Assembly of the nucleolus: in need of revision

News & Views Assembly o he nucleolus: in need o e ision Ma ia Ca mo-Fonseca Ou cu en iew o he nucleolus has been shaped by he concep ha he o ga- niza ion o his p ominen compa men wi hin he nucleus is p ima ily dic a ed by i s unc ion, he making o ibosome sub- uni s. Whe he ibosome biogenesis is amed by a dedica ed nucleola sca old has emained unclea . In his issue o The EMBO Jou nal, Caud on-He ge and collea- gues p esen e idence o a nucleola skele on composed o non-coding RNA en iched in Alu epea elemen s. See also: M Caud on-He ge e al (No embe 2015) P oduc ion o ibosomes is i al o li e, and euka yo ic cells e ol ed a specialized compa men , he nucleo- lus, whe e ibosomal RNA genes a e an- sc ibed, p e- ibosomal RNA is p ocessed, and p e- ibosomes a e assembled (Pede son, 2010). Mammalian cells ypically con ain a ew nucleoli ha disappea du ing mi osis and o m de no o in ea ly G1 a ound ibosomal genes ( DNA). In humans, long s e ches o DNA a ays loca ed on i e dis inc ch omo- somes coalesce o o m be ween one and h ee nucleoli (Sa ino e al, 2001). The DNA ansc ip ion uni s coupled wi h binding si es o he HMG-box p o ein UBF a e su icien o di ec o ma ion o a nucleolus (G ob e al, 2013). Howe e , wha d i es he clus e ing o DNA ansc ip ion uni s in o only one o a ew nucleoli pe nucleus emains elusi e. The new s udy by Caud on-He ge and colleagues iden i ies non-coding RNAs en iched in Alu epea elemen s as po en ially in ol ed in his p ocess. The e a e o e 1 million copies o Alu elemen s in he human genome, many o which localize wi hin in ons o p o ein- coding genes (Deininge , 2011). Acco ding o Caud on-He ge e al (2015), s able sho in on-de i ed RNAs con aining Alu epea elemen s accumula e in he nucleolus and a e essen ial o i s in eg i y. In he absence o so-called aluRNAs, nucleoli dispe sed in o smalle domains and DNA ansc ip ion was se e ely educed. Nascen in onic Alu RNAs we e p e iously shown o escape deg ada ion a e p e-mRNA splicing, being p ocessed and packaged in o me abolically s able small ibonucleop o ein pa icles (Ja ´dy e al, 2012). Howe e , hese p e i- ously desc ibed RNA species we e exclu- si ely de ec ed in he nucleoplasm (Ja ´dy e al, 2012). Caud on-He ge e al (2015) also ca ied ou RNA e he ing expe imen s using a lacO a ay consis ing o mul iple copies o he NOR NUCLEOLUS UBF NOR aluRNA Nucleolin/ Nucleophosmin Exon In onic Alu elemen Exon Figu e 1. Scheme illus a ing he aluRNA-sca old model. Splicing o p e-mRNAs con aining in onic Alu elemen s gene a es sho aluRNAs ha associa e wi h nucleola p o eins nucleolin and nucleophosmin. These aluRNAs, which a e capable o a ac ing genomic loci o he nucleolus, c ea e a sca old ha may con ibu e o clus e ing o nucleola o ganizing egions (NORs) om di e en ch omosomes ia in e ac ions wi h UBF. Ins i u o de Medicina Molecula , Faculdade de Medicina, Uni e sidade de Lisboa, Lisboa, Po ugal. E-mail: ca mo[email p o ec ed] DOI 10.15252/embj.201593185 | Published online 15 Oc obe 2015 ª2015 The Au ho The EMBO Jou nal Vol 34 |No22 |2015 2731 binding sequence o he lac ep esso p o ein, LacI (She so & Dund , 2011). AluRNA was agged wi h MS2 s em loops and immobilized a he lacO a ay h ough binding o MS2 p o ein used o LacI. This esul ed in ec ui men o he lacO genomic locus o he nucleolus. Nucleolin and nucleo- phosmin, wo abundan s uc u al p o eins o he nucleolus, we e u he shown o in e ac in i o wi h aluRNAs. Based on hese obse a ions, he ollowing model was p oposed: (i) p e-mRNA splicing o p o ein- coding genes con aining in onic Alu epea elemen s gene a es small RNA species e med aluRNAs; (ii) hese RNAs ac as a sca old ha ec ui s nucleolin and nucleo- phosmin; and (iii) in e ac ions be ween nucleolin, nucleophosmin, and UBF esul in coalescence o DNA a ays ha would o he wise dispe se h oughou he nucleus (Fig 1). A ema kable ea u e o his model is ha i explains he long-deba ed conund um ha speci ic inhibi ion o RNA polyme ase II (Pol II) causes disin eg a ion o nucleoli, al hough e y li le Pol II ac i i y is de ec ed in his nuclea compa men . Ine i ably, howe e , his new model aises a numbe o open ques ions. Fo example, Alu elemen s a e p ima e speci ic, and al hough a ge ed dis up ion o Alu- ela ed B1-con aining RNAs causes nucleola dispe sion in mouse cells (Caud on-He ge e al, 2015), i is in iguing how dis inc RNA species co- e ol ed a common nucleola sca olding ole. I is also puzzling ha a subse o in on-encoded Alu RNAs gene a e nucleo- plasmic box H/ACA RNPs (Ja ´dy e al, 2012), while o he s a e essen ial o nucleola o ga- niza ion. Addi ional mechanis ic issues conce n speci ici y o he in e ac ion be ween nucleola aluRNAs and he e y abundan nuclea p o eins nucleolin and nucleophosmin. A inal emaining enigma is how nucleola aluRNAs a e unc ionally coupled o Pol I ac i i y. Clea ly, he aluRNA-sca old model pa es he way o u he esea ch ha will likely expand ou unde s anding o how ibosome biogenesis is con olled by nucleola biology. Re e ences Caud on-He ge M, Panke T, Seile J, Néme h A, Voi R, G umm I, Rippe K (2015)Alu elemen - con aining RNAs main ain nucleola s uc u e and unc ion. EMBO J 34:2758 –2774 Deininge P (2011)Alu elemen s: know he SINES. Genome Biol 12:236 G ob A, Colle an C, MacS ay B (2013) Cons uc ion o syn he ic nucleoli in human cells e eals how a majo unc ional nuclea domain is o med and p opaga ed h ough cell di ision. Genes De 28:220 –230 Jády B, Ke ele A, Kiss T (2012) Human in on- encoded Alu RNAs a e p ocessed and packaged in o Wd 79-associa ed nucleoplasmic box H/ ACA RNPs. Genes De 26:1897 –1910 Pede son T (2010) The Nucleolus. Cold Sp ing Ha b Pe spec Biol 3:1 Sa ino TM, Geb ane-Younes J, De Mey J, Siba i a JB, He nandez-Ve dun D (2001) Nucleola assembly o he RNA p ocessing machine y in li ing cells. J Cell Biol 153:1097 –1110 She so SP, Dund M (2011) Nuclea ion o nuclea bodies by RNA. Na Cell Biol 13: 167 –173 The EMBO Jou nal Vol 34 |No22 |2015 ª2015 The Au ho The EMBO Jou nal Assembly o he nucleolus: in need o e ision Ma ia Ca mo-Fonseca 2732