Smad8B, a Smad8 splice variant lacking the SSXS site that inhibits Smad8-mediated signalling.
BACKGROUND
Members of the TGF-beta superfamily of ligands bind to and activate surface serine/threonine-kinase receptors. Transduction of these signals requires the Smad proteins, which transiently interact with the activated receptor complex and are phosphorylated on their C terminus, SSXS site, by the type I receptor. Smad8 is a downstream signalling mediator of ALK2/ActRIA.


RESULTS
We have cloned a splice variant of Smad8, designated Smad8B. The Smad8 and Smad8B cDNAs are identical in sequence, except that Smad8B lacks a portion encoding 47 amino acids, including the SSXS phosphorylation site, in the C-terminal MH2 region. Both Smad8 and Smad8B were expressed in many of the same cell types. Smad8B was capable of specific complex formation with either Smad8 or Smad4 in mammalian cells. In cells expressing constitutively activated ALK2, Smad8B was localized to the cytoplasmic region, whereas Smad8 was translocated into the nucleus. In mammalian cells, Smad8B acted as a dominant inhibitor of BMP signalling.


CONCLUSIONS
Smad8B, a splice variant of Smad8, was isolated and found to specifically associate with both Smad8 and Smad4. Smad8B inhibited BMP signalling. Smad8 and Smad8B thus represent novel signal transduction proteins that may regulate the BMP signalling pathway.
Nishita
M
M
Division of Morphogenesis, Department of Developmental Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan.
Ueno
N
N
Shibuya
H
H
D017398:Alternative Splicing; D000818:Animals; D019485:Bone Morphogenetic Proteins / Q000378:metabolism; D019556:COS Cells; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D006801:Humans; D009928:Organ Specificity; D011233:Precipitin Tests; D012333:RNA, Messenger / Q000032:analysis; D015398:Signal Transduction / Q000235:genetics* / Q000502:physiology; D051901:Smad4 Protein; D051904:Smad8 Protein; D015534:Trans-Activators / Q000235:genetics* / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000235:genetics* / Q000378:metabolism
D019485:Bone Morphogenetic Proteins; D004268:DNA-Binding Proteins; D012333:RNA, Messenger; C497796:SMAD4 protein, human; C107514:SMAD9 protein, human; D051901:Smad4 Protein; C490811:Smad4 protein, rat; D051904:Smad8 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta