Abstract
Eukaryotic cilia and flagella are evolutionarily conserved organelles involved in motility, signal transduction and disease. Their axoneme consists of a cylindrical array of 9 doublet microtubules (DMTs). The objective of this study was to analyze a specialized region of DMTs, i.e., “Ribbons” of 3-4 protofilaments that are stable after extraction with Sarkosyl detergent. Sea urchin sperm flagellar axonemes were analyzed for the distribution and stoichiometry of Ribbon-associated proteins by immuno-biochemistry and proteomics, and their structural arrangement by immuno-electron microscopy and immuno-/cryo-electron tomography. The proteins associated with Ribbons in a reproducible ratio include: acetylated α-tubulin, β-tubulin, conserved protein Rib45, >95% of the ciliary tektins, and >95% of two Ca2+-binding proteins, Rib74 and Rib85.5, whose human homologues are associated with juvenile myoclonic epilepsy. Ribbons can be further fractionated into filaments composed exclusively of tektins. Each DMT contains only one Ribbon, corresponding to protofilaments A11-12-13-1 of the ciliary A-tubule, with a prominent electron density on the side of the Ribbon facing the A-tubule lumen. Each Ribbon contains a single, 5-nm-diameter, hyperstable filament of equimolar tektins A, B and C. Diverse studies indicate that tektins interact with dynein and the nexin-dynein regulatory complex. Ribbon proteins are proposed to function in determining 3-dimensional ciliary architecture, stabilizing ciliary DMTs, and regulating ciliary motility.