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Ｓｍ ａｄ３ 和 Ｓｍ ａｄ４ 是将 Ｔ Ｇ Ｆβ的信号从细胞外传递到细胞核内的重要的信号传导蛋白． Ｔ Ｇ Ｆβ与其受体结合后，激活受体的磷酸激酶，使 Ｓｍ ａｄ３ 发生磷酸化，活化的 Ｓｍ ａｄ３ 与 Ｓｍ ａｄ４ 结合，形成异源复合物，进入到核中．然后 Ｓｍ ａｄ４ 以 Ｄ Ｎ Ａ 结合蛋白的形式与特定的 Ｄ Ｎ Ａ 结合，将 Ｔ Ｇ Ｆβ的信号传到核内．激活转录，诱导背中胚层的形成，抑制细胞的分化等．经研究利用酵母双杂交试验，鉴定了 Ｓｍ ａｄ３ 和 Ｓｍ ａｄ４ 相互作用的功能区域．构建 Ｓｍ ａｄ３ 和 Ｓｍ ａｄ４ 的 Ｃ 端、 Ｎ 端和中间连接区的突变体，将这些突变体克隆到 ｐ Ｇ Ａ Ｄ４２４ 和 ｐ Ｇ Ｂ Ｔ９ 载体中，并转化到 Ｈ Ｆ７ Ｃ 酵母中．通过 Ｌｅｕ－ ／ Ｔｒｐ－ ／ Ｈｉｓ－ Ｓ Ｄ 平板上菌落的形成，和 Ｘ－ ｇａｌ显色反应鉴定转化到酵母中的两个克隆质粒的相互作用．结果显示 Ｓｍ ａｄ４ 与 Ｓｍ ａｄ３ 异源相五作用时，主要是通过 Ｓｍ ａｄ４ 的中间连接区．在同源作用时， Ｓｍ ａｄ３ 是通过 Ｃ 端，而 Ｓｍ ａｄ４ 是通过中间连接区进行的．
Sm ad3 and Sm ad4 are important signaling proteins that transmit the signal of T G F-β from the extracellular to the nucleus. T G F β binding with its receptor, the activation of the receptor phosphokinase, Sm ad3 phosphorylation, activation of Sm ad3 and Sm ad4 binding, forming a heterologous complex, into the nucleus. Sm ad4 then binds to the specific D N A as a D N A-binding protein, signaling the T G F -β signal to the nucleus. Activate transcription, induce the formation of dorsal mesoderm, and inhibit the differentiation of cells. The yeast two-hybrid assay was used to identify the functional domains of Sm ad3 and Sm ad4 interactions. Mutants of the C-terminal, N-terminal and interlinked regions of Sm ad3 and Sm ad4 were constructed, cloned into pG A D424 and pG B T9 vectors and transformed into H7C yeast. The interaction of two cloned plasmids transformed into yeast was identified by colony formation on Leu- / Trp- / His-SD plates and X-gal chromogenic reaction. The results show that when Sm ad4 and Sm ad3 heterologous phase five role, mainly through Sm ad4 the middle connection region. At the time of homologation, Sm ad3 is through the C-terminus and Sm ad4 is through the mid-linker.