Dynamics of the p53-mdm2 loop in living cells, and the design-principles of biological feedback. 

Galit Shenhar, Ph.D.

Department of Molecular Cell Biology
Weizmann Institute of Science, Israel

www.weizmann.ac.il/mcb/UriAlon

A major goal of systems biology is to understand complex protein networks in cells. Great simplification would occur if networks could be broken down into basic recurring circuits, such as the recently defined 'network motifs'. We focused on a common network motif, where a transcription factor is negatively regulated on the protein level by one of its downstream gene products. We employ the well studied p53-mdm2 feedback loop to experimentally study the dynamics of this motif in single living cells. We constructed human cell lines expressing functional p53-CFP and mdm2-YFP fusions. Accurate measurements of protein levels, localizations and interaction were obtained at high temporal resolution by fluorescence microscopy. Detailed oscillatory kinetics following DNA damage was found.  We also studied the variability between the dynamics of individual cells, which can not be seen in assays on cell populations. The results allowed the construction of a mathematical model that captures the behavior of this regulatory module. We discuss the design-principles of biological feedback that were found in this and other systems.