Smad proteins

• Receptor Smad proteins (Smad 2 and 3)
• Cooperative Smad proteins (Co-Smads, Smad 4)
• Inhibitory Smad proteins (Smad 6 and 7).

All Smad proteins are relatively similar in structure. The highly conserved chain ends are connected by a linker of variable length and sequence. The N-terminus of Smad 4, the so-called MH1 domain, fulfils the function of binding to DNA promoters after activation and translocation into the cell nucleus. The C-terminal MH2 domain can bind to various proteins, for example to type 1 receptors, to other Smads and to transcription factors. After phosphorylation, the Smad complex permeates into the cell nucleus where it enters into interactions with transcription factors via the MH2 domain of R-Smads or directly mediates gene expression by binding from the MH1 domain of Smad 4 to promoters. Smad complexes also appear to have an inhibitory effect on DNA segments of growth-promoting genes such as c-myc.

The so-called receptor-Smad proteins (Smad 2 and Smad 3) bind to the receptor complex and are phosphorylated by the type 1 receptor. The phosphorylated R-Smad proteins form a complex with the cooperative Smad 4, which is able to penetrate the cell nucleus. Here the activated R-Smads attach themselves to DNA promoters and/or transcription factors and control transcription processes.

The inhibitory Smad proteins (Smad 6 and Smad 7) antagonize the attachment of the R-Smads to the receptor complex or to Smad 4. The receptor Smad proteins (R-Smads, Smad 2 and 3) interact directly with the type 1 receptor activated by the mechanisms described above. Only after phosphorylation can R-Smads bind the cytoplasmic cooperative Smad proteins (Co-Smads, Smad 4), which ultimately serve to attach the Smad complex to DNA promoters and to activate transcription.

The third group are the inhibitory Smad proteins (I-Smads, Smad 6 and 7). They are characterized by striking structural variations in comparison to the R- and Co-Smads. This is due to the fact that they competitively antagonize the TGF-mediated signal transduction. Both I-Smads are increasingly formed as a result of a large supply of growth factors, thus serving as negative feedback. They can compete with the R-Smads for receptor binding as well as prevent the interaction of R-Smad and Co-Smad.

The Activin Responsive Factor (ARF) can be cited for the combination of Smad proteins with transcription factors. This can interact with the activin responsive element (ARE) at the Mix2 gene only after cooperation with the Smad complex.

With the increasing establishment of the knock-out procedure and its application also to Smad proteins and TGF-β it became possible to observe the effects of the absence of these signal molecules on the orderly skin development. In this procedure, which specifically switches off genes for certain proteins, the blastocyst, which forms after natural fertilization, is removed and transfected with foreign DNA. The DNA is imported by transfer by retrovirus or microinjection. The embryonic cells are then implanted into a surrogate mother and their development is observed. The added DNA, which automatically passes from the cytoplasm into the cell nucleus, contains a segment that exactly corresponds to the gene to be eliminated. This segment attaches itself to the target sequence in the course of homologous recombination and fuses with it at the superposition site. Since the entire carrier structure is incorporated into the gene, transcription and the resulting protein synthesis are no longer possible.

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