In centrioles, what type of fibers connect the triplet microtubules?
Winey M, O'Toole E. 2014. Centriole structure. Phil Trans R Soc B 369:20130457


  Cross-sectional images of centrioles and basal bodies show a linker between the A- and C-tubules in many different organisms... Surprisingly, candidate molecules serving as constituents of these linkers have not yet been identified.


That's as of 2014. I don't have the time right now, but I'll try at some point to find if there are more recent developments.