A, Identification of the Bag6 complex as an interaction partner of gp78. Cell extracts from 293T cells transfected with FLAG-tagged gp78 or a control plasmid were subject to immunoprecipitation (IP) with FLAG antibody. IB, immunoblotting B, C, Endogenous interactions between the Bag6 and the gp78-p97 complexes. 293T cell extract was subject to IP analyses using the indicated antibodies. D, Bag6 directly binds the cytosolic domain of gp78 via the CUE domain. The indicated GST-tagged proteins were immobilized on glutathione beads and incubated with purified Bag6. E, The UBL domain in Bag6 is required for gp78 interaction. Purified Bag6 or a Bag6 variant lacking UBL was incubated with glutathione beads containing GST-gp78c or GST. The right panel shows the Ponceau S-stained membrane used for immunoblotting (left panel). See also Figure S1