Embedded in the nuclear membrane, the spindle pole body (SPB) duplicates before division. Based on indirect evidence, previous work had led to the conclusion that the new cell always got the new SPB. However, Elmar Schiebel of the Beatson Institute for Cancer Research in Glasgow, Scotland, and colleagues came to the opposite conclusion when they marked the old SPB selectively by timed folding of a red fluorescent protein and observed its movement during mitosis.
The old SPB also picked up a hitchhiker, the Bfa1p–Bub2p GAP complex. This complex is part of a checkpoint that halts cell division if the nucleus (and the complex) hasn't entered the bud, although the authors found that correct localization of the complex is controlled by cortex–microtubule interactions rather than by correct SPB inheritance.
What ensures that the old SPB moves to the bud? The most likely explanation, Schiebel says, is that the old SPB is the first to link to microtubules that reel it into the new cell. The checkpoint complex may then sense some difference between the two sets of astral microtubules. For example, the microtubules in the mother are probably pushing on the nucleus, whereas those in the bud are pulling on the nucleus. The molecular details underlying these differences are unknown.The findings may have implications beyond the world of beer and bread. In mammalian cells, completion of cytokinesis correlates with the movement of the old centriole (which has a function similar to the SPB) to the cytokinesis site. Thus, the timing of mammalian cytokinesis may be subject to similar pole-mediated controls. ▪