The ups and downs of p53: understanding protein dynamics in single cells

Response acceleration in post-translationally regulated genetic circuits

Transcription factors must often be chemically modified to perform their functions. Yet, it is not known whether the mechanisms that bring about such modifications impact the quantitative or kinetic properties of gene expression. Phosphorylation controls the activity of regulatory proteins of the two-component system family, which constitutes a prevalent form of bacterial signal transduction. These proteins are phosphorylated/dephosphorylated by cognate sensor proteins in response to specific signals. The phosphorylation level of the regulatory proteins is also modulated by small proteins-termed connectors-that are produced when a cell experiences signals other than those detected directly by the sensors. Here, we explore how differences in the targets (i.e., sensor or regulator) and the mechanisms used by connectors to generate phosphorylated regulatory proteins affect the output of two-component systems. Our mathematical modeling demonstrates that sensor-targeting mechanisms exhibit stronger response acceleration than those where the connector targets the regulator. These differences are robust to perturbations in kinetic parameters but dependent upon the specific sensor-to-regulator ratio and how the ratio is controlled in living cells. In contrast, the steady-state output levels of the circuits are determined primarily by the circuit parameters, and can be adjusted without affecting response acceleration. Likewise, the analyzed connector-mediated circuits exhibit similar noise generation properties. Our results highlight the relationship between the architecture of genetic regulatory circuits and their dynamic properties.

WIP1 phosphatase at the crossroads of cancer and aging

The PP2C family serine/threonine phosphatase WIP1 is characterized by distinctive oncogenic properties mediated by inhibitory functions on several tumor suppressor pathways, including ATM, CHK2, p38MAPK and p53. PPM1D, the gene encoding WIP1, is aberrantly amplified in different types of human primary cancers, and its deletion in mice results in a profound tumor-resistant phenotype. Numerous downstream targets of WIP1 have been identified, and genetic studies confirm that some play a part in tumorigenesis. Recent evidence highlights a new role for WIP1 in the regulation of a cell-autonomous decline in proliferation of certain self-renewing cell types, including pancreatic beta-cells, with advancing age. These emerging functions of WIP1 make it a potent therapeutic target against cancer and aging.

p53 Pre- and post-binding event theories revisited: stresses reveal specific and dynamic p53-binding patterns on the p21 gene promoter

p53 is a master transcription factor that prevents neoplasia and genomic instability. It is an important target for anticancer drug design. Understanding the molecular mechanisms behind its transcriptional activities in normal cells is a prerequisite to further understand the deregulation effected by mutant p53 in cancerous cells. Currently, how p53 coordinates transcription programs in response to stress remains unclear. One theory proposes that stresses induce pre-binding events that direct p53 to bind to specific response elements, whereas a second posits that, in response to stress, p53 binds most response elements and post-binding events then regulate transcription initiation. It is critical to establish the relevance of both theories and investigate whether stresses induce specific p53-binding patterns correlated with effector gene induction. Using unique in cellulo genomic footprinting experiments, we studied p53 binding to the five response elements of p21 in response to stresses and monitored p21 mRNA variant transcription. We show clear footprints of p53 bound to response elements in living cells and reveal that the binding of p53 to response elements is transient, subject to dynamic changes during stress responses, and influenced by response element pentamer orientations. We show further that stresses lead to specific p53-binding patterns correlated with particular p21 mRNA variant transcription profiles and that p53 binding is necessary but not sufficient to induce p21 transcription. Our results indicate that pre- and post-binding events act together to regulate adapted stress responses; this paves the way to the unification of pre- and post-binding event theories.