title: Targeting WW domains linker of HECT-type ubiquitin ligase Smurf1 for activation by CKIP-1.
abstract: E3 ubiquitin ligases are final effectors of the enzyme cascade controlling ubiquitylation. A central issue in understanding their regulation is to decipher mechanisms of their assembly and activity. In contrast with RING-type E3s, fewer mechanisms are known for regulation of HECT-type E3s. Smad ubiquitylation regulatory factor 1 (Smurf1), a C2-WW-HECT-domain E3, is crucial for bone homeostasis, in which it suppresses osteoblast activity. 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.
authors.affiliation: State Key Laboratory of Proteomics, Beijing Proteomics Research Center, Beijing Institute of Radiation Medicine, 27 Taiping Road, Beijing 100850, China.
authors.forename: Kefeng
authors.initials: K
authors.lastname: Lu
authors.forename: Xiushan
authors.initials: X
authors.lastname: Yin
authors.forename: Tujun
authors.initials: T
authors.lastname: Weng
authors.forename: Shenli
authors.initials: S
authors.lastname: Xi
authors.forename: Li
authors.initials: L
authors.lastname: Li
authors.forename: Guichun
authors.initials: G
authors.lastname: Xing
authors.forename: Xuan
authors.initials: X
authors.lastname: Cheng
authors.forename: Xiao
authors.initials: X
authors.lastname: Yang
authors.forename: Lingqiang
authors.initials: L
authors.lastname: Zhang
authors.forename: Fuchu
authors.initials: F
authors.lastname: He
mesh_terms: D000818:Animals; D001665:Binding Sites; D002352:Carrier Proteins / Q000378:metabolism / Q000502:physiology*; D004789:Enzyme Activation; D006801:Humans; D051379:Mice; D010012:Osteogenesis; D054730:Protein Interaction Domains and Motifs; D017434:Protein Structure, Tertiary; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism*; D054875:Ubiquitination; D047908:Intracellular Signaling Peptides and Proteins
chemical_list: D002352:Carrier Proteins; D044767:Ubiquitin-Protein Ligases; C492381:Smurf1 protein, mouse; C000620556:PLEKHO1 protein, mouse; D047908:Intracellular Signaling Peptides and Proteins