SIP1, a novel zinc finger homeodomain repressor, interacts with Smad proteins and binds to 5 '-CACCT sequences in candidate target genes

Activation of transforming growth factor beta receptors causes the phosphor ylation and nuclear translocation of Smad proteins, which then participate in the regulation of expression of target genes. We describe a novel Smad-i nteracting protein, SIP1, which was identified using the yeast two-hybrid s ystem. Although SIP1 interacts with the MH2 domain of receptor-regulated Sm ads in yeast and in vitro, its interaction with full length Smads in mammal ian cells requires receptor-mediated Smad activation. SIP1 is a new member of the delta EF1/Zfh-1 family of two-handed zinc finger/homeodomain protein s. Like delta EF1, SIP1 binds to 5'-CACCT sequences in different promoters, including the Xenopus brachyury promoter. Overexpression of either full-le ngth SIP1 or its C-terminal zinc finger cluster, which bind to the Xbra2 pr omoter in vitro, prevented expression of the endogenous Xbra gene in early Xenopus embryos. Therefore, SIP1, like delta EF1, is likely to be a transcr iptional repressor, which may be involved in the regulation of at least one immediate response gene for activin-depend ent signal transduction pathway s. The identification of this Smad-interacting protein opens new routes to investigate the mechanisms by which transforming growth factor beta members exert their effects on expression of target genes in responsive cells and in the vertebrate embryo.