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kinetochores of misoriented or unattachedchromosomes (reviewed in ref. 2). Cdc20,Mad2 and BubR1 cycle rapidly on and offkinetochores, suggesting that they might bepart of the diffusible checkpoint signal3,4.

Previous studies have focused on the ability of Mad2 to bind to Cdc20 and thecheckpoint protein Mad1. The Mad1–Mad2interaction localizes Mad2 to kinetochoresand seems to occur early in the checkpointpathway, whereas the binding of Mad2 toCdc20 blocks APC activation and happenslater in the checkpoint cascade (for reviewsee ref. 2). Upon associating with Mad1 orCdc20, the carboxy-terminal tail of Mad2rearranges from a disordered ‘open’ confor-mation (OMad2) to a closed conformation(CMad2), thereby encircling Mad1 or Cdc20peptides like a safety belt5,6. Mad1 and Cdc20compete for the same binding site on Mad2but, paradoxically, Mad1 is required for theformation of Cdc20–Mad2 complexes invivo. The high stability of Mad1–Mad2 com-plexes and the slow spontaneous inter-conversion of OMad2 and CMad2 seem atodds with the rapid kinetics of checkpointsignalling.

De Antoni et al.1 use Mad2 mutantslocked into open and closed conformationsto show that two Mad2 molecules participatein this complicated dance. Mad2 variantslacking ten amino acids at the carboxy ter-minus cannot close the safety belt, are stuckin the open state and do not bind to Mad1 or Cdc20. However, the authors show that‘always-open’ Mad2 can bind stably toMad1–CMad2 complexes.Strikingly,OMad2and CMad2 can partner each other, but twomolecules of the same conformation cannotassociate. In cells, the always-open OMad2localizes to kinetochores when normal Mad2is present, but it cannot engage the check-point, presumably because it cannot binddirectly to Cdc20. Importantly, Mad2mutants that cannot form partnerships with other Mad2 molecules do not bind to Mad1–CMad2, localize to kinetochores or support checkpoint function.

From these data, the authors propose amodel in which ‘core’ complexes of Mad1–CMad2 increase the concentration and stability of OMad2 at kinetochores (Fig. 1).When the checkpoint is activated, theOMad2 in Mad1–CMad2–OMad2 com-plexes can be transferred to Cdc20, closingup to form Cdc20–CMad2. Such complexesprevent APC activation and thus arrest celldivision.This is a striking shift from previousmodels in which just a single CMad2 mole-cule bound to Mad1 before unfolding itssafety belt and transferring to Cdc20. Thisnew model fits neatly with data showing thattwo distinct pools of Mad2 associate withkinetochores: one that remains stably boundthroughout cell division, and one that con-tinuously binds and dissociates with a half-life of about 20 seconds3,7. As predicted by

The maintenance of normal chromo-some number during cell divisionrequires the precise separation of

duplicated sister chromosomes into twoequal sets. Andrea Mussachio and col-leagues1, writing in Current Biology, shedlight on how a protein called Mad2 ensuresthe fidelity of this process. The authors pro-pose that two different structural forms ofMad2 collaborate to change one of the part-ners from an inactive to an active state.Such ‘self-templated’ changes in proteinconformation are a characteristic of prions,but represent a new mechanism for cell signalling.

To achieve the correct separation, eachchromosome is attached through the multi-protein ‘kinetochore’ complex to an intra-cellular scaffold called the spindle. In the‘anaphase’ stage of cell division, this struc-ture pulls the sister chromosomes apart toform two groups at opposite ends of the cell,each with one sister from each chromosomepair. The two groups will be surrounded by membranes to become the nuclei of the

daughter cells. The onset of anaphase isblocked until all chromosomes are boundcorrectly to the spindle. Defects in the pro-teins that make up this blockade (or ‘spindlecheckpoint’) lead to abnormal chromosomenumbers in the daughter cells, with poten-tially disastrous consequences — manytumour cells, for example, have abnormalchromosome complements. The spindlecheckpoint activates a cascade of signals thatconverge on Cdc20, a protein that regulatesthe anaphase-promoting complex (APC)2.If even a single kinetochore remains un-attached to the spindle, a diffusible signal isgenerated that is sufficient to restrain Cdc20and block cell division at the start ofanaphase for many hours3. But the nature ofthe signal and the mechanism by which it ispropagated remain elusive.

Two highly conserved Cdc20-bindingproteins, Mad2 and BubR1, are essential to inhibit APC (and therefore anaphase).These attach to kinetochores early in celldivision, and dissociate as the kinetochoreattaches to the spindle,but they persist on the

Cell biology

The more MAD, the merrierRobert S. Hagan and Peter K. Sorger

Cells must pass the correct number of chromosomes to their progenythrough the complex ballet of cell division. An unusual conformation-sensitive switch seems to maintain accurate chromosome segregation.

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Figure 1 An asymmetric Mad2 partnership at the centre of the spindle checkpoint machinery. Mad2encloses Mad1 to form a stable complex. Open-conformation Mad2 (OMad2) molecules bind to andare released from the Mad1–Mad2 complex in a cyclic fashion. At kinetochores (brown) that lackmicrotubule attachment, the OMad2 molecule encloses and binds activated Cdc20 (black), a step thatmay be promoted by the phosphorylation (PP) of Cdc20. The resulting closed Mad2 (CMad2)–Cdc20complex inhibits anaphase-promoting complex (APC) and may bind BubR1 and Bub3. This complexmay also act as a template for the formation of more CMad2–Cdc20 complexes from the cytosolicpool of OMad2, amplifying the checkpoint signal. Multiple protein complexes may be responsible forAPC inhibition; their exact composition remains unknown.

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ulator is BubR1, which is required for boththe kinetochore-dependent and kineto-chore-independent functions of Mad2. Anattractive possibility is that all of these inputsregulate a transition in Cdc20 between conformations that are susceptible to Mad2-dependent inhibition and those that areresistant. Although much remains to belearned about Cdc20 and the spindle check-point in general, the work of De Antoni et al.highlights the central role of Mad2 in guid-ing a cell through cell division with all of itschromosomes in order. ■

Robert S. Hagan is in the Department of Biology,and Peter K. Sorger is in the Department of Biologyand Biological Engineering Division, MassachusettsInstitute of Technology, 77 Massachusetts Avenue,68-371, Cambridge, Massachusetts 02139, USA.e-mail: psorger@mit.edu1. De Antoni, A. et al. Curr. Biol. 15, 214–225 (2005).

2. Rieder, C. L., Cole, R. W., Khodjakov, A. & Sluder, G. J. Cell Biol.

130, 941–948 (1995).

3. Howell, B. J., Hoffman, D. B., Fang, G., Murray, A. W. & Salmon,

E. D. J. Cell Biol. 150, 1233–1250 (2000).

4. Musacchio, A. & Hardwick, K. G. Nature Rev. Mol. Cell Biol. 3,731–741 (2002).

5. Luo, X., Tang, Z., Rizo, J. & Yu, H. Mol. Cell 9, 59–71 (2002).

6. Sironi, L. et al. EMBO J. 21, 2496–2506 (2002).

7. Shah, J. V. et al. Curr. Biol. 14, 942–952 (2004).

8. Meraldi, P., Draviam, V. M. & Sorger, P. K. Dev. Cell 7,

46–60 (2004).

9. Sudakin, V., Chan., G. K. & Yen, T. J. J. Cell Biol. 154,

925–936 (2001).

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between Solar System materials, such asplanets,meteorites, chondrules and CAIs.

We now know the oxygen isotopic com-positions of Earth, the Moon, Mars and theparent bodies of various kinds of meteorites.What we do not know is the composition inthe Sun, where more than 99% of oxygen inthe Solar System resides.The oxygen isotopicanomalies in Solar System materials werethought to be due to varying mixtures ofpresolar matter that gained their oxygen isotopic heterogeneity by nucleosynthesis indifferent kinds of stars. (Nucleosynthesis isthe process by which elements are formedthrough nuclear reactions.) However, labo-ratory experiments suggested that the anom-alies might be photochemical in origin3,4.

In an influential paper, Clayton5 sug- gested that the anomalies do indeed arisethrough a photochemical effect known asself-shielding.In this model,ultraviolet radi-ation from the early Sun dissociates carbonmonoxide — the most abundant oxygen-containing molecule in solar gas — in theinner part of the solar nebula, close to theSun.The radiation of a wavelength appropri-ate to dissociate C16O has a much shorterpath length than that needed to dissociateC17O and C18O, because C16O is so muchmore abundant. In the self-shielded portionof the nebula, only C17O and C18O would bedissociated. The resulting reactive 17O- and18O-enriched oxygen atoms would then formsilicates — a major constituent of most rocks— in an unspecified way.

The Sun contains most of the matter inthe Solar System, but surprisingly little is known about its isotopic com-

position. Spectroscopic methods can beapplied only to a few abundant elementsand are imprecise compared with labora-tory measurements on Earth. The onlypractical way to sample the Sun is to mea-sure the isotopic composition of matteremitted from it.

Such emissions can exist in one of twoforms: ‘solar wind’ consists of relatively low-energy charged particles from the Sun’scorona — mainly protons and helium ions,but also lesser quantities of every element —accelerated to speeds of 300 to 800 km s�1;and the less abundant ‘solar energetic parti-cles’ (SEPs), also comprising ions of all theelements, but with energies five to a hundredtimes greater than those of the solar-windions. Solar wind and SEPs can be implantedin materials exposed to the Sun in space or on the surface of planetary bodies withoutatmospheres, such as the Moon. In the latestin a series of studies using minerals in

lunar soils to infer isotopic compositions ofelements in the Sun, Hashizume and Chaussidon (page 619 of this issue)1 reportthat the Sun is enriched in the oxygen iso-tope 16O compared with most objects in theSolar System.

But why is the abundance of oxygen isotopes so important? Oxygen has three isotopes, 16O, 17O and 18O, that exist in theproportions 2,700:1:5. For a long time, only16O and 18O were measured for geo- and cosmochemical purposes. This was because,in most physical and chemical processes, thefractionation of oxygen isotopes from oneanother is a mass-dependent process, withthe 18O/16O ratio varying twice as much as the17O/16O ratio. Thus, it was thought that the17O/16O ratio in a sample could be inferredfrom the 18O/16O ratio. The situationchanged in 1973,with the discovery2 that cal-cium-aluminium-rich refractory inclusions(CAIs) in primitive chondrite meteorites areenriched in 16O alone (Fig. 1). Since that dis-covery, oxygen isotopes have proven to be apowerful tool for studying the relations

Cosmochemistry

A breath of solar airAndrew M. Davis

The abundance of different isotopes of oxygen provides clues to the origin of matter in the Solar System. Hence the importance of studies ofoxygen atoms from the Sun trapped in metal grains on the Moon.

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Figure 1 Oxygen in the Solar System. Thequantities �17O and �18O express deviations in the ratios 17O/16O and 18O/16O relative to the same ratios in ‘Standard Mean OceanWater’ (SMOW) on Earth, where �17O�[(17O/16O)sample/(17O/16O)SMOW�1]1,000 and�18O�[(18O/16O)sample/(18O/16O)SMOW�1]1,000.All terrestrial rocks and water lie along the linelabelled TF. Refractory inclusions in meteoriteslie along the line labelled CAIs. Most meteoriteslie slightly below or above the TF line. Becauseoxygen isotopic compositions of lunar metalgrains lie in the red field, and all lunar rocks lie along the TF line, the oxygen isotopiccomposition of the Sun is inferred to besomewhere in the light purple field, on or below the dark purple line.

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the model, Cdc20 also cycles on and offkinetochores with similar kinetics, whereasMad1 does not.

Despite the elegance of De Antoni andcolleagues’ model, many questions remainunanswered. It is not clear how kineto-chore–spindle attachment regulates Mad2-mediated checkpoint signalling. Alsounknown is the molecular nature of the dif-fusible signal; De Antoni et al. suggest thatCdc20–CMad2 may constitute this signaland may act as an amplifier by binding to andactivating other OMad2 molecules. Finally,the notion that Mad2 can inhibit Cdc20 onlyafter being activated by Mad1 does notexplain how cytosolic Mad2 regulates Cdc20early in the cell cycle in a Mad1-independentmanner8.

One promising approach to tacklingthese issues is to focus on Cdc20. Cdc20 isfound in cells in association with Mad2, andtwo other proteins, BubR1 and Bub3 (ref. 9),and is phosphorylated at multiple sites.Intriguingly, Mad2 binds tightly in vitro toCdc20 in the absence of any other proteins(most notably Mad1). In vivo, however,Mad2 requires kinetochores that are notattached to the spindle to bind to Cdc20,implying that access to the Mad2-bindingsite on Cdc20 is regulated. One possible reg-

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