Cilia are essential organelles that protrude from the cell body. Primary cilia act as biological sensors while motile cilia are essential to propel cells or move fluids. Cilia are made of a cylindrical, microtubule-based structure called the axoneme that contains nine microtubule doublets at the periphery. Motile cilia also have two microtubule singlets in the centre, known as the central pair. Cilia can be divided into different regions based on their morphology. The base is where the cilium is attached to the cell, the middle region constitutes the main part of the cilium, and the tip refers to the distal end. In most types of motile cilia, the ciliary tip is distinct from the rest of the cilium as it only contains the central pair. This region is of interest as it is thought to contain information on ciliary length regulation and ciliary growth, two processes that are not well understood. We used cryo-electron microscopy and tomography to obtain the structure of the central pair in the ciliary tip of the ciliate Tetrahymena thermophila.
​We identified de novo seven unique proteins binding to the microtubules of the central pair, including SPEF1, a conserved microtubule-binding protein.
We show that SPEF1 stabilises microtubules through a unique binding mode and is essential for correct ciliary assembly. Together these data provide the first structure of the tip central pair and shed light on its assembly.