Novel features of centriole polarity and cartwheel stacking revealed by cryo-tomography.

Centrioles are polarized microtubule-based organelles that seed the formation of cilia, and which assemble from a cartwheel containing stacked ring oligomers of SAS-6 proteins. A cryo-tomography map of centrioles from the termite flagellate Trichonympha spp. was obtained previously, but higher resolution analysis is likely to reveal novel features. Using sub-tomogram averaging (STA) in T. spp. and Trichonympha agilis, we delineate the architecture of centriolar microtubules, pinhead, and A-C linker. Moreover, we report ~25 Å resolution maps of the central cartwheel, revealing notably polarized cartwheel inner densities (CID). Furthermore, STA of centrioles from the distant flagellate Teranympha mirabilis uncovers similar cartwheel architecture and a distinct filamentous CID. Fitting the CrSAS-6 crystal structure into the flagellate maps and analyzing cartwheels generated in vitro indicate that SAS-6 rings can directly stack onto one another in two alternating configurations: with a slight rotational offset and in register. Overall, improved STA maps in three flagellates enabled us to unravel novel architectural features, including of centriole polarity and cartwheel stacking, thus setting the stage for an accelerated elucidation of underlying assembly mechanisms.