Centrioles are evolutionary conserved organelles that give rise to cilia and flagella as well as centrosomes. Centrioles display a characteristic ninefold symmetry imposed by the spindle assembly abnormal protein 6 (SAS-6) family. SAS-6 from Chlamydomonas reinhardtii and Danio rerio was shown to form ninefold symmetric, ring-shaped oligomers in vitro that were similar to the cartwheels observed in vivo during early steps of centriole assembly in most species. Here, we report crystallographic and EM analyses showing that, instead, Caenorhabotis elegans SAS-6 self-assembles into a spiral arrangement. Remarkably, we find that this spiral arrangement is also consistent with ninefold symmetry, suggesting that two distinct SAS-6 oligomerization architectures can direct the same output symmetry. Sequence analysis suggests that SAS-6 spirals are restricted to specific nematodes. This oligomeric arrangement may provide a structural basis for the presence of a central tube instead of a cartwheel during centriole assembly in these species.X-ray crystallography | electron microscopy | structure | centriolar architecture | SAS-5

Living organisms protect the genome against external influences by recognizing and repairing damaged DNA. A common source of gene mutation is the oxidized guanine, which undergoes base excision repair through cleavage of the glycosidic bond between the ribose and the nucleobase of the lesion. We unravel the repair mechanism utilized by bacterial glycosylase, MutM, using quantum-chemical calculations involving more than 1000 atoms of the catalytic site. In contrast to the base-protonated pathway currently favored in the literature, we show that the initial protonation of the lesion's ribose paves the way for an almost barrier-free glycosidic cleavage. The combination of theoretical and experimental data provides further insight into the selectivity and discrimination of MutM's binding site toward various substrates.

The ubiquitous occurrence of DNA damages renders its repair machinery a crucial requirement for the genomic stability and the survival of living organisms. Deficiencies in DNA repair can lead to carcinogenesis, Alzheimer, or Diabetes II, where increased amounts of oxidized DNA bases have been found in patients. Despite the highest mutation frequency among oxidized DNA bases, the base-excision repair process of oxidized and ring-opened guanine, FapydG (2,6-diamino-4-hydroxy-5-formamidopyrimidine), remained unclear since it is difficult to study experimentally. We use newly-developed linear-scaling quantum-chemical methods (QM) allowing us to include up to 700 QM-atoms and achieving size convergence. Instead of the widely assumed base-protonated pathway we find a ribose-protonated repair mechanism which explains experimental observations and shows strong evidence for a base-independent repair process. Our results also imply that discrimination must occur during recognition, prior to the binding within the active site.