Figure 3.

CA ALK-2 specifically phosphorylates Smad8. (A) CA ALK-2 but not CA ALK-4 phosphorylates Smad8. CHO cells were transiently transfected with control vector (pcDNA3), myc/Smad8, or myc/Smad8 with either CA ALK-2 or CA ALK-4. The transfected cells were labeled with [³²P]phosphorus for 3 h before harvesting. The cell lysate was immunoprecipitated with an anti-myc antibody, separated on SDS/PAGE, and detected by autoradiography. The migration of myc/Smad8 on the gel was determined by anti-myc Western blot analysis with the lysate from ³²P-unlabeled cells that expressed myc/Smad8 (data not shown). (B) CA ALK-4 but not CA ALK-2 phosphorylates Smad2. 293 cells were transiently transfected with control vector (pcDNA3), a HA-tagged Smad2 (HA/Smad2), and HA/Smad2 with either CA ALK-4 or CA ALK-2. The transfected cells were labeled with [³²P]phosphorus, and the cell lysate was immunoprecipitated with an anti-HA antibody followed by separation on SDS/PAGE. The phosphorylation products were detected by autoradiography.