Archaeal SegAB forms a bipolar structure that promotes chromosome segregation in spherical cells

ArchaealsegABoperons are thought to promote chromosome segregation, but their mechanism remains unknown. We employ comparative genomics, structural biology, genetic knockouts, and quantitative cell biology to investigate how SegA and SegB proteins work together to segregate chromosomes in the thermophilic archaeonSulfolobus acidocaldarius.In vitro, SegB binds a centromeric DNA sequence adjacent to thesegABoperon, andin vivoforms a distinct focus on each segregating chromosome. SegA, a ParA-like ATPase, binds DNA non-specificallyin vitroand promotes chromosome compaction and segregationin vivo. During division, SegA shifts from chromosome-associated puncta to form a single, elongated figure that runs between separating SegB foci. Late in division, SegA retreats to regions surrounding separated SegB foci. Elongated SegA figures appear insegBknockout cells but no longer lie perpendicular to the division plane. We propose that SegA and SegB interact to form a bipolar, DNA-segregating structure radically different from bacterial ParABS systems.