A structural road map to unveil basal body composition and assembly

The Basal Body (BB) acts as the template for the axoneme, the microtubule‐basedstructure of cilia and flagella. Although several proteins were recently implicatedin both centriole and BB assembly and function, their molecular mechanisms are stillpoorly characterized. In this issue of The EMBO journal, Li and coworkersdescribe for the first time the near‐native structure of the BB at 33 Åresolution obtained by Cryo‐Electron Microscopy analysis of wild‐type (WT) isolatedChlamydomonas BBs. They identified several uncharacterized non‐tubulinstructures and variations along the length of the BB, which likely reflect thebinding and function of numerous macromolecular complexes. These complexes areexpected to define BB intrinsic properties, such as its characteristic structure andstability. Similarly to the high‐resolution structures of ribosome and nuclear porecomplexes, this study will undoubtedly contribute towards the future analysis ofcentriole and BB biogenesis, maintenance and function.

The microtubule (MT)‐based structure of the cilium/flagellum grows from the distal partof the Basal Body (BB), which in many animal cells develops from the mature centriole inthe centrosome. Electron microscopic (EM) images of chemically fixed resin‐embeddedcentrioles and basal bodies (CBBs) suggest that their ultrastructure is similar, andthat their key components are MTs. The mechanisms underlying the organization of CBBMTs, comprising highly stable closed and open MTs, are likely to hold many surprises asthey are remarkably different from other microtubular structures in the cell.Additionally, non‐MT‐based structures are also part of the CBB, including a cartwheel inthe proximal lumen region that reinforces CBB symmetry (reviewed in Azimzadeh and Marshall, 2010 and Carvalho‐Santos etal, 2011).

Several centriole components and BB proteins were identified by comparative and/orfunctional genomics and proteomics studies of purified CBBs (reviewed in Azimzadeh and Marshall, 2010 and Carvalho‐Santos et al, 2011). Advances in our understanding of themolecular mechanisms of CBB assembly depend on high‐resolution comparative studies ofwild‐type (WT) and mutant structures, as well as characterization of the localization ofmolecular complexes within the small CBB structure. Despite the existence of beautifulultrastructure data acquired from chemically fixed specimens (Geimerand Melkonian, 2004; Ibrahim et al,2009), high‐resolution structures of native CBBs were missing. Usingelectron cryo‐tomography and 3D subtomogram averaging, Li et al (2012) solved thestructure of the near‐native BB triplet at 33 Å resolution. A pseudo‐atomic modelof the tubulin protofilaments at the core of the triplets was built by fitting theatomic structure of α/β‐tubulin monomers into the BB tomograms.

The 3D density map reveals several additional densities that represent non‐tubulinproteins attached, both internally and externally, to all triplet MTs, some linking MTsinside the triplets and/or MTs in consecutive triplets (Li et al, 2012; for asummary, see Table I). These structures are likely composed ofseveral proteins that have previously been isolated with CBBs. A Y‐shaped structure andlarge rod‐shaped structures emanate from the triplet A/B‐ and C‐tubules, respectively,and extend towards the BB central lumen. Possibly, these large inner circular structuresin the BB lumen function as a scaffold that stabilizes the entire BB barrel (Li etal, 2012; Table I). Linker structures had been observedbefore (Geimer and Melkonian, 2004; Ibrahim et al, 2009), but with less detail and complexity. Theauthors speculate that some of the additional densities present at the A‐ and B‐tubuleinner wall might correspond to proteins of the tektin family, probably conferringrigidity to the BB triplet (Amos, 2008).

Table 1. Characteristics of the non‐α/β‐tubulin structures reported in Liet al (2012) in this issue of The EMBO journal

The authors also show that the BB proximal and distal structures are significantlydifferent. The majority of the changes are confined to (1) the C‐tubule, (2) linkersbetween the adjacent triplets and (3) the twist angle of the triplets along the BBlength (Li et al, 2012; Table I; Figure 1). It is possible that together with thecartwheel, the linkers between consecutive triplets contribute to establishing andreinforcing the CBB nine‐fold symmetry, by defining the angles between triplets and inconsequence the available space to fit these MTs. The authors also propose that thestructural variations along the length of the BB suggest a sequential and coordinated BBassembly process. It will be important to obtain high‐resolution structures of thegrowing WT CBB and of mutants in genes associated with CBB stability and elongation,such as δ‐tubulin, POC5, CPAP, POC1 and Bld10 (reviewed in Azimzadeh and Marshall, 2010 and Carvalho‐Santos etal, 2011) to complement previous work (Pelletieret al, 2006; Guichard et al,2010) and to unveil CBB assembly mechanisms.

A comparison of the BB structure with that of the axoneme (resolved at 30 Å;Sui and Downing, 2006) revealed that the distributionof the accessory structures on the outer and inner surface of the A‐ and B‐tubules ofthe BB triplet are different from the axonemal doublet MTs for which they serve astemplate (Li et al, 2012). It will be important in the future to understand whatthose differences mean for CBB and axoneme function, including links with pericentriolarcomponents and motility.

The high‐resolution structure of ribosome and nuclear pore complexes, solved by singleparticle reconstruction electron cryo‐tomography, contributed immensely to our knowledgeon these organelles assembly and function (reviewed in Ramakrishnan,2009 and Ben‐Harush et al, 2010). TheBB high‐resolution structural analysis reported in this article (Li et al, 2012)will certainly pave the road for the identification of essential non‐MT BB components,and allow us to understand their molecular role in the context of CBB biogenesis,maintenance and function.

Conflict of Interest

The authors declare that they have no conflict of interest.