A minus end checkpoint

A minus end checkpoint B oth ends of microtubules get in on spindle checkpoint signaling. From kinetochores, proteins at microtubule plus ends relay the results of proper spindle-tochromosome attachments. Now, Hannah Müller, Bodo Lange (Max Planck Institute for Molecular Genetics, Berlin, Germany), and colleagues fi nd 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 fi nds a connection between γ-TuRC and known spindle assembly checkpoint proteins. Both Cdc20 and BubR1 copurifi ed with γ-tubulin in human and fl y 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. Reference: Müller, H., et al. 2006. Science. 314:654–657. Myosin’s need for speed T he fastest muscles are powered by a motor that is comparatively averse to energy-providing ATP, as shown by Douglas Swank (Rensselaer Polytechnic Institute, Troy, NY), Vivek Vishnudas, and David Maughan (University of Vermont, Burlington, VT). The muscles that power Drosophila fl ight contract over 200 times per second. To determine what makes their myosin motor work so quickly, Swank et al. compared contraction speeds of fl ight myosins with a slower embryonic myosin while varying levels of ATP, whose hydrolysis fuels contraction, and its byproducts, P i and ADP. Previous studies established that the speeds of slower myosins are limited by how quickly they releases ADP. But Swank found that the fl ight myosin is instead held back by P i release. “To increase speed,” Swank reasons, “the ADP release step must be faster. Here, it’s gotten so fast that P i release becomes limiting.” As ATP is needed for every contraction cycle, “you might think [fast myosin] would want to bind ATP [even tighter],” says Swank. “But that’s not the case.” The slow myosin had the higher affi nity for ATP. Flight myosin’s lower ATP affi nity might be a side effect of its faster release of structurally similar ADP. Flies probably compensate by maintaining very high ATP concentrations. Their fl ight muscles seem to have enough mitochondria for the task, but Swank plans to measure in vivo ATP levels directly to be sure. By swapping domains of fast and slow myosins, he also hopes to fi nd the structural differences that evolved to power such hustle. Reference: Swank, D.M., et al. 2006. Proc. Natl. Acad. Sci. USA. doi:10.1073/pnas.0604972103. Y east cells pee out amino acids to avoid ammonium toxicity, say David Hess (Princeton University, Princeton, NJ) and colleagues. The group was initially interested in the effect of potassium, not ammonium. But their microarray data suggested that low potassium is harmful to yeast because ammonium— a similarly sized and charged ion—seeps in through the battery of induced potassium channels. The yeast protect themselves from the intracellular ammonium by incorporating it into amino acids that they then secrete into the medium. Export seems to be through passive channels that also take up amino acids. The export may help natural yeast strains survive on ammonium-rich rotting vegetation (in which the external source of amino acids is probably also handy later on). Standard laboratory media have very high potassium levels that apparently masked ammonium’s toxic effects on yeast before now. The initial cellular response to high ammonium—converting it to glutamine or glutamate—is evolutionarily conserved. Since glutamine is a neurotoxin, however, mammals must further convert the excess amino acids to urea. “The root cause of ammonium toxicity is not understood in mammalian systems,” says Hess. At least now, he says, “we can use yeast as a model.” Reference: Hess, D.C., et al. 2006. PLoS Biol. doi:10.1371/journal.pbio.0040351.