Knowing when to let go

During meiosis I, some mechanism must allow homologous chromosomes to separate while keeping sister chromatids paired until meiosis II. How does a cell make this distinction? On page 219, Rogers et al. propose that in C. elegans the aurora-B kinase AIR-2 is largely responsible for ensuring that cohesion between chromosomes breaks down at the proper place and time. The authors also identified additional components in what is likely to be a conserved pathway controlling chromosome cohesion.

When AIR-2 activity is inhibited by RNAi, meiotic cells in the worm do not separate homologous chromosomes or sister chromatids. In metaphase I in normal meiotic cells, AIR-2 localizes distal to chiasmata, corresponding to the last points of contact between homologous chromosomes. In metaphase II, AIR-2 localizes to the last points of contact between sister chromatids. AIR-2 phosphorylates the cohesin protein REC-8 at a specific site in vitro, and inhibition of the CeGLC-7α or -β phosphatases causes AIR-2 to localize nonspecifically along chromosomes.

The authors suggest that CeGLC-7α/β phosphatases restrict AIR-2 localization temporally and spatially on meiotic chromosomes. AIR-2 phosphorylates REC-8 in its vicinity, causing the cohesin to be degraded and releasing chromosomal cohesion only in the appropriate location. ▪