Figure 5: Δ133p53α is ubiquitinated for degradation.

(a) Δ133p53α protein is polyubiquitinated. FLAG-tagged versions of wild-type Δ133p53α (wt) and mutant Δ133p53α (mut; from c below), along with vector control (−), were retrovirally transduced into MRC-5 fibroblasts. The wild-type Δ133p53α-expressing cells had a pair of STUB1 siRNA-transfected (+, siRNA no. 1 for 4 days) and untransfected (−) counterparts. These cells were maintained for 8 h under amino acid- and serum-starved conditions with bafilomycin A1 (100 nM). Protein lysates were used either in IP with anti-ubiquitin (Ub) antibody, followed by IB with anti-FLAG antibody (top), or directly in IB for FLAG-Δ133p53α, STUB1 and β-actin (lower three panels). Smear signals indicate polyubiquitinated FLAG-Δ133p53α protein (Poly-Ub). Asterisk indicates a nonspecific band in negative control. These experimental conditions resulted in similar amounts of FLAG-Δ133p53α in the presence and absence of STUB1 knockdown, allowing quantitative analysis of ubiquitination. The relative densitometric values of wild-type FLAG-Δ133p53α polyubiquitination in the presence and absence of STUB1 knockdown (normalized with total FLAG-Δ133p53α) are shown. (b) Δ133p53α is ubiquitinated at the C-terminal lysine residues. MRC-5 fibroblasts expressing wild-type FLAG-Δ133p53α in the absence of STUB1 knockdown were used in IP with anti-FLAG antibody, and the resulting immunoprecipitates were analysed using mass spectrometry. Representative MS/MS spectrum of an [M+3H]3+ peptide from FLAG-Δ133p53α shows sites of ubiquitin modification. The peptide sequence for the spectrum is shown, with the sites of modification marked as asterisks: amino-acid positions 248 and 249, which correspond to positions 381 and 382 in full-length p53. The same peptide was also observed in the unmodified state. (c) Lysine (K)-to-arginine (R) substitutions at amino-acid residues 248 and 249. (d) Mutation of the ubiquitination sites renders Δ133p53α resistant to degradation. The same three cells as in a were cultured under amino-acid- and serum-starved conditions for 8 h (without bafilomycin A1), transfected with STUB1 siRNA (no. 1 for 4 days) or untreated, and examined in IBs for FLAG-Δ133p53α, STUB1, p62/SQSTM1 and β-actin. The effect of starvation was confirmed by decreased p62/SQSTM1 in each cell. The upregulation of p62/SQSTM1 by wild-type and mutant FLAG-Δ133p53α is likely associated with enhanced cell proliferation by Δ133p53α overexpression³⁰.