THE JOURNAL OF CELL BIOLOGY View metadata,citationandsimilarpapersatcore.ac.uk http://www.jcb.org/cgi/doi/10.1083/jcb.200709152 The JournalofCell Biology, Vol. 179,No.2,October 22,2007179–181 © TheRockefellerUniversity Press Correspondence toDuaneA.Compton: [email protected] to befocusedatspindlepoles. These two mechanismsare andaresubsequently stabilizedandgrow outward attachments. Intheother, shortmicrotubule stubsassociatewith from centrosomescontactkinetochorestoformrelatively stable mechanism, thedynamicplusendsofmicrotubules emanating ment tokinetochores(Wadsworth andKhodjakov, 2004).Inone plementary mechanismswork togeneratemicrotubule attach- is marked bybreakdown ofthenuclearenvelope, andtwo com- at thetransitionfromprophasetoprometaphase. This transition no attachment untilthey arereleasedintothecytosol attachments. microtubule tein thatpromotesthecorrectionofinappropriatekinetochore– Porter etal.onp.187ofthisissue)thatidentifyBod1,apro- Insight intotheproblemisnow provided bynew data(see promotes theirconversion tocorrectbiorientedattachments. transi of inappropriateattachment(Fig.1). The intermediatesare is complex, anditfrequentlyproceedsthroughintermediatestages orien The processofestablishingandmaintainingtheproperbi- move tooppositedaughtercellswhenthey disjoininanaphase. tubules fromoppositepolesofthespindlesothatsisterchromatids sister kinetochoresoneachchromosomemustattachtomicro- centromeric heterochromatin(Fig.1). To segregate faithfully, called kinetochoresthatformaspairedstructuresadjacentto attachtochromosomesthroughuniquestructures responsible forchromosomesegregation duringmitosis.Spindle ploidy. The microtubule-based structurecalledthespindle is ganism viabilitybecauseerrorsareirreversible andcauseaneu- Accurate chromosomesegregation isrequiredforcellandor- from improperlyorientedmicrotubuleattachments. biorientationbyunleashingchromosomes of Bod1(BiorientationDefective1),aproteinthatpromotes murky. Someclarityisnowemergingwiththe identifi taining p involvedinestablishingandmain- The molecularmachinery emanating frombothpolesofthespindleapparatus. from theattachmentofchromosomestomicrotubules Precise chromosomesegregationduringcelldivisionresults ofBiochemistry,Department MedicalSchool,Hanover, Dartmouth NH03755 Duane A.Compton Chromosome orientation ted attachmentofchromosomestospindlemi cro tubules During mitosisinmammaliancells,chromosomeshold ent, but littleisknown aboutthemolecularmachinerythat roperly oriented microtubule attachments remains roperly orientedmicrotubuleattachmentsremains $30.00 cation cation fi proteins involved inchromosomestructure,but oneofthe some-associated proteins. This approach willsurelyidentify in live cells. in partbecausesyntelicattachmentsarediffi quency ofsyntelyinunperturbedcellsiscurrentlyunknown, tubules of from thesamepoleorifpole-focusingmechanismspullmicro- tion favors thecaptureofcentrosomalmicrotubule plusends and Degrassi, 2005).Syntelycanariseifchromosomeorienta- other c phase thatare tosis and,ifuncorrected,leadstolaggingchromatidsatana- and S chore f directly visualizedinliving cellsandariseswhenonekineto- of spindleassembly, asshown inFig.1.Monotelyhasbeen improper attachmentsarefrequentlymadeduringearlystages attachment, but they arebothinherentlystochastic.Consequently, that many unalignedchromosom somes onbipolarmitoticspindles.Carefulimagingrevealed in exiting mitosisanddisplaypersistent,unalignedchromo- cells aredepletedofthisproteinusingRNAi, they aredelayed izes tokinetochoresandspindlepolesduringmitosis. When suffi of –microtubule attachment. This distinctionisnot rect syntelicsthatordinarilyarise outofthestochasticprocess syntelic attachmentsatkinetochores orwhetheritworks tocor- is astochasticprocess. but thatisexpected becausekinetochore–microtubule attachment in Bod1-defi that explicitly. The frequency ofchromosomealignmentdefects defective anaphasespresented,but theauthorsdidnotscore cells alsohave persistentmerotelic attachmentsbasedonthe attachments inmitosis.Itishighlylikely thatBod1-defi protein isknown tocausepersistentsyntelicchromosome of thesefi fail toresolve intoamphitelicattachments. The strikingaspect retain force-generatingcapacity. However, syntelic attachments to spindlemicrotubules anddemonstratingthatkinetochores ward andantipoleward, validating theirpersistentattachment syntelic chromosomesinBod1-defi name thisproteinBod1forBiorientationDefective 1. The ments tospindlemicrotubules, leadingPorter rst uniqueproteinsidentifi ciently robust toensureeffi Porter etal.(2007)usedproteomicstoidentifychromo- An openquestioniswhether Bod1 actively discourages almon, 1998).Merotelyisrelatively commoninearlymi- orms microtubule attachmentsbeforeitssister(Rieder hromatids have disjoinedandmoved poleward (Cimini ndings isthat,todate,the defi sister kinetochorestoward thesamepole. The fre- cient mitoticcellswas variable fromcelltocell, easily detectedinthespindlemidzoneafterall ed through this approachlocal- cient kinetochore–microtubule cient es possesssyntelic cient cellsoscillatepole- JCB: ciency ofnoother cult tovisualize et al.(2007)to COMMENT brought toyouby attach- provided byPubMedCentral cient JCB 179 CORE Figure 1. Chromosome orientation on mitotic spindles. In mammalian cells, multiple microtubules attach to each kinetochore (red). Precise chromo- some segregation requires sister kinetochores on to attach to microtubules from opposite spindle poles, leading to biorientation or amphitely. Incorrect orientations can range from single unattached kineto- chores (montely) or single kinetochores with microtubule attachments to both spindle poles (merotely) to both sister kinetochores attached to the same spindle pole (syntely). Figure 2. Potential mechanisms for the correction of syntelic attachments by Bod1. (A) Bod1 may induce the release of microtubules from one kineto- chore so that appropriate attachments can be made to the opposite spindle pole. (B) Bod1 may promote the shortening of kinetochore microtubules, easy to resolve experimentally because syntelic attachments are bringing the chromosome to the spindle pole. The chromosome could then diffi cult to detect in live cells, but the data hint that Bod1 serves release short microtubules and move to the plate for biorienta- to correct improper attachments. For example, forcing spindle tion via kinetochore transport on adjacent microtubules (arrows), as demon- strated previously (Kapoor et al., 2006). (C) Bod1 may induce the release formation to proceed through a monopolar intermediate sub- of microtubules from both kinetochores to permit new attachments with stantially increases the frequency of syntelic chromosomes correct orientation toward both spindle poles. (Lampson et al., 2004). Under those conditions, Bod1-defi cient cells establish a bipolar spindle but display elevated numbers of To gain insight into why Bod1-defi cient cells sustain im- syntelic chromosomes, indicating a failure to correct improper proper kinetochore–microtubule attachments, Porter et al. (2007) attachments. Moreover, there is persistent syntelic chromosome examined the localization of the microtubule-depolymerizing attachment in Bod1-defi cient cells despite chromosome oscilla- kinesin-13 protein mitotic -associated kinesin (MCAK). tion and movement. This suggests that correction mechanisms MCAK localizes to and inner kinetochores and are lacking or weakened in Bod1-defi cient cells, and possible participates in releasing microtubules from kinetochores through pathways for Bod1-mediated syntelic correction are presented a mechanism regulated by the conserved in Fig. 2. This point may seem subtle, but it is important be- (Gorbsky, 2004). Bod1-defi cient cells display no substantial cause Bod1-defi cient cells may provide a means to estimate the change in total MCAK at centromeres but are reduced in the relative frequency of syntelic attachments during early stages of quantity of phosphorylated MCAK. Moreover, localization of spindle formation. the remaining phosphorylated MCAK is disturbed at centromeres. Correction of improper kinetochore attachments is inti- Thus, Bod1 defi ciency alters the behavior of MCAK, although mately tied to the rate at which kinetochores release micro- this most likely represents an indirect effect because Bod1 and tubules (Nicklas and Ward, 1994) and is regulated by Aurora B MCAK localize to different positions on the kinetochore. Bod1 kinase activity (Lampson et al., 2004; Pinsky and Biggins, 2005). may be an Aurora kinase substrate, infl uence the substrate se- The release rate is probably tied to tension such that strong lection of Aurora kinase, or alter the activity of one of the other tension across amphitelic kinetochores suppresses micro tubule kinesin-13 family members that are expressed in human cells release, favoring the retention of kinetochore micro tubules (Manning et al., 2007). and stabilizing appropriate attachments (Pinsky and Biggins, Because of the stochastic nature of microtubule binding 2005). Syntelic kinetochores do not experience the same to kinetochores, the correction of inappropriate attachments is level of tension and fail to suppress microtubule release, which critical for faithful chromosome segregation. The identifi cation of favors the correction of improper attachments. In this context, it Bod1 opens new doors to the molecular analysis of this process. is interesting to note that the quantity of GFP-Bod1 at kineto- Bod1 is one component of a large complex, and the identities of its chores decreases in metaphase and compared with binding partners may reveal further molecular details and pro- prometaphase. If Bod1 plays a role in correcting improper at- vide additional tools. Correction of kinetochore–microtubule tachments by infl uencing microtubule release, as suggested attachment errors is likely to be a complex process for mamma- above, perhaps high levels at kinetochores are unnecessary once lian cells in which each kinetochore binds multiple micro- stable, amphitelic attachments are attained. This idea raises sev- tubules. Bod1 appears to be a key piece that will help solve this eral testable hypotheses. First, Bod1-defi cient cells should have complex puzzle. reduced rates of kinetochore–microtubule turnover compared with control cells. Second, the abundance of Bod1 at kineto- Submitted: 24 September 2007 chores may respond to tension. Accepted: 2 October 2007

180 JCB • VOLUME 179 • NUMBER 2 • 2007 References Cimini, D., and F. Degrassi. 2005. : a matter of bad connections. Trends Cell Biol. 15:442–451. Gorbsky, G.J. 2004. : MCAK under the aura of Aurora B. Curr. Biol. 14:R346–R348. Kapoor, T.M., M.A. Lampson, P. Hegert, L. Cameron, D. Cimini, E.D. Salmon, B.F. McEwen, and A. Khodjakov. 2006. Chromosomes can congress to the metaphase plate before biorientation. Science. 311:388–391. Lampson, M.A., K. Renduchitala, A. Khodjakov, and T.M. Kapoor. 2004. Correcting improper chromosome-spindle attachments during . Nat. Cell Biol. 6:232–237. Manning, A.L., N.J. Ganem, S.F. Bakhoum, M. Wagenbach, L. Wordeman, and D.A. Compton. 2007. The kinesin-13 proteins Kif2a, Kif2b, and Kif2c/ MCAK have distinct roles during mitosis in human cells. Mol. Biol. Cell. 18:2970–2979. Nicklas, R.B., and S.C. Ward. 1994. Elements of error correction in mitosis: microtubule capture, release and tension. J. Cell Biol. 126:1241–1253. Pinsky, B.A., and S. Biggins. 2005. The spindle checkpoint: tension versus attachment. Trends Cell Biol. 15:486–493. Porter, I.M., S.E. McClelland, G.A. Khoudoli, C.J. Hunter, J.S. Andersen, A.D. McAinsh, J.J. Blow, and J.R. Swedlow. 2007. Bod1, a novel ki- netochore protein required for chromosome biorientation. J. Cell Biol. 179:187–197. Rieder, C.L., and E.D. Salmon. 1998. The vertebrate cell kinetochore and its roles during mitosis. Trends Cell Biol. 8:310–318. Wadsworth, P., and A. Khodjakov. 2004. E pluribus unum: towards a universal mechanism for spindle assembly. Trends Cell Biol. 14:413–419.

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