A structural road map to unveil basal body composition and assembly |
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Authors: | Jana Swadhin C Machado Pedro Bettencourt-Dias Mónica |
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Institution: | Instituto Gulbenkian de Ciência, Oeiras, Portugal. |
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Abstract: | EMBO J
31
3, 552–562 (2012);
published online December132011The Basal Body (BB) acts as the template for the axoneme, the microtubule-based
structure of cilia and flagella. Although several proteins were recently implicated
in both centriole and BB assembly and function, their molecular mechanisms are still
poorly characterized. In this issue of The EMBO journal, Li and coworkers
describe for the first time the near-native structure of the BB at 33 Å
resolution obtained by Cryo-Electron Microscopy analysis of wild-type (WT) isolated
Chlamydomonas BBs. They identified several uncharacterized non-tubulin
structures and variations along the length of the BB, which likely reflect the
binding and function of numerous macromolecular complexes. These complexes are
expected to define BB intrinsic properties, such as its characteristic structure and
stability. Similarly to the high-resolution structures of ribosome and nuclear pore
complexes, this study will undoubtedly contribute towards the future analysis of
centriole and BB biogenesis, maintenance and function.The microtubule (MT)-based structure of the cilium/flagellum grows from the distal part
of the Basal Body (BB), which in many animal cells develops from the mature centriole in
the centrosome. Electron microscopic (EM) images of chemically fixed resin-embedded
centrioles and basal bodies (CBBs) suggest that their ultrastructure is similar, and
that their key components are MTs. The mechanisms underlying the organization of CBB
MTs, comprising highly stable closed and open MTs, are likely to hold many surprises as
they are remarkably different from other microtubular structures in the cell.
Additionally, non-MT-based structures are also part of the CBB, including a cartwheel in
the proximal lumen region that reinforces CBB symmetry (reviewed in Azimzadeh and Marshall, 2010 and Carvalho-Santos et
al, 2011).Several centriole components and BB proteins were identified by comparative and/or
functional genomics and proteomics studies of purified CBBs (reviewed in Azimzadeh and Marshall, 2010 and Carvalho-Santos et al, 2011). Advances in our understanding of the
molecular mechanisms of CBB assembly depend on high-resolution comparative studies of
wild-type (WT) and mutant structures, as well as characterization of the localization of
molecular complexes within the small CBB structure. Despite the existence of beautiful
ultrastructure data acquired from chemically fixed specimens (Geimer
and Melkonian, 2004; Ibrahim et al,
2009), high-resolution structures of native CBBs were missing. Using
electron cryo-tomography and 3D subtomogram averaging, Li et al (2012) solved the
structure of the near-native BB triplet at 33 Å resolution. A pseudo-atomic model
of the tubulin protofilaments at the core of the triplets was built by fitting the
atomic structure of α/β-tubulin monomers into the BB tomograms.The 3D density map reveals several additional densities that represent non-tubulin
proteins attached, both internally and externally, to all triplet MTs, some linking MTs
inside the triplets and/or MTs in consecutive triplets (Li et al, 2012; for a
summary, see ; ; Ibrahim et al, 2009), but with less detail and complexity. The
authors speculate that some of the additional densities present at the A- and B-tubule
inner wall might correspond to proteins of the tektin family, probably conferring
rigidity to the BB triplet (Amos, 2008).Table 1Characteristics of the non-α/β-tubulin structures reported in Li
et al (2012) in this issue of The EMBO journalOpen in a separate windowThe authors also show that the BB proximal and distal structures are significantly
different. The majority of the changes are confined to (1) the C-tubule, (2) linkers
between the adjacent triplets and (3) the twist angle of the triplets along the BB
length (Li et al, 2012; ). It is possible that together with the
cartwheel, the linkers between consecutive triplets contribute to establishing and
reinforcing the CBB nine-fold symmetry, by defining the angles between triplets and in
consequence the available space to fit these MTs. The authors also propose that the
structural variations along the length of the BB suggest a sequential and coordinated BB
assembly process. It will be important to obtain high-resolution structures of the
growing 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 et
al, 2011) to complement previous work (Pelletier
et al, 2006; Guichard et al,
2010) and to unveil CBB assembly mechanisms.Open in a separate windowProximal and distal views of the reconstructed basal body model. MT triplets
are represented in blue and non-tubulin proteins attached to the triplets
are represented in yellow. Note the structural differences between the
proximal and distal regions of the BB at the level of the C-tubule and
non-tubulin structures. Lower images represent 3 × magnified view of
the box marked area; white arrowheads—indicate the changes in the
C-Tubule configuration; black arrowheads—indicate changes in the
non-MT structures. Distal view is mirrored to facilitate the comparison with
proximal view. Images were kindly provided by Sam Li.A comparison of the BB structure with that of the axoneme (resolved at 30 Å;
Sui and Downing, 2006) revealed that the distribution
of the accessory structures on the outer and inner surface of the A- and B-tubules of
the BB triplet are different from the axonemal doublet MTs for which they serve as
template (Li et al, 2012). It will be important in the future to understand what
those differences mean for CBB and axoneme function, including links with pericentriolar
components and motility.The high-resolution structure of ribosome and nuclear pore complexes, solved by single
particle reconstruction electron cryo-tomography, contributed immensely to our knowledge
on these organelles assembly and function (reviewed in Ramakrishnan,
2009 and Ben-Harush et al, 2010). The
BB 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. |
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