The SAS-5 N-terminal domain is a tetramer, with implications for centriole assembly in C. elegans

The centriole is a conserved microtubule-based organelle essential for both centrosome formation and cilium biogenesis. It has a unique 9-fold symmetry and its assembly is governed by at least five component proteins (SPD-2, ZYG-1, SAS-5, SAS-6 and SAS-4), which are recruited in a hierarchical order. Recently published structural studies of the SAS-6 N-terminal domain have greatly advanced our understanding of the mechanisms of centriole assembly. However, it remains unclear how the weak interaction between the SAS-6 N-terminal head groups could drive the assembly of a closed ring-like structure, and what determines the stacking of multiple rings on top one another in centriole duplication. We recently reported that SAS-5 binds specifically to a very narrow region of the SAS-6 central coiled coil through its C-terminal domain (CTD, residues 391-404). Here, we further demonstrate by both static light scattering and small angle X-ray scattering that the SAS-5 N-terminal domain (NTD, residues 1-260) forms a tetramer. Specifically, we found that the tetramer is formed by SAS-5 residues 82-260, whereas residues 1-81 are intrinsically disordered. Taking these results together, we propose a working model for SAS-5-mediated assembly of the multi-layered central tube structure.