Figure 2

Schematic structure of Smad family members. (a) The schematic structure of the indicated Smads shows the homology between family members and the localization and conservation of various critical functional domains. The MH1 (red) and MH2 (green) domains are conserved among Smads, but the linker region (blue) is not conserved between the R-Smads and Smad4. The main splice variant of Smad2 contains two inserts in its MH1 domain (L1 and exon 3) that are not found in other R-Smads. For details on the functions of the indicated domains and motifs, see the 'Smad domains and function' and 'Regulation of Smads by phosphorylation' sections of the text. (b) A more detailed representation of the MH1 and linker domains of Smad3 shows various phosphorylation sites and, as examples, some of the kinases and other modifying enzymes acting on these sites and domains. Co-Smad, common-partner Smad; GSK-3, glycogen synthase kinase-3; I-Smad, inhibitory Smad; MH, mad homology; PKC, protein kinase C; R-Smad, receptor-regulated Smad.