ALK7 can potentiate activin B signaling. A, LβT2 cells seeded in 24-well plates were transfected with 450 ng/well promoter-reporter and 200 ng/well of the indicated receptor expression vector. Cells were then treated with 25 ng/ml activin A or B in serum-free medium for 24 h. Cell lysates were collected and assayed for luciferase activity. Data are presented as fold difference from the control condition (pcDNA3, Control) set to 1 and represent the mean (± s.d.) of three independent experiments performed in duplicate. The arrowhead shows the potentiation of the activin B response in the presence of ALK7. B, Cells were transfected with kinase-deficient forms (KR) of ALK4 and ALK7 alone and treated with activin B as described in A. Data are from two independent experiments with treatments performed in triplicate. In both panels, bars with different letters differed significantly from one another.