K Tada1, H Inoue, T Ebisawa, M Makuuchi, M Kawabata, T Imamura, K Miyazono. 1. Department of Biochemistry, The Cancer Institute of the Japanese Foundation for Cancer Research (JFCR), and Research for the Future Program, Japan Society for the Promotion of Science, 1-37-1 Kami-ikebukuro, Toshima-ku, Tokyo 170-8455, Ja.
Abstract
BACKGROUND: Smad4 has a unique region of 35 amino acids between alpha-helices 3 and 4 (termed H3/4 loop) of the Mad homology (MH) 2 domain. In order to elucidate the functional importance of the H3/4 loop, we prepared chimeric constructs of Smad4 containing the region corresponding to the alpha-helix 3, H3/4 loop and alpha-helix 4 of different Smads, including a chimera containing that of Smad2 (Smad4-HL2). RESULTS: Smad4-HL2 constitutively induced the transcriptional activation of p3TP-Lux, a TGF-beta-responsive reporter construct. However, co-transfection of Smad2 with Smad4-HL2 did not induce a further increase in the activation of p3TP-Lux. Smad4-HL2 did not induce the activation of pAR3-Lux, which contains FAST1-binding sites and is activated by a complex composed of FAST1, Smad2 and Smad4. Smad4-HL2 formed a homo-oligomer more efficiently than wild-type Smad4 in mammalian cells. Moreover, Smad4-HL2 bound to DNA containing the Smad-binding sites with a gretaer affinity than the wild-type Smad4. CONCLUSION: Smad4-HL2 spontaneously forms a homo-oligomer, which may bind to DNA with relatively high affinity and induce transcriptional activation of p3TP-Lux. The H3/4 loop of Smad4 may thus play a role in precluding the spontaneous oligomer formation of Smad4.
BACKGROUND:Smad4 has a unique region of 35 amino acids between alpha-helices 3 and 4 (termed H3/4 loop) of the Mad homology (MH) 2 domain. In order to elucidate the functional importance of the H3/4 loop, we prepared chimeric constructs of Smad4 containing the region corresponding to the alpha-helix 3, H3/4 loop and alpha-helix 4 of different Smads, including a chimera containing that of Smad2 (Smad4-HL2). RESULTS:Smad4-HL2 constitutively induced the transcriptional activation of p3TP-Lux, a TGF-beta-responsive reporter construct. However, co-transfection of Smad2 with Smad4-HL2 did not induce a further increase in the activation of p3TP-Lux. Smad4-HL2 did not induce the activation of pAR3-Lux, which contains FAST1-binding sites and is activated by a complex composed of FAST1, Smad2 and Smad4. Smad4-HL2 formed a homo-oligomer more efficiently than wild-type Smad4 in mammalian cells. Moreover, Smad4-HL2 bound to DNA containing the Smad-binding sites with a gretaer affinity than the wild-type Smad4. CONCLUSION:Smad4-HL2 spontaneously forms a homo-oligomer, which may bind to DNA with relatively high affinity and induce transcriptional activation of p3TP-Lux. The H3/4 loop of Smad4 may thus play a role in precluding the spontaneous oligomer formation of Smad4.