other species.