Abstract
E3 ubiquitin ligases are final effectors of the enzyme cascade controlling ubiquitylation1. A central issue in understanding their regulation is to decipher mechanisms of their assembly and activity2. In contrast with RING-type E3s, fewer mechanisms are known for regulation of HECT-type E3s2,3,4. Smad ubiquitylation regulatory factor 1 (Smurf1), a C2-WW-HECT-domain E3, is crucial for bone homeostasis, in which it suppresses osteoblast activity5,6. However, whether and how its activity is regulated remains unclear. Here we show that Smurf1, but not Smurf2, interacts with casein kinase-2 interacting protein-1 (CKIP-1), resulting in an increase in its E3 ligase activity. Surprisingly, CKIP-1 targets specifically the linker region between the WW domains of Smurf1, thereby augmenting its affinity for and promoting ubiquitylation of the substrate. Moreover, CKIP-1-deficient mice undergo an age-dependent increase in bone mass as a result of accelerated osteogenesis and decreased Smurf1 activity. These findings provide evidence that the WW domains linker is important in complex assembly and in regulating activity of HECT-type E3s and that CKIP-1 functions as the first auxiliary factor to enhance the activation of Smurf1.
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Change history
27 April 2010
In the version of this letter initially published online, Fig. 1c, Fig. 3a and Fig. 3c were incorrectly labelled. These errors have been corrected in both the HTML and PDF versions of the letter.
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Acknowledgements
We thank N. Hou and Y. Tie for technical help; D. Chen, C. X. Deng, D. W. Litchfield, M. Liu, K. Miyazono and Y. Xiong for providing materials; and X. F. Wang, X. Guo and C. Lu for helpful advice. This study was supported by Chinese National Basic Research Programs 2006CB910802 (F.H., L.Z.) and 2007CB914601 (L.Z.), Chinese National Natural Science Foundation Projects 30621063 (F.H.) and 30570366 (L.Z.), and Beijing Science and Technology NOVA Program 2007A063 (L.Z.).
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L.Z. and F.H. co-organized and performed project planning, data analysis, discussion and writing; K.L. performed the main experimental work of CKIP-1–Smurf1 interaction, CKIP-1 regulation on Smurf1 activity and molecular mechanism investigations; X. Yin performed the main experimental work of generating CKIP-1 gene knockout mice and bone phenotype analysis; T.W., L.L., X.C. and X. Yang participitated in the establishment of CKIP-1 gene knockout mice; T.W. and X. Yang were also involved in the bone phenotype analysis; S.X. participated in the in vitro GST pull-down and in vitro ubiquitylation assays; and G.X. contributed to the protein interaction studies.
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Lu, K., Yin, X., Weng, T. et al. Targeting WW domains linker of HECT-type ubiquitin ligase Smurf1 for activation by CKIP-1. Nat Cell Biol 10, 994–1002 (2008). https://doi.org/10.1038/ncb1760
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DOI: https://doi.org/10.1038/ncb1760