60 Binding and transactivation by the tumor suppression protein p53 are encoded in the differential structural properties of its response elements

Abstract

The tumor suppressor p53 is one of the most central hubs in human cells, connected to a complex network in the living cell. Mutations in the p53 tumor suppressor gene are the most frequent genetic alterations in human cancer. In response to cellular stress, p53 acts as a transcription factor by binding to defined DNA targets, thereby activating the expression of genes leading to diverse cellular outcomes. How p53 decides between its various cellular functions and what determines the strength of binding of p53 to its response elements (REs) at various levels of protein is currently unknown. We will discuss our recent studies and will show that differential structural properties of p53 REs affect both p53 binding mechanism as well as the transactivation (TA) level (Beno et al. 2011; Jordan et al. 2012; Menendez et al. 2012). Transactivation by p53 is considered to be dependent on its cellular level, but until recently how the binding strength of p53 to its REs is related to transactivation level was unknown. We recently discovered that this relationship is protein-level dependent. At high protein level, we observed the expected positive relationship between TA level and binding affinity of p53 to its REs, but at low protein level this relationship breaks down and instead we observed a positive relationship between TA level and the torsional flexibility of p53 REs. In addition, we discovered two sequences that supported high TA levels even at basal p53 concentration (“super-transactivating” REs, or STA-REs), in both a highly controlled functional assay in yeast and when transfected into human cells. The transcriptional capacity of many p53 mutants associated with breast cancers could be discriminated by transactivation from these REs. Our findings establish that p53 transactivation is kinetically determined at low levels and thermodynamically driven at high levels, and that the DNA sequence itself can strongly affect p53 transactivation even under conditions where the number of p53 molecules is small, suggesting that DNA structural properties can be critical factors in p53-dependent gene regulation. The mechanistic implications of these findings for p53 binding on its target sequences will be discussed.