p11 is polyubiquitinated in the absence of A2. A, HEK 293
cells were transiently transfected with FLAG-p11 for 48 h. The cells were
pretreated with cycloheximide (CHX) (10 μg/ml, 30 min) and then
incubated with or without MG132 (10 μm) as indicated. Lysates
were immunoblotted for FLAG-p11, A2, and GAPDH. B, HEK 293 cells were
cotransfected with plasmids encoding FLAG-p11 and HA-ubiquitin
(HA-Ub). Cell lysates (500 μg) were either immunoprecipitated
(IP) with anti-HA and immunoblotted (WB) with anti-FLAG IgG
(left panel) or immunoprecipitated with anti-FLAG and blotted with
anti-HA IgG (right panel). C, dominant negative ubiquitin
mutants, K48R and K29R, or empty control vectors were cotransfected with
FLAG-p11, and cell lysates were subjected to immunoblotting with anti-p11 and
anti-actin IgG after 36 h. D, FLAG-p11 was transfected with either
HA-wt- or HA-K48R-ubiquitin into HEK 293 cells. Cell lysates were
coimmunoprecipitated with anti-FLAG IgG and either probed with anti-HA
antibody (top panel) or immunoblotted with anti-p11 (lower
panel).