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.

  • Review Article
  • Published:

New insights into ubiquitin E3 ligase mechanism

Abstract

E3 ligases carry out the final step in the ubiquitination cascade, catalyzing transfer of ubiquitin from an E2 enzyme to form a covalent bond with a substrate lysine. Three distinct classes of E3 ligases have been identified that stimulate transfer of ubiquitin and ubiquitin-like proteins through either a direct or an indirect mechanism. Only recently have the catalytic mechanisms of E3 ligases begun to be elucidated.

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: RING E3 ligases.
Figure 2: Mechanism by which RING E3s stimulate ubiquitin transfer.
Figure 3: E2 back-side binding by additional domains in RING E3 ligases.
Figure 4: HECT E3 ligases.
Figure 5: RBR E3 ligase mechanism.

Similar content being viewed by others

Accession codes

Accessions

Protein Data Bank

References

  1. Kerscher, O., Felberbaum, R. & Hochstrasser, M. Modification of proteins by ubiquitin and ubiquitin-like proteins. Annu. Rev. Cell Dev. Biol. 22, 159–180 (2006).

    Article  CAS  PubMed  Google Scholar 

  2. Ulrich, H.D. & Walden, H. Ubiquitin signalling in DNA replication and repair. Nat. Rev. Mol. Cell Biol. 11, 479–489 (2010).

    Article  CAS  PubMed  Google Scholar 

  3. Komander, D. & Rape, M. The ubiquitin code. Annu. Rev. Biochem. 81, 203–229 (2012).

    Article  CAS  PubMed  Google Scholar 

  4. Pickart, C.M. & Eddins, M.J. Ubiquitin: structures, functions, mechanisms. Biochim. Biophys. Acta 1695, 55–72 (2004).

    Article  CAS  PubMed  Google Scholar 

  5. Tokunaga, F. et al. Involvement of linear polyubiquitylation of NEMO in NF-κB activation. Nat. Cell Biol. 11, 123–132 (2009).

    Article  CAS  PubMed  Google Scholar 

  6. Scaglione, K.M. et al. The ubiquitin-conjugating enzyme (E2) Ube2w ubiquitinates the N terminus of substrates. J. Biol. Chem. 288, 18784–18788 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  7. Tatham, M.H., Plechanovová, A., Jaffray, E.G., Salmen, H. & Hay, R.T. Ube2W conjugates ubiquitin to α-amino groups of protein N-termini. Biochem. J. 453, 137–145 (2013).

    Article  CAS  PubMed  Google Scholar 

  8. Deshaies, R.J. & Joazeiro, C.A. RING domain E3 ubiquitin ligases. Annu. Rev. Biochem. 78, 399–434 (2009).

    Article  CAS  PubMed  Google Scholar 

  9. Li, W. et al. Genome-wide and functional annotation of human E3 ubiquitin ligases identifies MULAN, a mitochondrial E3 that regulates the organelle's dynamics and signaling. PLoS ONE 3, e1487 (2008).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  10. Metzger, M.B., Hristova, V.A. & Weissman, A.M. HECT and RING finger families of E3 ubiquitin ligases at a glance. J. Cell Sci. 125, 531–537 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  11. Budhidarmo, R., Nakatani, Y. & Day, C.L. RINGs hold the key to ubiquitin transfer. Trends Biochem. Sci. 37, 58–65 (2012).

    Article  CAS  PubMed  Google Scholar 

  12. Huibregtse, J.M., Scheffner, M., Beaudenon, S. & Howley, P.M. A family of proteins structurally and functionally related to the E6-AP ubiquitin-protein ligase. Proc. Natl. Acad. Sci. USA 92, 2563–2567 (1995).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  13. Wenzel, D.M. & Klevit, R.E. Following Ariadne's thread: a new perspective on RBR ubiquitin ligases. BMC Biol. 10, 24 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  14. van Wijk, S.J. & Timmers, H.T. The family of ubiquitin-conjugating enzymes (E2s): deciding between life and death of proteins. FASEB J. 24, 981–993 (2010).

    Article  CAS  PubMed  Google Scholar 

  15. Wenzel, D.M., Stoll, K.E. & Klevit, R.E. E2s: structurally economical and functionally replete. Biochem. J. 433, 31–42 (2011).

    Article  CAS  PubMed  Google Scholar 

  16. Wu, P.Y. et al. A conserved catalytic residue in the ubiquitin-conjugating enzyme family. EMBO J. 22, 5241–5250 (2003).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  17. Berndsen, C.E., Wiener, R., Yu, I.W., Ringel, A.E. & Wolberger, C. A conserved asparagine has a structural role in ubiquitin-conjugating enzymes. Nat. Chem. Biol. 9, 154–156 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  18. Yunus, A.A. & Lima, C.D. Lysine activation and functional analysis of E2-mediated conjugation in the SUMO pathway. Nat. Struct. Mol. Biol. 13, 491–499 (2006).

    Article  CAS  PubMed  Google Scholar 

  19. Plechanovová, A., Jaffray, E.G., Tatham, M.H., Naismith, J.H. & Hay, R.T. Structure of a RING E3 ligase and ubiquitin-loaded E2 primed for catalysis. Nature 489, 115–120 (2012).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  20. Jin, L., Williamson, A., Banerjee, S., Philipp, I. & Rape, M. Mechanism of ubiquitin-chain formation by the human anaphase-promoting complex. Cell 133, 653–665 (2008).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  21. Wickliffe, K.E., Lorenz, S., Wemmer, D.E., Kuriyan, J. & Rape, M. The mechanism of linkage-specific ubiquitin chain elongation by a single-subunit E2. Cell 144, 769–781 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  22. Sakata, E. et al. Crystal structure of UbcH5bubiquitin intermediate: insight into the formation of the self-assembled E2Ub conjugates. Structure 18, 138–147 (2010).

    Article  CAS  PubMed  Google Scholar 

  23. Eddins, M.J., Carlile, C.M., Gomez, K.M., Pickart, C.M. & Wolberger, C. Mms2–Ubc13 covalently bound to ubiquitin reveals the structural basis of linkage-specific polyubiquitin chain formation. Nat. Struct. Mol. Biol. 13, 915–920 (2006).

    Article  CAS  PubMed  Google Scholar 

  24. McKenna, S. et al. An NMR-based model of the ubiquitin-bound human ubiquitin conjugation complex Mms2.Ubc13: the structural basis for lysine 63 chain catalysis. J. Biol. Chem. 278, 13151–13158 (2003).

    Article  CAS  PubMed  Google Scholar 

  25. Pruneda, J.N., Stoll, K.E., Bolton, L.J., Brzovic, P.S. & Klevit, R.E. Ubiquitin in motion: structural studies of the ubiquitin-conjugating enzyme approximately ubiquitin conjugate. Biochemistry 50, 1624–1633 (2011).

    Article  CAS  PubMed  Google Scholar 

  26. Lorick, K.L. et al. RING fingers mediate ubiquitin-conjugating enzyme (E2)-dependent ubiquitination. Proc. Natl. Acad. Sci. USA 96, 11364–11369 (1999).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  27. Bentley, M.L. et al. Recognition of UbcH5c and the nucleosome by the Bmi1/Ring1b ubiquitin ligase complex. EMBO J. 30, 3285–3297 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  28. Dou, H., Buetow, L., Sibbet, G.J., Cameron, K. & Huang, D.T. BIRC7–E2 ubiquitin conjugate structure reveals the mechanism of ubiquitin transfer by a RING dimer. Nat. Struct. Mol. Biol. 19, 876–883 (2012).

    Article  CAS  PubMed  Google Scholar 

  29. Yin, Q. et al. E2 interaction and dimerization in the crystal structure of TRAF6. Nat. Struct. Mol. Biol. 16, 658–666 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  30. Zheng, N., Wang, P., Jeffrey, P.D. & Pavletich, N.P. Structure of a c-Cbl-UbcH7 complex: RING domain function in ubiquitin-protein ligases. Cell 102, 533–539 (2000).

    Article  CAS  PubMed  Google Scholar 

  31. Liew, C.W., Sun, H., Hunter, T. & Day, C.L. RING domain dimerization is essential for RNF4 function. Biochem. J. 431, 23–29 (2010).

    Article  CAS  PubMed  Google Scholar 

  32. Brzovic, P.S., Rajagopal, P., Hoyt, D.W., King, M.C. & Klevit, R.E. Structure of a BRCA1–BARD1 heterodimeric RING–RING complex. Nat. Struct. Biol. 8, 833–837 (2001).

    Article  CAS  PubMed  Google Scholar 

  33. Buchwald, G. et al. Structure and E3-ligase activity of the Ring–Ring complex of polycomb proteins Bmi1 and Ring1b. EMBO J. 25, 2465–2474 (2006).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  34. Campbell, S.J. et al. Molecular insights into the function of RING finger (RNF)-containing proteins hRNF8 and hRNF168 in Ubc13/Mms2-dependent ubiquitylation. J. Biol. Chem. 287, 23900–23910 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  35. Ohi, M.D., Vander Kooi, C.W., Rosenberg, J.A., Chazin, W.J. & Gould, K.L. Structural insights into the U-box, a domain associated with multi-ubiquitination. Nat. Struct. Biol. 10, 250–255 (2003).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  36. Eletr, Z.M., Huang, D.T., Duda, D.M., Schulman, B.A. & Kuhlman, B. E2 conjugating enzymes must disengage from their E1 enzymes before E3-dependent ubiquitin and ubiquitin-like transfer. Nat. Struct. Mol. Biol. 12, 933–934 (2005).

    Article  CAS  PubMed  Google Scholar 

  37. Lydeard, J.R., Schulman, B.A. & Harper, J.W. Building and remodelling Cullin–RING E3 ubiquitin ligases. EMBO Rep. 14, 1050–1061 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  38. Skaar, J.R., Pagan, J.K. & Pagano, M. Mechanisms and function of substrate recruitment by F-box proteins. Nat. Rev. Mol. Cell Biol. 14, 369–381 (2013).

    Article  CAS  PubMed  Google Scholar 

  39. Hibbert, R.G., Huang, A., Boelens, R. & Sixma, T.K. E3 ligase Rad18 promotes monoubiquitination rather than ubiquitin chain formation by E2 enzyme Rad6. Proc. Natl. Acad. Sci. USA 108, 5590–5595 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  40. Mace, P.D. et al. Structures of the cIAP2 RING domain reveal conformational changes associated with ubiquitin-conjugating enzyme (E2) recruitment. J. Biol. Chem. 283, 31633–31640 (2008).

    Article  CAS  PubMed  Google Scholar 

  41. Ozkan, E., Yu, H. & Deisenhofer, J. Mechanistic insight into the allosteric activation of a ubiquitin-conjugating enzyme by RING-type ubiquitin ligases. Proc. Natl. Acad. Sci. USA 102, 18890–18895 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  42. Plechanovová, A. et al. Mechanism of ubiquitylation by dimeric RING ligase RNF4. Nat. Struct. Mol. Biol. 18, 1052–1059 (2011).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  43. Spratt, D.E., Wu, K., Kovacev, J., Pan, Z.Q. & Shaw, G.S. Selective recruitment of an E2ubiquitin complex by an E3 ubiquitin ligase. J. Biol. Chem. 287, 17374–17385 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  44. Dou, H. et al. Structural basis for autoinhibition and phosphorylation-dependent activation of c-Cbl. Nat. Struct. Mol. Biol. 19, 184–192 (2012).

    Article  CAS  PubMed  Google Scholar 

  45. Reverter, D. & Lima, C.D. Insights into E3 ligase activity revealed by a SUMO–RanGAP1–Ubc9–Nup358 complex. Nature 435, 687–692 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  46. Pruneda, J.N. et al. Structure of an E3:E2Ub complex reveals an allosteric mechanism shared among RING/U-box ligases. Mol. Cell 47, 933–942 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  47. Bruice, T.C. & Pandit, U.K. Intramolecular models depicting the kinetic importance of “fit” in enzymatic catalysis. Proc. Natl. Acad. Sci. USA 46, 402–404 (1960).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  48. Jencks, W.P. Catalysis in Chemistry and Enzymology (Dover, New York, 1987).

  49. Saha, A., Lewis, S., Kleiger, G., Kuhlman, B. & Deshaies, R.J. Essential role for ubiquitin-ubiquitin-conjugating enzyme interaction in ubiquitin discharge from cdc34 to substrate. Mol. Cell 42, 75–83 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  50. Das, R. et al. Allosteric activation of E2-RING finger-mediated ubiquitylation by a structurally defined specific E2-binding region of gp78. Mol. Cell 34, 674–685 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  51. Brzovic, P.S., Lissounov, A., Christensen, D.E., Hoyt, D.W. & Klevit, R.E.A. UbcH5/ubiquitin noncovalent complex is required for processive BRCA1-directed ubiquitination. Mol. Cell 21, 873–880 (2006).

    Article  CAS  PubMed  Google Scholar 

  52. Das, R. et al. Allosteric regulation of E2:E3 interactions promote a processive ubiquitination machine. EMBO J. 32, 2504–2516 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  53. Notenboom, V. et al. Functional characterization of Rad18 domains for Rad6, ubiquitin, DNA binding and PCNA modification. Nucleic Acids Res. 35, 5819–5830 (2007).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  54. Metzger, M.B. et al. A structurally unique E2-binding domain activates ubiquitination by the ERAD E2, Ubc7p, through multiple mechanisms. Mol. Cell 50, 516–527 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  55. Huang, L. et al. Structure of an E6AP-UbcH7 complex: insights into ubiquitination by the E2–E3 enzyme cascade. Science 286, 1321–1326 (1999).

    Article  CAS  PubMed  Google Scholar 

  56. Verdecia, M.A. et al. Conformational flexibility underlies ubiquitin ligation mediated by the WWP1 HECT domain E3 ligase. Mol. Cell 11, 249–259 (2003).

    Article  CAS  PubMed  Google Scholar 

  57. Kamadurai, H.B. et al. Insights into ubiquitin transfer cascades from a structure of a UbcH5Bubiquitin-HECT(NEDD4L) complex. Mol. Cell 36, 1095–1102 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  58. Kamadurai, H.B. et al. Mechanism of ubiquitin ligation and lysine prioritization by a HECT E3. Elife 2, e00828 (2013).

    Article  PubMed  PubMed Central  Google Scholar 

  59. Maspero, E. et al. Structure of a ubiquitin-loaded HECT ligase reveals the molecular basis for catalytic priming. Nat. Struct. Mol. Biol. 20, 696–701 (2013).

    Article  CAS  PubMed  Google Scholar 

  60. Ronchi, V.P., Klein, J.M. & Haas, A.L. E6AP/UBE3A ubiquitin ligase harbors two E2ubiquitin binding sites. J. Biol. Chem. 288, 10349–10360 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  61. Kim, H.C. & Huibregtse, J.M. Polyubiquitination by HECT E3s and the determinants of chain type specificity. Mol. Cell. Biol. 29, 3307–3318 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  62. Aguilera, M., Oliveros, M., Martinez-Padron, M., Barbas, J.A. & Ferrus, A. Ariadne-1: a vital Drosophila gene is required in development and defines a new conserved family of ring-finger proteins. Genetics 155, 1231–1244 (2000).

    CAS  PubMed  PubMed Central  Google Scholar 

  63. Dawson, T.M. & Dawson, V.L. The role of parkin in familial and sporadic Parkinson's disease. Mov. Disord. 25, S32–S39 (2010).

    Article  PubMed  PubMed Central  Google Scholar 

  64. Wenzel, D.M., Lissounov, A., Brzovic, P.S. & Klevit, R.E. UBCH7 reactivity profile reveals parkin and HHARI to be RING/HECT hybrids. Nature 474, 105–108 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  65. Duda, D.M. et al. Structure of HHARI, a RING-IBR-RING ubiquitin ligase: autoinhibition of an Ariadne-family E3 and insights into ligation mechanism. Structure 21, 1030–1041 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  66. Riley, B.E. et al. Structure and function of Parkin E3 ubiquitin ligase reveals aspects of RING and HECT ligases. Nat. Commun. 4, 1982 (2013).

    Article  CAS  PubMed  Google Scholar 

  67. Trempe, J.F. et al. Structure of parkin reveals mechanisms for ubiquitin ligase activation. Science 340, 1451–1455 (2013).

    Article  CAS  PubMed  Google Scholar 

  68. Wauer, T. & Komander, D. Structure of the human Parkin ligase domain in an autoinhibited state. EMBO J. 32, 2099–2112 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  69. Stieglitz, B. et al. Structural basis for ligase-specific conjugation of linear ubiquitin chains by HOIP. Nature 503, 422–426 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  70. Kirisako, T. et al. A ubiquitin ligase complex assembles linear polyubiquitin chains. EMBO J. 25, 4877–4887 (2006).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  71. Chaugule, V.K. et al. Autoregulation of Parkin activity through its ubiquitin-like domain. EMBO J. 30, 2853–2867 (2011).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  72. Burchell, L., Chaugule, V.K. & Walden, H. Small, N-terminal tags activate Parkin E3 ubiquitin ligase activity by disrupting its autoinhibited conformation. PLoS ONE 7, e34748 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  73. Cookson, M.R. Parkinsonism due to mutations in PINK1, parkin, and DJ-1 and oxidative stress and mitochondrial pathways. Cold Spring Harb. Perspect. Med. 2, a009415 (2012).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  74. Hofmann, K. & Bucher, P. The UBA domain: a sequence motif present in multiple enzyme classes of the ubiquitination pathway. Trends Biochem. Sci. 21, 172–173 (1996).

    Article  CAS  PubMed  Google Scholar 

  75. Walden, H. & Martinez-Torres, R.J. Regulation of Parkin E3 ubiquitin ligase activity. Cell. Mol. Life Sci. 69, 3053–3067 (2012).

    Article  CAS  PubMed  Google Scholar 

  76. Komander, D., Clague, M.J. & Urbe, S. Breaking the chains: structure and function of the deubiquitinases. Nat. Rev. Mol. Cell Biol. 10, 550–563 (2009).

    Article  CAS  PubMed  Google Scholar 

  77. Smit, J.J. et al. The E3 ligase HOIP specifies linear ubiquitin chain assembly through its RING-IBR-RING domain and the unique LDD extension. EMBO J. 31, 3833–3844 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  78. Stieglitz, B., Morris-Davies, A.C., Koliopoulos, M.G., Christodoulou, E. & Rittinger, K. LUBAC synthesizes linear ubiquitin chains via a thioester intermediate. EMBO Rep. 13, 840–846 (2012).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  79. Smit, J.J. et al. Target specificity of the E3 ligase LUBAC for ubiquitin and NEMO relies on different minimal requirements. J. Biol. Chem. 288, 31728–31737 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  80. Vijay-Kumar, S., Bugg, C.E. & Cook, W.J. Structure of ubiquitin refined at 1.8 Å resolution. J. Mol. Biol. 194, 531–544 (1987).

    Article  CAS  PubMed  Google Scholar 

  81. Komander, D. et al. Molecular discrimination of structurally equivalent Lys 63-linked and linear polyubiquitin chains. EMBO Rep. 10, 466–473 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  82. Datta, A.B., Hura, G.L. & Wolberger, C. The structure and conformation of Lys63-linked tetraubiquitin. J. Mol. Biol. 392, 1117–1124 (2009).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  83. Cook, W.J., Jeffrey, L.C., Carson, M., Chen, Z. & Pickart, C.M. Structure of a diubiquitin conjugate and a model for interaction with ubiquitin conjugating enzyme (E2). J. Biol. Chem. 267, 16467–16471 (1992).

    CAS  PubMed  Google Scholar 

  84. Wu, G. et al. Structure of a β-TrCP1-Skp1-β-catenin complex. Mol. Cell 11, 1445–1456 (2003).

    Article  CAS  PubMed  Google Scholar 

  85. Zheng, N. et al. Structure of the Cul1–Rbx1–Skp1–F boxSkp2 SCF ubiquitin ligase complex. Nature 416, 703–709 (2002).

    Article  CAS  PubMed  Google Scholar 

  86. Duda, D.M. et al. Structural insights into NEDD8 activation of cullin-RING ligases: conformational control of conjugation. Cell 134, 995–1006 (2008).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

Download references

Acknowledgements

We thank R. Hay, A. Plechanovová and B. Schulman for providing coordinates of modeled complexes. C.W. acknowledges grant support from the US National Institutes of Health (GM095822) and National Science Foundation (MCB0920082).

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to Cynthia Wolberger.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Berndsen, C., Wolberger, C. New insights into ubiquitin E3 ligase mechanism. Nat Struct Mol Biol 21, 301–307 (2014). https://doi.org/10.1038/nsmb.2780

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nsmb.2780

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