Fig. 5. Smad nucleocytoplasmic shuttling. The five pathways shown are: Smad2 nuclear import after release from SARA (pathway 1); Smad3 nuclear import mediated by importin-ß1 and Ran (pathway 2); Smad4 shuttling mediated by the exportin Crm1 (pathway 3); putative Smad2 (4) and Smad3 (5) export pathways marked with question marks. Horizontal double arrowheaded lines indicate possibilities of Smad oligomerisation in the cytoplasm or nucleus. Smad2 (S2), Smad3 (S3) and Smad4 (S4) are shown as monomers and the actual stoichiometry of the Smad complexes is not depicted here. Small black circles represent the di-phosphate modification of the SXS motif. For references, see Kurisaki et al. (Kurisaki et al., 2001) and Massagué (Massagué, 2000).