The Centrosome and the Spindle Pole Body

Yes I wrote about centrosome kissing and then ... another paper appears in Nature Cell Biology.

But this time it's not in mammalian cells but in yeast. Remember what I once said:

prokaryotic (bacterial) strategy: out-multiply your neighbors
eukaryotic strategy: out-sophisticate your neighbors
yeast strategy: shed many of your eukaryotic tools and go back to out-multiplying your neighbors

Durring cell division in eukaryotes, microtubules that eminate from two centrosomes form a spindle. The spindle microtubules act to yank the duplicated chromosomes to each daugther cell. But microtubules are on the outside of the nucleus, chromosomes are inside the nucleus. How to bring the two together? Braek down the nuclear membrane just before you partition your chromosomes.

Here's a good movie that illustrates this point:

On the other hand yeast waste no time in dividing, they double once every 2hrs (as compared with your cells, about once a day). They don't even bother to brake down their nucleus. (To be honest I don't know whether anyone has figured out if yeast save time by skipping the whole nuclear disassembly/reassembly process). Yeast do not have centrosomes, they lost 'em at some point. So how is this all done (and what about the centrosome smooch?)

Well yeast have modified two aspects of mitosis.

1) They import tubulin (the building block of microtubules) into the nucleus.
2) They have a centrosome "replacement", called the spindle pole body (or SPB). Click here to read about my recent entry on centrosomes.

Here's a nice picture of the SPB:

Notice how the actual SPB structure (a dark linear structure in the image) is embedded into the nuclear double membrane. Also microtubules (the long hollow tubes) emanate from the SPB's cytoplasmic (right) and nuclear (left) sides. Nuclear microtubules pull appart the chromosomes (inside the intact nucleus) and cytoplasmic microtubules drags the nuceus arround the cell. The two sets of microtubules are coordinated by the duplicated SPBs.

But is the SPB equivalent to the centrosome?

They share genes. They duplicate (although the process seems bizare). They both nucleate and regulate microtubules. And it seems like in budding yeast, components that end mitosis and activate cytokinesis (the Mitotic Exit Network or MEN) interact with the SPB. But what is the final proof?

Blast the damn thing with a big ass laser and see if cytokinesis is inhibited.

And now Alexey Khodjakov did that in fission yeast. Cool.

Valentin Magidson, Fred Chang & Alexey Khodjakov
Regulation of cytokinesis by spindle-pole bodies
Nature Cell Biology (2006) 8:891 - 893