AIP4 mediates K48-linked ubiquitination of M1. (A) Immunoblot analysis of lysates of 293T cells transfected with Flag-vector, Flag-AIP4, Flag-TRIM25, or Flag-Smurf-1, along with Myc-M1 and HA-Ub for 36 h, followed by immunoprecipitation (IP) with anti-M1 antibody. (B) Immunoblot analysis of lysates of 293T cells transfected with HA-Ub, HA-Ub-K48 or HA-Ub-K63, along with Flag-AIP4 and Myc-M1 for 36 h, followed by IP with anti-M1 antibody.