SMADS ARE THE CENTRAL-COMPONENT IN TRANSFORMING-GROWTH-FACTOR-BETA SIGNALING

Until recently, little was known about how transforming growth factor (TGF)-beta signals are transduced to the nucleus. With the discovery o f the Smad proteins initially in Drosophila and C. elegans, the unrave ling of the pathway has begun. Nine different vertebrate members also have been reported, indicating that Smads are a conserved component of the TGF-beta pathway. Currently, there are three functional classes o f Smads. Class I Smads are phosphorylated by TGF-beta receptors and mo ve to the nucleus. The Class II Smads function with Class I Smads, whi le Class III Smads antagonize the function of Class I Smads. New evide nce shows that Smads bind specific DNA sequences and induce transcript ion of downstream target genes, thus placing the Smads at the center o f the TGF-beta signaling pathway. (C) 1998 Elsevier Science Inc.