Figure 1

The isolation of Smad1 interacting proteins suggests a functional link between Smad1 and the 26S proteasome-mediated protein degradation. A. Yeast two-hybrid test of the interaction specificity between thirteen isolated Smad1 interactors and Smad1, Smad2, Smad3 and Smad4. The "Protein Trap" system was used for the test [27]. Yeast EGY48 (leu2,his3,trp1,ura3) was first transformed with LexA fusion constructs of Smads in pEG202 vector and then transformed again with each of the thirteen different cDNA clones in pJG4-5 vector. The transformants were streaked onto selective plates of either galactose/raffinose (top) or glucose (bottom) lacking uracil, histidine and tryptophan (U^-H^-W^-) but containing X-Gal. The expression of the cDNA encoded fusion proteins is under the control of the GAL1 promoter. The light blue detected on the glucose plates reflects a basal level of transcriptional activation by the LexA-Smad fusion proteins. B. The thirteen Smad1 interactors were grouped into three groups based upon their known functions (top panel). The four clones that have functions along the proteasome-mediated degradation pathways are marked by different colors in the top panel and matches with the colored symbol in the bottom panel, which illustrates the proteasomal targeting pathways. See text for details.