The N domain of Smad7 is essential for specific inhibition of transforming growth factor-β signaling

Inhibitory Smads (I-Smads) repress signaling by cytokines of the transforming growth factor-β (TGF-β) superfamily. I-Smads have conserved carboxy-terminal Mad homology 2 (MH2) domains, whereas the amino acid sequences of their amino-terminal regions (N domains) are highly divergent from those of other Smads. Of the two different I-Smads in mammals, Smad7 inhibited signaling by both TGF-β and bone morphogenetic proteins (BMPs), whereas Smad6 was less effective in inhibiting TGF-β signaling. Analyses using deletion mutants and chimeras of Smad6 and Smad7 revealed that the MH2 domains were responsible for the inhibition of both TGF-β and BMP signaling by I-Smads, but the isolated MH2 domains of Smad6 and Smad7 were less potent than the full-length Smad7 in inhibiting TGF-β signaling. The N domains of I-Smads determined the subcellular localization of these molecules. Chimeras containing the N domain of Smad7 interacted with the TGF-β type I receptor (TβR-I) more efficiently, and were more potent in repressing TGF-β signaling, than those containing the N domain of Smad6. The isolated N domain of Smad7 physically interacted with the MH2 domain of Smad7, and enhanced the inhibitory activity of the latter through facilitating interaction with TGF-β receptors. The N domain of Smad7 thus plays an important role in the specific inhibition of TGF-β signaling

Functional Analysis of Basic Transcription Element Binding Protein by Gene Targeting Technology

Basic transcription element binding protein (BTEB) is a transcription factor with a characteristic zinc finger motif and is most remarkably enhanced by thyroid hormone T(3) treatment (R. J. Denver et al., J. Biol. Chem. 272:8179-8188, 1997). To investigate the function of BTEB per se and to touch on the effects of T(3) (3,5,3′-triiodothyronine) on mouse development, we generated BTEB-deficient mice by gene knockout technology. Homologous BTEB(−/−) mutant mice were bred according to apparently normal Mendelian genetics, matured normally, and were fertile. Mutant mice could survive for at least 2 years without evident pathological defects. From the expression of lacZ, which was inserted into the reading frame of the BTEB gene, BTEB showed a characteristic tissue-specific expression profile during the developmental process of brain and bone. Dramatically increased expression of BTEB was observed in Purkinje cells of the cerebellum and pyramidal cell layers of the hippocampus at P7 when synapses start to form in the brain. Although general behavioral activities such as locomotion, rearing, and speed of movement were not so much affected in the BTEB(−/−) mutant mice, they showed clearly reduced activity levels in rotorod and contextual fear-conditioning tests; this finding was probably due to defective functions of the cerebellum, hippocampus, and amygdala