(G) Luciferase and MIG-6 knockdown 293 cells were transfected with the indicated plasmids and subjected to MG132 treatment. Afterward, cells were harvested for IP with HA antibody, followed by immunoblotting analysis to determine the level of K48-linked ubiquitination of HIF1α. MG132 was used to preserve the degradative K48-linked ubiquitination signals. 