Structural insights into SSNA1 self-assembly and its microtubule binding for centriole maintenance.

SSNA1 is a fibrillar protein involved in dynamic microtubule remodeling, including nucleation, co-polymerization, and microtubule branching. The underlying molecular mechanism has remained unclear due to a lack of structural information. Here, we determine the cryo-EM structure of C.elegans SSNA-1 at 4.55-Å resolution and evaluate its role in embryonic development. We find that SSNA-1 forms an anti-parallel coiled-coil, with self-assembly facilitated by an overhang of 16 C-terminal residues that form a triple-stranded helical junction. The microtubule-binding region is within the triple-stranded junction, suggesting that self-assembly of SSNA-1 creates hubs for effective microtubule interaction. Genetical analysis elucidates that SSNA-1 deletion significantly reduces embryonic viability, and causes multipolar spindles during cell division. Interestingly, impairing SSNA-1 self-assembly has a comparable effect on embryonic viability as the knockout strain. Our study provides molecular insights into SSNA-1's self-assembly and its role in microtubule binding and cell division regulation through centriole stability.