Ntain a DNA-binding domain, i.e., the MH1 (Mad homology 1) domain, which is connected via a linker to a transactivation domain, i.e., the MH2 domain. SMAD1, 2, 3, five, and 8, representing the R-SMADs, directly interact with type I receptors and are activated by those by way of phosphorylation in the C-terminus of their MH2 domain, i.e., the SSXS motif. They subsequently form heterotrimeric complexes using the shared SMAD4 by means of the MH2 domain and the phosphorylated SSXS motif. These complexes then act as transcription aspects to regulate gene transcription. The specificity on the interaction in between R-SMADs and form I receptors determines which R-SMAD branch is activated. R-SMADs 1, five, and eight associate with BMP signaling upon activation by the form I receptors 5-HT3 Receptor custom synthesis activin receptor like kinase (ALK)1, ALK2, ALK3 and ALK6 and R-SMADs two and three are linked to activin and TGF signaling (at the same time as some GDFs) upon activation by the form I receptors ALK4, ALK5, and ALK7. This functional separation is backed by phylogenetic analyses clustering the R-SMADs into a SMAD1/5/8 along with a SMAD2/3 branch [11]. Despite the fact that SMAD proteins have been found to become Bfl-1 Species hugely homologous (specifically within their MH1 and MH2 domains), the 3 as well as the two SMAD members within one branch usually do not share identical amino acid sequences thereby supplying a possibility for a receptor-specific activation. Biochemical analyses, having said that, suggested that the specificity from the TGF/BMP form I receptor-SMAD interaction could be solely mediated by a quick loop sequence inside the receptor (L45 loop) and also the R-SMAD protein (L3 loop), which differs only by several amino acid residues amongst the type I receptors activating a diverse SMAD branch and two amino acid residues between SMAD1/5/8 and SMAD2/3 [7,12,13]. Moreover, the L45 loop sequences show no amino acid distinction in between the form I receptors ALK3 and ALK6, which each activate SMAD1/5/8, or involving ALK4, ALK5 and ALK7 known to activate SMAD2/3. This suggests that these type I receptors may possibly not be able to differentially activate R-SMAD proteins of 1 branch [12]. Only the L45 loops of ALK1/ALK2 differ from that of ALK3/ALK6 indicating that ALK1 and ALK2 may possibly activate R-SMADs in the SMAD1/5/8 branch differently in comparison with ALK3 and ALK6 [12]. Thus, ALK1/ALK2 might produce a distinct pattern of activated R-SMADs than ALK3/ALK6 which could provide the basis for additional signal specification. Nevertheless, to create matters worse, structural analyses of complexes of SMAD MH1 domains bound to DNA, i.e., of SMAD1, SMAD2, SMAD3, and SMAD5 showed that the DNA-recognizing element, i.e., a -hairpin harboring residues 75 to 82, is identical among all R-SMADs and engages in identical interactions with DNA [146]. Although this exceptional getting could insinuate that all R-SMADs share comparable DNA binding properties, one has to keep in mind that R-SMADs are acting as heterotrimeric complexes and differences in the architecture of these complexes can considerably alter DNA recognition and binding. Unfortunately, no structure information are yet readily available for such larger full-length R-SMAD/Co-SMAD4 assemblies in complex with DNA creating predictions on a mechanistic scale, how SMAD recognizes DNA to modulate gene transcription, impossible so far. The phosphorylation of R-SMADs in their C-terminal SSXS motif undoubtedly describes the initial activation event in canonical TGF/BMP signaling, but many further phosphorylation sitesCells 2019, 8,4 ofhave been mapped within the DNA-bin.