Unraveling the structure and function of a novel SegC protein interacting with the SegAB chromosome segregation complex in Archaea.

Genome segregation is a fundamental process that preserves the genetic integrity of all organisms, but the mechanisms driving genome segregation in archaea remain enigmatic. This study delved into the unknown function of SegC (SSO0033), a novel protein thought to be involved in chromosome segregation in archaea. Using fluorescence polarization DNA binding assays, we discovered the ability of SegC to bind DNA without any sequence preference. Furthermore, we determined the crystal structure of SegC at 2.8 Å resolution, revealing the multimeric configuration and forming a large positively charged surface that can bind DNA. SegC has a tertiary structure folding similar to those of the ThDP-binding fold superfamily, but SegC shares only 5-15% sequence identity with those proteins. Unexpectedly, we found that SegC has nucleotide triphosphatase (NTPase) activity. We also determined the SegC-ADP complex structure, identifying the NTP binding pocket and relative SegC residues involved in the interaction. Interestingly, images from negative-stain electron microscopy revealed that SegC forms filamentous structures in the presence of DNA and NTPs. Further, more uniform and larger SegC-filaments are observed, when SegA-ATP was added. Notably, the introduction of SegB disrupts these oligomers, with ATP being essential for regulating filament formation. These findings provide insights into the functional and structural role of SegC in archaeal chromosome segregation.