Functional analysis of CDK inhibitor p21WAF1.

p21WAF1 was originally identified as a protein that binds and inhibits cyclin-dependent kinases (CDKs). p21WAF1 is recognized to have at least two separate roles-first as a CDK inhibitor, and second as an inhibitor of PCNA, an accessory protein of DNA polymerase delta. p21WAF1 plays a critical role in the cellular response to DNA damage. Additionally, p21WAF1 plays a role in DNA repair, apoptosis, cellular senescence, terminal differentiation, and cell cycle arrest upon extracellular signaling. p21WAF1 protein levels are regulated both by transcriptional control by p53 and by factors other than p53, as well as by posttranscriptional regulation. Although the role of p21WAF1 has been explained so far only by its interaction with CDKs and with PCNA, it has several other binding partners. The ability of p21WAF1 to participate in several cellular functions has been widely studied by transfection of cells with p21WAF1 vectors. We describe here procedures for analysis of p21WAF1 function in mammalian cells after transfection of p21 plasmids. The procedures include inhibition of DNA synthesis, cellular localization, association with binding partners, and half-life measurements.