Centrioles are cylindrical, ninefold symmetrical structures with peripheral triplet microtubules strictly required to template cilia and flagella. The highly conserved protein SAS-6 constitutes the center of the cartwheel assembly that scaffolds centrioles early in their biogenesis. We determined the x-ray structure of the amino-terminal domain of SAS-6 from zebrafish, and we show that recombinant SAS-6 self-associates in vitro into assemblies that resemble cartwheel centers. Point mutations are consistent with the notion that centriole formation in vivo depends on the interactions that define the self-assemblies observed here. Thus, these interactions are probably essential to the structural organization of cartwheel centers.

No abstract

Human CA150, a transcriptional activator, binds to and is co-deposited with huntingtin during Huntington's disease. The second WW domain of CA150 is a three-stranded β-sheet that folds in vitro in microseconds and forms amyloid fibers under physiological conditions. We found from exhaustive alanine scanning studies that fibrillation of this WW domain begins from its denatured conformations, and we identified a subset of residues critical for fibril formation. We used high-resolution magic-angle-spinning NMR studies on site-specific isotopically labeled fibrils to identify abundant long-range interactions between side chains. The distribution of critical residues identified by the alanine scanning and NMR spectroscopy, along with the electron microscopy data, revealed the protofilament repeat unit: a 26-residue nonnative β-hairpin. The structure we report has similarities to the hairpin formed by the A β (1–40) protofilament, yet also contains closely packed side-chains in a “steric zipper” arrangement found in the cross-β spine formed from small peptides from the Sup35 prion protein. Fibrillation of unrelated amyloidogenic sequences shows the common feature of zippered repeat units that act as templates for fiber elongation.