Abstract
Centrioles are conserved organelles fundamental for the organisation of microtubules in animal cells. Oligomerisation of the spindle assembly abnormal protein 6 (SAS-6) is an essential step in the centriole assembly process and may act as trigger for the formation of these organelles. SAS-6 oligomerisation is driven by two independent interfaces, comprising an extended coiled coil and a dimeric N-terminal globular domain. However, how SAS-6 oligomerisation is controlled remains unclear. Here, we show that in the Caenorhabditis elegans SAS-6, a segment of the N-terminal globular domain, unresolved in crystallographic structures, comprises a flexible loop that assists SAS-6 oligomerisation. Atomistic molecular dynamics simulations and nuclear magnetic resonance experiments suggest that transient interactions of this loop across the N-terminal dimerisation interface stabilise the SAS-6 oligomer. We discuss the possibilities presented by such flexible SAS-6 segments for the control of centriole formation.
Highlights
Centrioles are conserved organelles widespread in the eukaryotic kingdom[1,2,3]
SAS-6 is co-recruited to the site of centriole assembly and interacts with the protein SAS-5 in C. elegans[25,26,29], while in insects and vertebrates binding to SAS-6 is reported for the proteins Ana[230,31] and STIL32,33, respectively
The resulting Δ103–130 variant of CeSAS-6N remained folded and showed only minimal structural changes compared to the WT protein as judged by X-ray crystallography (Cα RMSD of 0.3 Å)[25]; nuclear magnetic resonance (NMR) 15N heteronuclear single quantum coherence (HSQC) spectra of CeSAS-6N variants showed a very high degree of similarity, suggesting limited long-range structural changes to this domain as a result of modifications (Supplemental Fig. 1)
Summary
Centrioles are conserved organelles widespread in the eukaryotic kingdom[1,2,3]. In animals, a pair of centrioles comprise the structured core of centrosomes, which direct formation of the microtubule network and the mitotic spindle during cell division[4,5]. Compared to the SAS-6 coiled-coil dimer, which spans hundreds of amino acids[18], the N-terminal dimer principally depends on the interaction of a single amino acid, I154 in C. elegans, with a hydrophobic cavity across the NN dimerisation interface[18,20] In this manner, it offers an attractive target for a relatively small, trigger-like molecular event to exert maximum influence on the oligomerisation propensity of SAS-6. We set out to explore the effect of this C. elegans SAS-6 segment on the protein properties
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