Structure of the SAS-6 cartwheel hub from Leishmania major

Abstract

Centrioles are cylindrical cell organelles with a ninefold symmetric peripheral microtubule array that is essential to template cilia and flagella. They are built around a central cartwheel assembly that is organized through homo-oligomerization of the centriolar protein SAS-6, but whether SAS-6 self-assembly can dictate cartwheel and thereby centriole symmetry is unclear. Here we show that Leishmania major SAS-6 crystallizes as a 9-fold symmetric cartwheel and provide the X-ray structure of this assembly at a resolution of 3.5 Å. We furthermore demonstrate that oligomerization of Leishmania SAS-6 can be inhibited by a small molecule in vitro and provide indications for its binding site. Our results firmly establish that SAS-6 can impose cartwheel symmetry on its own and indicate how this process might occur mechanistically in vivo. Importantly, our data also provide a proof-of-principle that inhibition of SAS-6 oligomerization by small molecules is feasible.DOI: http://dx.doi.org/10.7554/eLife.01812.001

Self-Assembly of Open Protein Systems: A Comprehensive View Based onthe Interactions between 3d Hydrophobic and Electric Dipole MomentVectors

“…e), f). Procentriole assembly, PDBid: 4CKP, [33,34]. According to these authors, this ring interacts with others both by stacking and by lateral association.…”

Coupling Form and Function: How the Oligomerisation Symmetry of the SAS-6 Protein Contributes to the Architecture of Centriole Organelles

“…X-ray crystallography studies showed that SAS-6 comprises a globular N-terminal domain followed by a long coiled coil ( Figure 1) and a C-terminal disordered region. SAS-6 strongly dimerises in vitro and in vivo via its coiled-coil domain, and protein dimers further interact via their N-terminal domains to form oligomers with 9-fold radial symmetry on average [28][29][30][31]. The SAS-6 N-terminal domain bears strong structural resemblance to similar domains of XRCC4 and XLF (Figure 2), proteins involved in DNA repair and non-homologous end joining.…”

“…For example, there exist three crystallographic models of the CrSAS-6 NN interface and one of the CrSAS-6 CC interface [31]; thus, three different oligomer variants can be built depending on the NN interface model employed resulting in 9.6-to 10.1-fold radial symmetry. Similarly, when DrSAS-6 oligomers are built from available crystal structures a variation in oligomer architecture is observed, with DrSAS-6 forming either an 8-fold symmetric shallow spiral or an 8-fold symmetric ring [29], Most SAS-6 oligomers resolved to date are highly reminiscent of cartwheel rings, with a central hub derived from the N-terminal globular domains and the SAS-6 coiled coils comprising the cartwheel spokes [28,29,31]. Furthermore, disruption of SAS-6 oligomerisation abrogates formation of the central centriole scaffold and strongly hinders organelle assembly [29,31].…”

“…For example, Caenorhabditis elegans SAS-6 (CeSAS-6) self-assembles into a spiral [30] rather than the planar rings observed in the Chlamydomonas reinhardtii (CrSAS-6; [31]), Danio rerio (DrSAS-6; [29]) and Leishmania major (LmSAS-6; [28]) variants of the same protein. Furthermore, there is substantial heterogeneity in the intrinsic symmetry of SAS-6 oligomers, both inferred by X-ray crystallography and directly visualised by microscopy.…”

“…Further, similar to SAS-6, XRCC4 forms dimers via a coiled-coil domain, and it hetero-oligomerises with XLF via the globular N-terminal domains to form a spiral [32][33][34][35][36]. Most SAS-6 oligomers resolved to date are highly reminiscent of cartwheel rings, with a central hub derived from the N-terminal globular domains and the SAS-6 coiled coils comprising the cartwheel spokes [28,29,31]. Furthermore, disruption of SAS-6 oligomerisation abrogates formation of the central centriole scaffold and strongly hinders organelle assembly [29,31].…”