Securin a new role for itselfAnil K. Rustgi

Departments of Medicine & Genetics, Abramson Cancer Center, University of Pennsylvania, Philadelphia, Pennsylvania 19104-2144, USA.e-mail: anil2@mail.med.upenn.edu

Securin has biological functions in both cellular transformation and sister chromatid separation. A new study shows that it alsointeracts with p53 and regulates p53-mediated transcription and apoptosis.

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222 nature genetics • volume 32 • october 2002

indicated by crystal structures12, orextended portions of one of the proteinsinvolved in silencing.

SIN similaritiesThis is not the full story, however: four ofthe affected residues around H3 Lys79are directly involved in protein–DNAinteractions, either by burrowing intoone particular compressed minor grooveof the bound DNA or by contacting itsphosphate backbone. The other sevenresidues are either partially or completelyburied in the H3–H4 interface and main-tain the conformation of the protein toenable local protein–DNA interactions atthat site11. Earlier screens for yeastmutants that alleviate transcriptiondefects caused by inactivating a particu-lar ATP-dependent chromatin remodel-ing factor, SWI-SNF13, identifiedSWI-SNF independent (SIN) mutationsin histones H3 and H4. Unexpectedly, theSIN mutations and the mutations identi-fied by Park et al.5 cluster at opposite,structurally equivalent ends of the quasi-symmetric, crescent-shaped H3–H4 het-erodimer. These two regions fulfillequivalent roles in DNA binding, anduse similar structural elements andamino acid side chains for the task. Evenmore surprisingly, four of the mutationsidentified by Park et al.5 are the exactstructural equivalents of the four SINmutants that most severely affectDNA–protein interactions, either directlyor indirectly.

This cannot be a coincidence. ATP-dependent chromatin remodeling factors(multiple variations of which are found inall eukaryotes studied so far) alleviate therepressive effect of chromatin on transcrip-tion, presumably by mobilizing and relo-cating the histone octamer on the DNA14.One consequence of the SIN mutations isthat the histone octamer offers less resis-tance to being repositioned along the DNAduring a process known as nucleosomesliding (A. Flaus and T. Owen-Hughes,manuscript in preparation). Inspection ofthe SIN mutant crystal structures showsthat bonds between the histone mainchain and the DNA backbone are dis-rupted in a complex and subtle manner(U. Muthurajan and K.L., in preparation).Incorporated into model in vitro nucleo-some arrays, one of the SIN mutantsexhibits a striking defect in its ability tocondense into higher order structures15.By analogy, it is tempting to speculate thatat least some of the mutants identified byPark et al.5 could similarly affect silencingby making nucleosomes intrinsicallymobile and preventing them from order-ing in nucleosomal arrays conducive tocondensation by means of accessory tailbinding proteins such as Sir3/4. In thiscontext, it would be interesting to testwhether the equivalent mutations identi-fied by Park et al.5 also confer a SIN phe-notype, and whether nucleosomesharboring these mutant histones alsoshow an increased propensity to slideand/or inability to form condensed arrays.

Chromatin beyond the nucleosomeFrom a broader perspective, these observa-tions may be glimpses of an emerging newpattern. Far from the histone core actingmerely as an edifice for DNA compactionwhile the histone tails direct complexprocesses, this new study demonstratesclearly that the tail does not always wag thedog. In light of this, it is important to con-sider that detailed structural informationon higher order chromatin organization isvirtually absent. To understand these andother recent findings in a structural con-text, it is essential that we learn more aboutthe dynamics of nucleosomes and howthey interact to form higher order chro-matin structure. �1. Luger, K., Maeder, A.W., Richmond, R.K., Sargent,

D.F. & Richmond, T.J. Nature 389, 251–259 (1997).2. Moazed, D. Mol. Cell. 8, 489–498 (2001).3. Jenuwein, T. & Allis, C.D. Science 293, 1074–1080

(2001).4. Berger, S.L. Curr. Opin. Genet. Dev. 12, 142–148

(2002).5. Park, J.-H., Cosgrove, M.S., Youngman, E.,

Wolberger, C. & Boeke, J.D. Nature Genet. 32(2002); advance online publication, 16 September2002 (doi:10.1038/ng982).

6. Ng, H.H. et al. Genes Dev. 16, 1518–1527 (2002).7. van Leeuwen, F., Gafken, P.R. & Gottschling, D.E.

Cell 109, 745–756 (2002).8. Lacoste, N., Utley, R.T., Hunter, J.M., Poirier, G.G. &

Cote, J. J. Biol. Chem. 277, 30421–30424 (2002).9. Briggs, S.D. et al. Nature 418, 498 (2002).10. Varga-Weisz, P.D. & Dalgaard, J.Z. Mol. Cell 9,

1154–1156 (2002).11. White, C.L., Suto, R.K. & Luger, K. EMBO J. 20,

5207–5218 (2001).12. Luger, K. & Richmond, T.J. Curr. Opin. Genet. Dev. 8,

140–146 (1998).13. Kruger, W. et al. Genes Dev. 9, 2770–2779 (1995).14. Flaus, A. & Owen-Hughes, T. Curr. Opin. Genet. Dev.

11, 148–154 (2001).15. Horn, P.J., Crowley, K.A., Carruthers, L.M., Hansen,

J.C. & Peterson, C.L. Nature Struct. Biol. 9, 167–171(2002).

Expression of the protein securin peaks inmitosis and is phosphorylated in a cellcycle–dependent manner1. It also sharesbiochemical properties with yeast proteinsthat mediate in cell cycle control.

Securin is expressed in adult testis,thymus, colon, small intestine, brain,lung and fetal liver, and abundant levels

are found in different cancer cell lines.Notably, there is overexpression in sometumors, such as pituitary adenomas, pri-mary epithelial neoplasms and hemopoi-etic malignancies2. Transfection ofPTTG, encoding securin, into NIH 3T3cells results in anchorage-independenttransformation in vitro and tumor for-

mation when transfectants are injectedinto athymic nude mice. In addition,PTTG expression is upregulated inrapidly proliferating cells but is down-regulated in conditions of serum starva-tion or cell confluence3. Mice lackingsecurin show aberrant cell cycle progres-sion, premature centromere division and

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nature genetics • volume 32 • october 2002 223

problems with chromosomal stability,apart from tissue-specific phenotypes,such as testicular and splenic hypoplasiaand thymic hyperplasia5. Expression of astabilized mutant securin in Xenopus lae-vis embryo extracts, however, blocks sis-ter chromatid separation but not othermitotic events5.

How does securin function?Juan Bernal and colleagues6 now reportthat securin binds p53 and thereby regu-lates its apoptosis and transcriptionactivity. By screening a phage displaylibrary, the p53 tumor suppressor geneproduct was found to interact withsecurin. Follow-up studies indicated thatthis interaction occurred not only invitro, but in vivo as well. The interactionrequires the carboxy terminus of p53and the amino terminus of securin.Functional studies showed that theinteraction of these proteins preventsp53 from binding DNA, thereby inhibit-ing p53-mediated transcriptional activ-ity. In particular, Bax promoter activityis diminished by the p53–securin inter-action, leading to a decrease in apopto-sis. Complementary experiments showthat p53-mediated transcriptional acti-vation and apoptosis are augmented inPTTG-null cells. Thus, the oncogeniceffects of securin that were observed ini-tially may involve, at least in part, thedownregulation and inhibition of p53.

Recent findings tell us more about therole of securin in sister chromatid sepa-ration at the metaphase-to-anaphasetransition in both yeast and human cells.Human somatic cells lacking securin losechromosomes at high frequency7. Thishas been linked to abnormal anaphasesduring which cells could not separatetheir chromosomes. Sister chromatidseparation involves the proteolytic cleav-age of cohesin (a protein that links sis-ter chromatids), a process that ismediated by separase, a cysteine pro-tease. Separase interacts with securin, itsinhibitor, until anaphase initiation. Atthat point, securin is degraded by ananaphase-promoting complex/cyclo-some (APC/C), which is a cell-cycle reg-ulated ubiquitin ligase that facilitates

degradation of securin and cyclins by the26S proteasome8. Yeast Pds1 (securin) isa substrate of Cdc28 cyclin-dependentkinase. Phosphorylation of Pds1 is cru-cial for efficient binding to Esp1/sepa-rase and for nuclear localization of thelatter9. As securin prevents the bindingof separase to its substrates10, we canconclude that securin supports separaseactivity in anaphase and that securinmust be destroyed by ubiquitinationbefore separase becomes active.

The destruction of securin is regulatedat the spindle checkpoint and coincideswith the switch in APC activators (cdc20to cdh1) during mitosis11. A D-boxmutant of securin that cannot bedegraded in metaphase inhibits sisterchromatid separation. It should be noted,however, that in X. laevis, APC activationis not exclusively dependent upon securindestruction; rather, it also involvesdecreased Cdk1 activity.

The p53 linkLoss of securin or failure to destroy italters chromosome segregation, whichcould result in aneuploidy as observed incancer. One potential corollary to theseobservations is the speculated linkbetween the DNA response pathway andsister chromatid separation. The interac-tion could be mediated through Ku, aregulatory subunit of the DNA depen-dent protein kinase that interacts with

securin12. DNA damage functionally dis-rupts this association. It should benoted, however, that yeast Pds1 is phos-phorylated in vitro in a DNA damage–independent manner9. Could p53 beinduced during DNA damage and bindsecurin after securin dissociates fromKu, which may be important duringmalignant transformation?

The roles of p53 in G1/S and G2/Mcell-cycle checkpoints are diverse. Inresponse to a disrupted mitotic spindle,cells induce a p53-independent check-point that acts at the metaphase–anaphase transition. Cells cannot ini-tially progress through mitosis untilspindle formation is completed. Abnor-mal mitosis then ensues, in which sisterchromatids fail to segregate. After adelay, p53 responds to this mitotic fail-ure by instituting a G1-like growtharrest13. However, the molecular mecha-nism by which p53 senses mitotic failureis still unknown. One supposition is thatthere is direct interaction between p53and the mitotic spindle apparatus. Forexample, p53 associates with micro-tubules and is transported to the nucleusby dynein, the latter to increase its accu-mulation after DNA damage14. Alterna-tively, p53 might detect aneuploidy aftera cell exits mitosis. In fact, there may beoverlap between the p53-dependentpostmitotic checkpoint and the check-point which ensues DNA damage15.

BOB CRIMI

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Securin in the cell cycle. a, Securin inhibits separase,a cysteine protease, until anaphase initiation. TheAPC (anaphase-promoting complex), a cell-cycleregulated ubiquitin ligase, modulates securindegradation. Separase mediates sister chromatidseparation during mitosis. b, In the transformedcell, securin may interact with p53 to affect p53-mediated transciptional activity and, possibly, p53induction with DNA damage.

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Although securin overexpression mayconfer growth advantages to the cellthrough interaction with p53, it is unclearwhether p53 responds to mitotic failurethrough securin. Might securin destruc-tion at anaphase liberate p53 for a post-mitotic checkpoint function? It will beimportant to examine the effects ofsecurin on transcriptional targets of p53and to test these effects in cells that do notexpress those targets. Another point ofinvestigation is to establish the subcellular

localization and the phase of the cell cycleduring which the securin–p53 interactionoccurs, and how this interaction isaffected by DNA damage and other per-turbations of mitosis. �1. Ramos-Morales, F. et al. Oncogene 19, 403–409

(2000).2. Saeg, C. et al. Oncogene 18, 5473–5476 (1999).3. Mei, J., Huang, H. & Zhang, P. Curr. Biol. 11,

1197–1201 (2001).4. Wang, Z., Yu, R. & Melmed, S. Mol. Endocrinol. 15,

1870–1879 (2001).5. Zou, H., McGarry, T.J., Bernal, T. & Kirschner, M.W.

Science 285, 418–422 (1999).

6. Bernal, J.A. et al. Nat. Genet. 32 (2002); advanceonline publication, 23 September 2002(doi:10.1038/ng997).

7. Jallepalli, P.V. et al. Cell 105, 445–457 (2001).8. Peters, J.M. Mol. Cell 9, 931–943 (2002).9. Agarwal, R. & Cohen-Fix, O. Genes Dev. 16,

1371–1382 (2002).10. Hornig, N.C., Knowles, P.P., McDonald, N.Q. &

Uhlmann, F. Curr. Biol. 12, 973 (2002).11. Hagting, A. et al. J. Cell Biol. 24, 1125–1137 (2002).12. Romero, F. et al. Nucleic Acids Res. 29, 1300–1307

(2001).13. Cicciarello, M. et al. J. Biol. Chem. 276, 19205–19213

(2001).14. Giannakakou, P. et al. Nat. Cell Biol. 2, 709–717

(2000).15. Lanni, J.S. & Jacks, T. Mol. Cell. Biol. 18, 1055–1064

(1998).

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