1. Adipose PancreasAorta MuscleHeart Liver LungKidney Testis Spleen Brian 18S p55 γ p55 α Figure S1 Tissue distribution of p55 . (A) Representative RT-PCR showing that p55  is expressed in aorta. (B) Representative RT-PCR showing the expression of p55  mRNA is dominant in aorta and VSMCs rather than p55 . A. B. 18Sp55 γ p55 α Aorta VSMCs Supplementary Figures

2. p53  -actin Scrambled p53 siRNA1 p53 siRNA2 + - - - + - - - + Figure S2 Representative western blots and averaged data showing the knockdown of p53 in cultured VSMCs infected with scrambled or two sets of p53 siRNA (p53 siRNA1 and p53 siRNA2; n = 6; **P <0.01 versus scrambled).

3. p53 MG132 + - + Ad-p55  + - - - Ad-GFP + β- actin Figure S3 The regulation of p53 by p55  with or without proteasome inhibitor MG132. Cells were treated with MG132 (20  M) for 4 hours before harvesting. For all panels, HCT116 p53 +/+ cells were infected with Ad-GFP or Ad- p55  (100 m.o.i., 48 hours). n = 6; **P <0.01 versus Ad-GFP; †P <0.05 versus Ad-p55 . †

4. Figure S4 p55  reverses MDM2-induced p53 reduction. HCT116 p53 +/+ cells were infected with Ad-GFP or Ad-p55  (100 m.o.i., 48 hours), and cotransfected with MDM2 expressing vector as indicated. n = 5; *P <0.05, **P <0.01 versus Ad-GFP; †P <0.05 versus Ad-GFP/MDM2. + - + Ad-p55 γ MDM2 + + - - - - Ad-GFP + - + p53 GAPDH Flag (p55 γ) Flag (MDM2) 90 kDa 55 kDa GFP †

5. Ad-GFP Ad-p55  Hoechst staining A. B.C. Figure S5 Overexpression of p55  does not affect cell survival. (A) Representative image of Hoechst staining in VSMCs transfected with Ad-GFP or Ad-p55  (n = 5; 100 m.o.i. for 48 hours). (B and C) Overexpression of p55  does not affect PDGF-BB-induced Akt and ERK phosphorylation. Representative western blot and averaged data showing the total or phosphorylated Akt (B) and ERK (C) protein level in the VSMCs transfected with Ad-GFP or Ad-p55  (n = 6). p-ERK t-ERK Ad-GFP Ad-p55  - - + - - - - + - - + - - - - + PDGF-BB p-Akt t-Akt Ad-GFP Ad-p55  - - + - - - - + - - + - - - - + PDGF-BB

6. Figure S6 Overexpression of p55  and knockdown of p55  does not affect PI3K activity. (A) PI3K activity in VSMCs transfected with Ad-GFP or Ad-p55  (n = 5; 100 m.o.i. for 48 hours). (B) PI3K activity in VSMCs transfected with Ad- scrambled or Ad-p55  -shRNA1 or Ad-p55  -shRNA2 (n = 5; 100 m.o.i. for 48 hours). A.B. Ad-GFP Ad-p55  - - + - - - - + Ad-scrambled Ad-p55  -shRNA1 Ad-p55  -shRNA2 + - - - + - - - +

7. Table S1 Identify p53 as one of the p55  associated proteins by mass spectrum SequenceDescriptiveConf% IPI00025087.3K.KKPLDGEYFTLQIR.GTP53 Isoform 1 of Cellular tumor antigen p53 100.00% IPI accession number Three independent experiments were performed for p55  interacting proteins totally. Cells without infection of Ad-p55  -flag served as control. The p53 protein was identified in two independent experiments.