Modeling and analysis of MH1 domain of Smads and their interaction with promoter DNA sequence motif.

The Smads are a group of related intracellular proteins critical for transmitting the signals to the nucleus from the transforming growth factor-beta (TGF-beta) superfamily of proteins at the cell surface. The prototypic members of the Smad family, Mad and Sma, were first described in Drosophila and Caenorhabditis elegans, respectively. Related proteins in Xenopus, Humans, Mice and Rats were subsequently identified, and are now known as Smads. Smad protein family members act downstream in the TGF-beta signaling pathway mediating various biological processes, including cell growth, differentiation, matrix production, apoptosis and development. Smads range from about 400-500 amino acids in length and are grouped into the receptor-regulated Smads (R-Smads), the common Smads (Co-Smads) and the inhibitory Smads (I-Smads). There are eight Smads in mammals, Smad1/5/8 (bone morphogenetic protein regulated) and Smad2/3 (TGF-beta/activin regulated) are termed R-Smads, Smad4 is denoted as Co-Smad and Smad6/7 are inhibitory Smads. A typical Smad consists of a conserved N-terminal Mad Homology 1 (MH1) domain and a C-terminal Mad Homology 2 (MH2) domain connected by a proline rich linker. The MH1 domain plays key role in DNA recognition and also facilitates the binding of Smad4 to the phosphorylated C-terminus of R-Smads to form activated complex. The MH2 domain exhibits transcriptional activation properties. In order to understand the structural basis of interaction of various Smads with their target proteins and the promoter DNA, we modeled MH1 domain of the remaining mammalian Smads based on known crystal structures of Smad3-MH1 domain bound to GTCT Smad box DNA sequence (1OZJ). We generated a B-DNA structure using average base-pair parameters of Twist, Tilt, Roll and base Slide angles. We then modeled interaction pose of the MH1 domain of Smad1/5/8 to their corresponding DNA sequence motif GCCG. These models provide the structural basis towards understanding functional similarities and differences among various Smads.