A parasitic plasmid poaches a centromeric protein from its host, as shown by Hajra et al. on page 779. The protein helps the plasmid segregate during mitosis.
The plasmid is a benign yeast parasite, called the 2-μm plasmid, that replicates and segregates precisely to maximize its numbers while restricting itself to nontoxic levels. The plasmid's segregation locus, STB, contains chromatin-remodeling proteins, suggesting that STB's architecture is different from that of the rest of the plasmid—much as the centromere is unique within a host chromosome.
The new results reveal how the plasmid makes the STB locus distinctive: it marks it with the same histone H3 variant, Cse4p, that its host uses to tag centromeres. Cse4p is a rapidly degraded protein but is somehow protected from the proteasome at the STB locus, as it is at yeast centromeres. Plasmid mutants that do not bind this core histone do not load the remodeling proteins or yeast cohesin and thus missegregate.
Spindle microtubules are required for plasmid Cse4p loading, although just how they participate is not clear. Perhaps they position the plasmid at its known locale near host centromeres at spindle poles—a prime spot for poaching some Cse4p and cohesin. As STB lacks other true kinetochore proteins, the plasmid might need to hitchhike on yeast chromosomes rather than grab onto the spindle directly.