title: Expression of a novel form of p21Cip1/Waf1 in UV-irradiated and transformed cells.
abstract: The tumor suppressor p53 and its target the CDK inhibitor p21 (Cip1/Waf1) are key components of the cellular response to DNA damage. Insight into how p21 is regulated in normal cells, and how it may be deregulated in tumor cells is important for the understanding of tumorigenesis. p21 was induced in normal human diploid fibroblasts after UV irradiation-induced DNA damage, but, at a high dose of UV irradiation, a faster mobility form of p21 on SDS-PAGE (designated p21delta) was expressed. Surprisingly, in a variety of growing transformed cell lines, the level of p21 was low but p21delta was prominent. We found that p21delta appeared to be derived through a loss of around 10 amino acids from the C-terminus of p21, which theoretically would remove the PCNA binding domain, a second cyclin binding domain and the nuclear localization signal sequence. Several characteristics distinguish p21 from p21delta. Both the full length p21 and p21delta could be stabilized by a proteasome inhibitor, but only the full length p21 was associated with Cdk2 and PCNA. Consistent with this, gel filtration chromatography revealed that all the full length p21 in the cell was complexed to other proteins, whereas a significant portion of p21delta was in monomeric form. Moreover, p21 was mainly localized to the nucleus, but p21delta was mainly localized to the cytoplasm. We propose that the decrease in p21 and increase in p21delta could contribute to the deregulation of the cell cycle, and could be a mechanism involved in cellular transformation.
authors.affiliation: Department of Biochemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon.
authors.forename: R Y
authors.initials: RY
authors.lastname: Poon
authors.forename: T
authors.initials: T
authors.lastname: Hunter
mesh_terms: D000595:Amino Acid Sequence; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle / Q000528:radiation effects; D002461:Cell Line, Transformed; D002478:Cells, Cultured; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases / Q000096:biosynthesis*; D016213:Cyclins / Q000096:biosynthesis* / Q000737:chemistry; D004249:DNA Damage; D004791:Enzyme Inhibitors; D005347:Fibroblasts; D005786:Gene Expression Regulation / Q000528:radiation effects*; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D016133:Polymerase Chain Reaction; D018809:Proliferating Cell Nuclear Antigen / Q000096:biosynthesis; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry; D017384:Sequence Deletion; D012867:Skin; D014162:Transfection; D014407:Tumor Cells, Cultured; D014466:Ultraviolet Rays
chemical_list: C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004791:Enzyme Inhibitors; D018809:Proliferating Cell Nuclear Antigen; D011994:Recombinant Proteins; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases