Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Structural basis and function of XRN2 binding by XTB domains

Abstract

The RNase XRN2 is essential in RNA metabolism. In Caenorhabditis elegans, XRN2 functions with PAXT-1, which shares a putative XRN2-binding domain (XTBD) with otherwise unrelated mammalian proteins. Here, we characterize the structure and function of an XTBD–XRN2 complex. Although XTBD stably interconnects two XRN2 domains through numerous interacting residues, mutation of a single critical residue suffices to disrupt XTBD–XRN2 complexes in vitro and to recapitulate paxt-1–null mutant phenotypes in vivo. Demonstrating conservation of function, vertebrate XTBD-containing proteins bind XRN2 in vitro, and human CDKN2AIPNL (HsC2AIL) can substitute for PAXT-1 in vivo. In vertebrates, which express three distinct XTBD-containing proteins, XRN2 may partition into distinct stable heterodimeric complexes, which probably differ in subcellular localization or function. In C. elegans, complex formation with PAXT-1, the sole XTBD protein, serves to preserve the stability of XRN2 in the absence of substrate.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Architecture of the XTBD–XRN2ΔZLC complex.
Figure 2: Tyr56 is critical for stability of the highly conserved XTBD-XRN2ΔZLC interface.
Figure 3: XTBD is a general XRN2 binder.
Figure 4: XTBD or RNA substrate stabilize XRN2 by promoting PBS folding.

Similar content being viewed by others

Accession codes

Primary accessions

Protein Data Bank

Referenced accessions

Protein Data Bank

References

  1. Petfalski, E., Dandekar, T., Henry, Y. & Tollervey, D. Processing of the precursors to small nucleolar RNAs and rRNAs requires common components. Mol. Cell. Biol. 18, 1181–1189 (1998).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  2. Wang, M. & Pestov, D.G. 5′-end surveillance by Xrn2 acts as a shared mechanism for mammalian pre-rRNA maturation and decay. Nucleic Acids Res. 39, 1811–1822 (2011).

    Article  PubMed  Google Scholar 

  3. Couvillion, M.T., Bounova, G., Purdom, E., Speed, T.P. & Collins, K. A Tetrahymena Piwi bound to mature tRNA 3′ fragments activates the exonuclease Xrn2 for RNA processing in the nucleus. Mol. Cell 48, 509–520 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  4. Geerlings, T.H., Vos, J.C. & Raué, H.A. The final step in the formation of 25S rRNA in Saccharomyces cerevisiae is performed by 5′→3′ exonucleases. RNA 6, 1698–1703 (2000).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  5. Chanfreau, G., Rotondo, G., Legrain, P. & Jacquier, A. Processing of a dicistronic small nucleolar RNA precursor by the RNA endonuclease Rnt1. EMBO J. 17, 3726–3737 (1998).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  6. Zakrzewska-Placzek, M., Souret, F.F., Sobczyk, G.J., Green, P.J. & Kufel, J. Arabidopsis thaliana XRN2 is required for primary cleavage in the pre-ribosomal RNA. Nucleic Acids Res. 38, 4487–4502 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  7. West, S., Gromak, N. & Proudfoot, N.J. Human 5′ → 3′ exonuclease Xrn2 promotes transcription termination at co-transcriptional cleavage sites. Nature 432, 522–525 (2004).

    Article  CAS  PubMed  Google Scholar 

  8. Kim, M. et al. The yeast Rat1 exonuclease promotes transcription termination by RNA polymerase II. Nature 432, 517–522 (2004).

    Article  CAS  PubMed  Google Scholar 

  9. Davidson, L., Kerr, A. & West, S. Co-transcriptional degradation of aberrant pre-mRNA by Xrn2. EMBO J. 31, 2566–2578 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  10. Chernyakov, I., Whipple, J.M., Kotelawala, L., Grayhack, E.J. & Phizicky, E.M. Degradation of several hypomodified mature tRNA species in Saccharomyces cerevisiae is mediated by Met22 and the 5′-3′ exonucleases Rat1 and Xrn1. Genes Dev. 22, 1369–1380 (2008).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  11. Chatterjee, S. & Großhans, H. Active turnover modulates mature microRNA activity in Caenorhabditis elegans. Nature 461, 546–549 (2009).

    Article  CAS  PubMed  Google Scholar 

  12. Miki, T.S. & Großhans, H. The multifunctional RNase XRN2. Biochem. Soc. Trans. 41, 825–830 (2013).

    Article  CAS  PubMed  Google Scholar 

  13. Nagarajan, V.K., Jones, C.I., Newbury, S.F. & Green, P.J. XRN 5′→3′ exoribonucleases: structure, mechanisms and functions. Biochim. Biophys. Acta 1829, 590–603 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  14. Amberg, D.C., Goldstein, A.L. & Cole, C.N. Isolation and characterization of RAT1: an essential gene of Saccharomyces cerevisiae required for the efficient nucleocytoplasmic trafficking of mRNA. Genes Dev. 6, 1173–1189 (1992).

    Article  CAS  PubMed  Google Scholar 

  15. Miki, T.S., Rüegger, S., Gaidatzis, D., Stadler, M.B. & Großhans, H. Engineering of a conditional allele reveals multiple roles of XRN2 in Caenorhabditis elegans development and substrate specificity in microRNA turnover. Nucleic Acids Res. 42, 4056–4067 (2014).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  16. Parker, R. & Sheth, U. P bodies and the control of mRNA translation and degradation. Mol. Cell 25, 635–646 (2007).

    Article  CAS  PubMed  Google Scholar 

  17. Kenna, M., Stevens, A., McCammon, M. & Douglas, M.G. An essential yeast gene with homology to the exonuclease-encoding XRN1/KEM1 gene also encodes a protein with exoribonuclease activity. Mol. Cell. Biol. 13, 341–350 (1993).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  18. Stevens, A. & Poole, T.L. 5′-exonuclease-2 of Saccharomyces cerevisiae: purification and features of ribonuclease activity with comparison to 5′-exonuclease-1. J. Biol. Chem. 270, 16063–16069 (1995).

    Article  CAS  PubMed  Google Scholar 

  19. Poole, T.L. & Stevens, A. Comparison of features of the RNase activity of 5′-exonuclease-1 and 5′-exonuclease-2 of Saccharomyces cerevisiae. Nucleic Acids Symp. Ser. 79–81 (1995).

  20. Jinek, M., Coyle, S.M. & Doudna, J.A. Coupled 5′ nucleotide recognition and processivity in Xrn1-mediated mRNA decay. Mol. Cell 41, 600–608 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  21. Xiang, S. et al. Structure and function of the 5′→3′ exoribonuclease Rat1 and its activating partner Rai1. Nature 458, 784–788 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  22. Xue, Y. et al. Saccharomyces cerevisiae RAI1 (YGL246c) is homologous to human DOM3Z and encodes a protein that binds the nuclear exoribonuclease Rat1p. Mol. Cell. Biol. 20, 4006–4015 (2000).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  23. Miki, T.S., Richter, H., Rüegger, S. & Großhans, H. PAXT-1 promotes XRN2 activity by stabilizing it through a conserved domain. Mol. Cell 53, 351–360 (2014).

    Article  CAS  PubMed  Google Scholar 

  24. Brannan, K. et al. mRNA decapping factors and the exonuclease Xrn2 function in widespread premature termination of RNA polymerase II transcription. Mol. Cell 46, 311–324 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  25. Close, P. et al. DBIRD complex integrates alternative mRNA splicing with RNA polymerase II transcript elongation. Nature 484, 386–389 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  26. Söding, J., Biegert, A. & Lupas, A.N. The HHpred interactive server for protein homology detection and structure prediction. Nucleic Acids Res. 33, W244–W248 (2005).

    Article  PubMed  PubMed Central  Google Scholar 

  27. Buchan, D.W., Minneci, F., Nugent, T.C., Bryson, K. & Jones, D.T. Scalable web services for the PSIPRED Protein Analysis Workbench. Nucleic Acids Res. 41, W349–W357 (2013).

    Article  PubMed  PubMed Central  Google Scholar 

  28. Linding, R. et al. Protein disorder prediction: implications for structural proteomics. Structure 11, 1453–1459 (2003).

    Article  CAS  PubMed  Google Scholar 

  29. Chang, J.H., Xiang, S., Xiang, K., Manley, J.L. & Tong, L. Structural and biochemical studies of the 5′→3′ exoribonuclease Xrn1. Nat. Struct. Mol. Biol. 18, 270–276 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  30. Holm, L. & Rosenström, P. Dali server: conservation mapping in 3D. Nucleic Acids Res. 38, W545–W549 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  31. Krissinel, E. & Henrick, K. Inference of macromolecular assemblies from crystalline state. J. Mol. Biol. 372, 774–797 (2007).

    Article  CAS  PubMed  Google Scholar 

  32. Duarte, J.M., Srebniak, A., Schärer, M.A. & Capitani, G. Protein interface classification by evolutionary analysis. BMC Bioinformatics 13, 334 (2012).

    Article  PubMed  PubMed Central  Google Scholar 

  33. Ashkenazy, H., Erez, E., Martz, E., Pupko, T. & Ben-Tal, N. ConSurf 2010: calculating evolutionary conservation in sequence and structure of proteins and nucleic acids. Nucleic Acids Res. 38, W529–W533 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  34. Celniker, G. et al. ConSurf: using evolutionary data to raise testable hypotheses about protein function. Isr. J. Chem. 53, 199–206 (2013).

    Article  CAS  Google Scholar 

  35. Arribere, J.A. et al. Efficient marker-free recovery of custom genetic modifications with CRISPR/Cas9 in Caenorhabditis elegans. Genetics 198, 837–846 (2014).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  36. Dickinson, D.J., Ward, J.D., Reiner, D.J. & Goldstein, B. Engineering the Caenorhabditis elegans genome using Cas9-triggered homologous recombination. Nat. Methods 10, 1028–1034 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  37. Cheung, C.T., Singh, R., Kalra, R.S., Kaul, S.C. & Wadhwa, R. Collaborator of ARF (CARF) regulates proliferative fate of human cells by dose-dependent regulation of DNA damage signaling. J. Biol. Chem. 289, 18258–18269 (2014).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  38. Hasan, M.K. et al. CARF is a novel protein that cooperates with mouse p19ARF (human p14ARF) in activating p53. J. Biol. Chem. 277, 37765–37770 (2002).

    Article  CAS  PubMed  Google Scholar 

  39. Hasan, M.K. et al. Alternative reading frame protein (ARF)-independent function of CARF (collaborator of ARF) involves its interactions with p53: evidence for a novel p53-activation pathway and its negative feedback control. Biochem. J. 380, 605–610 (2004).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  40. Querol, E., Perez-Pons, J.A. & Mozo-Villarias, A. Analysis of protein conformational characteristics related to thermostability. Protein Eng. 9, 265–271 (1996).

    Article  CAS  PubMed  Google Scholar 

  41. Feng, X. et al. Identification of a negative response element in the human inducible nitric-oxide synthase (hiNOS) promoter: the role of NF-kappa B-repressing factor (NRF) in basal repression of the hiNOS gene. Proc. Natl. Acad. Sci. USA 99, 14212–14217 (2002).

    Article  CAS  PubMed  Google Scholar 

  42. Uhlén, M. et al. Proteomics: tissue-based map of the human proteome. Science 347, 1260419 (2015).

    PubMed  Google Scholar 

  43. Rother, S. et al. NF-κB-repressing factor phosphorylation regulates transcription elongation via its interactions with 5′→3′ exoribonuclease 2 and negative elongation factor. FASEB J. doi:10.1096/fj.15-270256 (4 September 2015).

  44. Vieille, C. & Zeikus, G.J. Hyperthermophilic enzymes: sources, uses, and molecular mechanisms for thermostability. Microbiol. Mol. Biol. Rev. 65, 1–43 (2001).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  45. Ghaemmaghami, S. & Oas, T.G. Quantitative protein stability measurement in vivo. Nat. Struct. Biol. 8, 879–882 (2001).

    Article  CAS  PubMed  Google Scholar 

  46. McLendon, G. & Radany, E. Is protein turnover thermodynamically controlled? J. Biol. Chem. 253, 6335–6337 (1978).

    CAS  PubMed  Google Scholar 

  47. Parsell, D.A. & Sauer, R.T. The structural stability of a protein is an important determinant of its proteolytic susceptibility in Escherichia coli. J. Biol. Chem. 264, 7590–7595 (1989).

    CAS  PubMed  Google Scholar 

  48. Kumar, S., Tsai, C.J. & Nussinov, R. Factors enhancing protein thermostability. Protein Eng. 13, 179–191 (2000).

    Article  CAS  PubMed  Google Scholar 

  49. Wheeler, T.J., Clements, J. & Finn, R.D. Skylign: a tool for creating informative, interactive logos representing sequence alignments and profile hidden Markov models. BMC Bioinformatics 15, 7 (2014).

    Article  PubMed  PubMed Central  Google Scholar 

  50. Blommel, P.G. & Fox, B.G. A combined approach to improving large-scale production of tobacco etch virus protease. Protein Expr. Purif. 55, 53–68 (2007).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  51. Kabsch, W. XDS. Acta Crystallogr. D Biol. Crystallogr. 66, 125–132 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  52. McCoy, A.J. et al. Phaser crystallographic software. J. Appl. Crystallogr. 40, 658–674 (2007).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  53. Cowtan, K. The Buccaneer software for automated model building. 1. Tracing protein chains. Acta Crystallogr. D Biol. Crystallogr. 62, 1002–1011 (2006).

    Article  PubMed  Google Scholar 

  54. Afonine, P.V. et al. Towards automated crystallographic structure refinement with phenix.refine. Acta Crystallogr. D Biol. Crystallogr. 68, 352–367 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  55. Emsley, P., Lohkamp, B., Scott, W.G. & Cowtan, K. Features and development of Coot. Acta Crystallogr. D Biol. Crystallogr. 66, 486–501 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  56. Sinturel, F. et al. Real-time fluorescence detection of exoribonucleases. RNA 15, 2057–2062 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  57. Brenner, S. The genetics of Caenorhabditis elegans. Genetics 77, 71–94 (1974).

    CAS  PubMed  PubMed Central  Google Scholar 

  58. Gibson, D.G. et al. Enzymatic assembly of DNA molecules up to several hundred kilobases. Nat. Methods 6, 343–345 (2009).

    Article  CAS  PubMed  Google Scholar 

  59. Friedland, A.E. et al. Heritable genome editing in C. elegans via a CRISPR-Cas9 system. Nat. Methods 10, 741–743 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  60. Katic, I. & Großhans, H. Targeted heritable mutation and gene conversion by Cas9-CRISPR in Caenorhabditis elegans. Genetics 195, 1173–1176 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  61. Katic, I., Xu, L. & Ciosk, R. CRISPR/Cas9 genome editing in Caenorhabditis elegans: evaluation of templates for homology-mediated repair and knock-ins by homology-independent DNA repair. G3 (Bethesda) 5, 1649–1656 (2015).

    Article  CAS  Google Scholar 

  62. Frøkjaer-Jensen, C. et al. Single-copy insertion of transgenes in Caenorhabditis elegans. Nat. Genet. 40, 1375–1383 (2008).

    Article  PubMed  PubMed Central  Google Scholar 

  63. Frøkjær-Jensen, C., Davis, M.W., Ailion, M. & Jorgensen, E.M. Improved Mos1-mediated transgenesis in C. elegans. Nat. Methods 9, 117–118 (2012).

    Article  PubMed  PubMed Central  Google Scholar 

  64. de la Mata, M. et al. Potent degradation of neuronal miRNAs induced by highly complementary targets. EMBO Rep. 16, 500–511 (2015).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

Download references

Acknowledgements

We thank J. Keusch for technical support, D. Hess for mass spectrometry analysis, M. de la Mata for assistance with cell transfections, M. Zou (Friedrich Miescher Institute for Biomedical Research) for zebrafish lysate and J.A. Chao and N. Thomä for critical comments on the manuscript. We are grateful to the staff of the Swiss Light Source, Paul Scherrer Institute, Villingen, where parts of the experiments were performed. The research leading to these results received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement number 241985 (European Research Council 'miRTurn'; to H. Großhans), the Swiss National Science Foundation (SNF 31003A_143313; to H. Großhans) and the Novartis Research Foundation through the Friedrich Miescher Institute (to I.K., H. Gut and H. Großhans).

Author information

Authors and Affiliations

Authors

Contributions

H.R. and H. Großhans designed research. H.R. designed and performed in vitro and in vivo experiments. H. Gut and H.R. performed crystallographic structure determination and analyses. I.K. performed clustered regularly interspaced short palindromic repeats (CRISPR) procedures, injected worms and assisted in crossing worm lines. H.R., H. Großhans and H. Gut wrote the manuscript. All authors edited the manuscript.

Corresponding author

Correspondence to Helge Großhans.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Integrated supplementary information

Supplementary Figure 1 Complexes of XRNs and their binding partners.

(a) A Coomassie-stained SDS-PAGE gel of PAXT-1 truncation mutants (asterisks) co-expressed with XRN2 (arrowhead). All mutant proteins bind XRN2 to similar extents. For each PAXT-1 mutant, two clones were expressed and pulled on their His6-tag to co-elute XRN2. (b) Structural representation of the XTBD – XRN2 complex highlighting the tower domain and the PBS-protruding loop. XTBD and the remaining parts of XRN2 are in light colors. (c) Structural representation of XTBD bound to XRN2, with helices and loops indicated. (d) Superposition of C. elegans XTBD – XRN2 complex and S. pombe Rai1p – Rat1p complex reveals that XTBD and Rai1p bind to distinct sites on XRN2. (e) Superposition of C. elegans XTBD – XRN2 complex and D. melanogaster XRN1 reveals potential structural clashes of XRN1 with XTBD (dashed red circles in close-up, top).

Supplementary Figure 2 Characterization of XRN2-XTBD interaction.

(a) SEC-MALS analysis of the XTBD –XRN2ΔZLC complex. The blue line indicates the gel filtration elution profile, monitored by UV light absorption at 280 nm. Red dots indicate molar mass values under the peak. The measured molar mass for the complex (80.9 kDa) was calculated over peak fractions covering the elution volume 14.8 ml – 16.3 ml, which represent a completely monodisperse sample (polydispersity = 1.000). (b) Close-up of the binding interface of XTBD (yellow) and XRN2ΔZLC (cyan/gray). Key residues mediating interaction are represented as sticks with oxygens and nitrogens colored in red and blue (atom colors), respectively. (c) Zoom-in on residue Cys54, which forms hydrophobic interactions with Pro656 and Cα / Cβ of Phe659 as well as a weak hydrogen bond with Leu675. (d) Biological replicate of the western blot shown in Fig. 2g, detecting XRN2 and PAXT-1 from worms grown at 26°C. (e), (f) Small scale expression of His6-PAXT-1, -PAXT-1_Y56A and- PAXT-1_C54G. (e) For each construct, cleared lysate, unbound and Ni-NTA pull-down fraction were loaded. (f) Western blot using antibodies against XRN2 and PAXT-1 was performed on the cleared lysates.

Supplementary Figure 3 Homology of XTBDs and XRN2 PBS domains.

(a) A human C2AIL homology model (magenta) superimposed onto the XTBD - XRN2 complex (yellow and cyan, respectively). (b) Pfam alignment of selected XTBDs from PAXT-1 of C. elegans (Uniprot identifier Q21738) and C. briggsae (A8XFX8), respectively, and CDKN2AIP from D. rerio (F1QZX8), D. melanogaster (Q9VF83), H. sapiens (Q9NXV6), and M. musculus (Q8BI72), respectively, on which the homology model is based. The critical residue Tyr56 is found at position 104 in this alignment. (c) Alignment based on HMM-search of XRN2 PBS. Shown are representative sequences from C. elegans (Q9U299), C. briggsae (Q60SG7), D. rerio (Q802V7), D. melanogaster (Q9VM71), H. sapiens (Q9H0D6) and M. musculus (Q9DBR1) XRN2.

Supplementary Figure 4 Kinetic analysis of XRN2 and its complex.

(a) Michaelis-Menten kinetic curves of XRN2 and the PAXT-1 – XRN2 complex, respectively. XRN2’s catalytic activity was determined on a broad range of substrate concentrations using an assay based on a FAM fluorophore-coupled RNA substrate and a quencher-coupled DNA primer (Sinturel et al., RNA, 15, 2057-2062, 2009). As the substrate range accessible by this assay is limited by the dynamic range of measurable FAM fluorescence by the qPCR machine, we extended it by adding a four-fold excess of unlabeled 22 nt let-7 miRNA to the 30 nt RNA – DNA duplex. The curves are extrapolations from measurements shown in the inset. The corresponding KM values are marked as colored squares. At 0.013 s-1, the kcat of XRN2 alone is comparable to that of the PAXT-1 – XRN2 complex (table), and turnover rates are identical at substrate concentrations up to 0.5 · 10-6 M (inset). A small difference in Michaelis-Menten constants KM (table) suggests a modestly increased affinity of PAXT-1-bound XRN2 to its substrate relative to XRN2 alone. We conclude that PAXT-1 has little or no stimulatory activity on XRN2, at least under the conditions of this assay. (b) Replicates of the assay shown in Figure 4d.

Supplementary information

Supplementary Text and Figures

Supplementary Figures 1–4 (PDF 1561 kb)

Supplementary Data Set 1

Uncropped gels and blots (PDF 4789 kb)

Source data

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Richter, H., Katic, I., Gut, H. et al. Structural basis and function of XRN2 binding by XTB domains. Nat Struct Mol Biol 23, 164–171 (2016). https://doi.org/10.1038/nsmb.3155

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nsmb.3155

This article is cited by

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing