Staged maturation converts a probasal body template complex into a nucleation-competent basal plate

The central pair (CP) of microtubules is essential for flagellar motility, yet the molecular events underpinning its nucleation remain poorly defined. Here, we report a comprehensive screen of flagellar transition zone proteins that identifies new central pair assembly proteins (CPAPs). Using expansion microscopy, we reveal a concentric architecture at the basal plate, with themelin and basalin forming nested punctae with radial symmetry surrounding a central core containing TZP103.8, TZP48 and {gamma}-tubulin. A subset of CPAPs pre-assemble at the probasal body, a precursor structure that has not yet built a flagellum, with themelin functioning as the architectural template and master recruiter, and TZP103.8 and {gamma}-tubulin IFT-dependant addition being the final step of basal plate activation. Strikingly, in the absence of TZP103.8, {gamma}-tubulin still reaches the basal plate but is insufficient for central pair nucleation. We propose the staged assembly of CPAPs progressively constructs a structural template that geometrically constrains {gamma}-tubulin into the positions and orientations required for CP nucleation.