A minus end checkpoint

Both ends of microtubules get in on spindle checkpoint signaling. From kinetochores, proteins at microtubule plus ends relay the results of proper spindle-to-chromosome attachments. Now, Hannah Müller, Bodo Lange (Max Planck Institute for Molecular Genetics, Berlin, Germany), and colleagues find that, at minus ends, γ-tubulin signals that all is well with microtubule nucleation.

Spindle microtubule minus ends are focused at the centrosome, where the microtubule-nucleating γ-tubulin ring complex (γ-TuRC) resides. Müller et al. found that the loss of γ-TuRC proteins, including γ-tubulin, activates the spindle checkpoint. The problem does not seem to stem from fewer spindle-to-kinetochore attachments, as spindle microtubule density was not strongly reduced.

The group instead finds a connection between γ-TuRC and known spindle assembly checkpoint proteins. Both Cdc20 and BubR1 copurified with γ-tubulin in human and fly cell extracts. Loss of a functional checkpoint, via knockdown of BubR1 for example, overcame the mitotic stall caused by γ-tubulin loss.

The arrest could not, however, be overcome by disrupting centrosomes. Thus the checkpoint relies on γ-TuRC for microtubule nucleation, but the centrosome as a “molecular hub” is not required for this particular process.

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