Two Short Segments of Smad3 Are Important for Specific Interaction of Smad3 with c-Ski and SnoN

Abstract

Cytokines of the transforming growth factor-␤ (TGF-␤)1 superfamily bind to two different serine/threonine kinase receptors, type I and type II, and transmit intracellular signals through Smad proteins (1). Receptor-regulated Smads (RSmads) are directly phosphorylated by type I receptors, and form complexes with common-partner Smad (Co-Smad). The R-Smad⅐Co-Smad complexes translocate into the nucleus, where they regulate transcription of target genes through interacting with various transcription factors and transcriptional co-activators or co-repressors. Of the eight different mammalian Smad proteins, Smad1, Smad5, and Smad8 are R-Smads activated by bone morphogenetic proteins (BMPs) and anti-Mü llerian hormone, whereas Smad2 and Smad3 are R-Smads activated by TGF-␤s, activins, and nodal. Smad4 is the only Co-Smad in mammals.Transcription of target genes by TGF-␤ is up-regulated by binding of Smads to transcriptional co-activators, including p300 and CBP, which induce acetylation of histones (2-4). In contrast, transcriptional co-repressors, including c-Ski and its related protein SnoN, physically interact with Smads and repress TGF-␤ superfamily signaling through recruitment of histone deacetylases (5-9). Ski was originally identified as the oncogene present in the avian Sloan-Kettering retroviruses (10). Although c-Ski regulates transcriptional activities of some other proteins (11,12), Smads may be one of the most important partners of c-Ski in transcriptional regulation.R-Smads and Co-Smads are composed of the N-terminal MH1 domains, linker regions, and the C-terminal MH2 domains. Among the various regions of Smads, the MH2 domains play important roles in binding to type I receptors, formation of Smad oligomers, and interaction with DNA-binding proteins (13). Upon ligand stimulation, R-Smads and Co-Smads may form heterotrimers or heterodimers through their MH2 domains, although their exact oligomeric structures have not been fully determined (14 -16). Smads interact with various transcription factors and transcriptional co-activators/co-repressors through the MH2 domains. Among them, Mixer and FoxH3 (originally termed FAST1) have been shown to bind to ␣-helix 2 (H2) of the MH2 domains of Smad2/3 (17-19). However, sites of binding to other proteins in Smads have not been determined in detail. Interestingly, SnoN was shown to be degraded by a HECT type E3 ubiquitin ligase Smurf2, which binds to the linker region of Smad2 (20). SnoN does not directly bind to Smurf2, but it may be located in the vicinity of Smurf2 when it binds to Smad2. It is thus important to determine through which regions c-Ski/SnoN bind to Smad2 and Smad3.Among mammalian Smad proteins, Smad2, Smad3, and Smad4 interact with c-Ski, but Smad1 or Smad5 does so only weakly (6). In the present study, we have shown that the SE and QPSMT parts in the MH2 domain of Smad3 interact with c-Ski and SnoN. SE and QPSMT are located on the N-terminal upper side of the toroidal structure of the MH2 oligomer, toward which the MH1 domain and the linker region f...

Interaction with Smad4 Is Indispensable for Suppression of BMP Signaling by c-Ski

“…This result is consistent with our previous observation that c-Ski interacts with Smad2/3 through the upper surface of the MH2 toroid structure of Smad2/3 trimer (Mizuide et al, 2003). We further examined how c-Ski affects Smad heteromeric complex using c-Ski ⌬S2/3, which interacts with Smad4 but not with R-Smads.…”

“…In the model, c-Ski binds to L3 loop of Smad4, a loop that is responsible for interaction with R-Smads; thus, c-Ski inhibits formation of active Smad complex between R-Smads and Co-Smad. Although BMP-specific R-Smads bind to c-Ski only weakly (Akiyoshi et al, 1999;Mizuide et al, 2003), it has been shown that c-Ski inhibits BMP signaling in mammals and in Xenopus (Amaravadi et al, 1997;Wang et al, 2000).The ARPG mutant of Ski protein was first described in v-Ski as a transformation-defective mutant (Colmenares et al, 1991). The ARPG mutation is an insertional mutation of four amino acids (Ala-Arg-Pro-Gly), replacing Asp in a po- et al, 1991).…”

The RING-H2 protein RNF11 is overexpressed in breast cancer and is a target of Smurf2 E3 ligase

“…The PPPPY motif sequence of RNF11 is identical to that of Smads 2, 3 and 7, which have been shown to bind WW domains of Smurf2 Ub ligase and mediate the ubiquitination and degradation of various target proteins (Coopman et al, 2000;Longnecker et al, 2000). Smurf2 plays a key role in the ubiquitination and proteasomal degradation of receptor-activated Smad2 and Smad3, and corepressor SnoN (Bonni et al, 2001;Mizuide et al, 2003). The Smurf2 and Smad7 complex ubiquitinates TbRI, a key event resulting in degradation of the receptor and TGFb resistance in cancer cells (Kavsak et al, 2000).…”

Direct Interaction of Ski with Either Smad3 or Smad4 Is Necessary and Sufficient for Ski-mediated Repression of Transforming Growth Factor-β Signaling

“…Taken together, our data strongly suggest that under these experimental conditions, two distinct domains in Ski can independently mediate interaction of Ski with Smad3. Interestingly, Mizuide et al (15) recently identified two separate sites in the MH2 domain of Smad2/3, which mediate interaction with Ski/ Sno. Thus, it is possible that two distinct interfaces of Ski/Sno and Smad2/3 facilitate interaction with each other.…”