Hrs1p in mitotic cells induces a horsetail astral array (inset) and nuclear oscillation (bottom vs. top).


Addition of a single fission yeast protein can induce nuclear oscillations in mitotic cells, even though these movements are normally seen only in meiotic cells, according to Kayoko Tanaka, Masayuki Yamamoto, and colleagues (University of Tokyo, Japan).

Fission yeast meiosis includes a thrashing about of the nucleus (also called horsetail nuclear movement [HNM]) that helps align homologous chromosomes. This oscillation is driven by a dynein–dynactin system. Cortex-localized dynein may be switched on at first one end of the cell and then the other. The dynein tugs on an astral array of microtubules called the horsetail astral array (HAA).

Mitotic cells lack both the HAA and any significant amounts of dynein and dynein-anchoring proteins. So it was a surprise when the Japanese group succeeded in inducing the mitotic appearance of the HAA and the HNM by expressing a single meiotic protein. That protein, called Hrs1p or Mcp6p, was discovered by this and another group as a meiosis-specific protein required for HAA formation.

The HNM oscillations were somewhat less organized in the mitotic cells, probably because of the low levels of dynein and associated proteins. But the HAA was quite robust. Hrs1p/Mcp6p appears to interact with both spindle pole body components and microtubule-nucleating components. Tanaka hopes to assemble in vitro complexes with these two activities to see if Hrs1p/Mcp6p can link them together.


Tanaka, K., et al.
Curr. Biol.