Universität zu Köln 

Prof. Dr. Martin Scheffner

Zentrum für Biochemie der Medizinischen Fakultät der Universität Köln
since 01/2004: Universität Konstanz
website

Running time: 07/2003 – 12/2003

Abstract

The MdmX protein shares significant structural and functional similarity with the proto-oncoprotein Mdm2. Both bind to the tumour suppressor protein p53, both contain a C-terminal RING finger motif, and both Mdm2 null mice and MdmX null mice are only viable in a p53 null background. Thus, Mdm2 and MdmX are major antagonists of p53. Similar to many other RING finger proteins, Mdm2 has the activity of an ubiquitin-protein ligase E3 and specifically targets p53 for proteasome-mediated degradation and, thus, inactivation. Despite their similarity, MdmX does not induce p53 degradation.

In fact, overexpression of MdmX was reported to interfere with Mdm2-mediated p53 degradation, presumably via heterodimer formation with Mdm2. To obtain insight into the mechanism by which MdmX inhibits p53 degradation we set up an in vitro system to study the effect of MdmX on the E3 activity of Mdm2. Surprisingly, this revealed that, in vitro, MdmX stimulates Mdm2-mediated ubiquitination of p53 and acts as a stimulator of Mdm2 E3 activity in general, challenging the prevailing hypothesis that MdmX counteracts the E3 activity of Mdm2.

In this project, the role of MdmX in Mdm2-dependent ubiquitination processes will be further characterised by biochemical means and by RNA interference-based methods. Furthermore, potential substrate proteins of this putative E3 complex shall be identified. These studies should contribute to the elucidation of the role of MdmX in Mdm2-facilitated ubiquitination in general and in p53 degradation and stability regulation in particular.

Publications resulting from the project:

Linares, L.K., Hengstermann, A., Ciechanover, A., Müller, S., Scheffner, M. (2003). HdmX stimulates Hdm2-mediated ubiquitination and degradation of p53. Proc. Natl. Acad. Sci. USA 100, 12009-12014.

Esser, C., Scheffner, M., Höhfeld, J. (2005). The chaperone associated ubiquitin ligase CHIP is able to target p53 for proteasomal degradation. J. Biol. Chem. 280, 27443-27448.