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This file contain Supplementary Information that is relevant to the main body of the text. Supplementary Figure S1. Supplementary Figure S2. Half-lives of WT and mutant I332fs*14 p53 in U2OS and HCT116 cells. Supplementary Figure S3. Subcellular localization of p53 FS mutants expressed in U2OS and HCT116 cells. Supplementary Figure S4. Cross-linked species detected in cells grown in the absence or presence of Photo-Met. Supplementary Figure S5. Transcriptional activity of p53 FS mutants in U2OS and HCT116 cells. Supplementary Figure S6. Transactivation potential of N345K* and N345fs*26 mutants in U2OS cells. Supplementary Figure S7. Cell cycle profiles of I332fs*14-expressing HCT116 and U2OS cells. Supplementary Figure S8. Pro-apoptotic activity of I332fs*14 p53.

p53 mutations that result in loss of transcriptional activity are commonly found in numerous types of cancer. While the majority of these are missense mutations that map within the central DNA-binding domain, truncations and/or frameshift mutations can also occur due to various nucleotide substitutions, insertions, or deletions. These changes result in mRNAs containing premature stop codons that are translated into a diverse group of C-terminally truncated proteins. Here we characterized three p53 frameshift mutant proteins expressed from the endogenous TP53 locus in U2OS osteosarcoma and HCT116 colorectal cancer cell lines. These mutants retain intact DNA-binding domains but display altered oligomerization properties. Despite their abnormally high expression levels, they are mostly transcriptionally inactive and unable to initiate a stimuli-induced transcriptional program characteristic of wild-type p53. However, one of these variant p53 proteins, I332fs*14, which resembles naturally expressed TAp53 isoforms β and γ, retains some residual antiproliferative activity and can induce cellular senescence in HCT116 cells. Cells expressing this mutant also display decreased motility in migration assays. Hence, this p53 variant exhibits a combination of loss-of-gain and gain-of-function characteristics, distinguishing it from both wild type p53 and p53 loss.Implications:p53 frameshift mutants display a mixture of residual antiproliferative and neomorphic functions that may be differentially exploited for targeted therapy.