After two yeast cells fuse, their genomes come together via oppositely oriented microtubule sets whose plus ends face each other at the fusion site. The old model of this DNA congression suggested that the Kar3p microtubule minus end–directed motor pulled the plus end of one microtubule toward the minus end of its counterpart. Upon reaching the spindle pole body, Kar3p was thought to depolymerize the microtubule minus ends.
Recent findings put doubt on this 15-yr-old model. For one, minus end–directed motors generally accumulate on plus ends. Kar3p was indeed shown in vitro to depolymerize plus ends, where most microtubule dynamics is thought to occur.
Molk and colleagues now find that Kar3 does not track to the poles during congression. Instead, it remained at the interface between two microtubule sets, where it held together opposing plus ends (with the help of a plus-end binding protein called Bik1p). No microtubule overlap, which would indicate sliding, was seen.
The group proposes that Kar3p depolymerizes the plus ends while linking them, thus pulling the DNA to the center. The same mechanism might also apply to mammalian fertilization, in which the male pronucleus uses microtubules to migrate to the female genome.