Conserved Structural Motifs of the Central Pair Complex Projections in Eukaryotic Flagella

Flagellar/ciliary motility requires the coordination of thousands of axonemal dyneins. This coordination is achieved in part by the central pair complex (CPC). The CPC is composed of two singlet microtubules, each with a set of associated projections and is located in the center of the axoneme, surrounded by the nine doublet microtubules. Its role as a key regulator of motility is demonstrated by the paralyzed flagella phenotype exhibited by many CPC-defective mutants. Our current knowledge of the CPC’s structure has come primarily from studies utilizing classical electron microscopy (EM) techniques as well as 2D EM averaging. Although we have learned a great deal about the general structure of the CPC, these imaging techniques do not represent the native state of the CPC and provide little to no 3D structural information. Using state-of-the-art cryo-electron tomography (cryo-ET) coupled with subtomogram averaging we have obtained a higher resolution, in situ structure of the CPC from Chlamydomonas and S. purpuratus. Our results show that the CPC projections are highly conserved between these two distantly related organisms, lending further credence to the CPC’s essential role in flagellar/ciliary motility.