Allosteric Regulation of MDMX by Protein Disorder

MDMX and MDM2 are homologues that regulate the p53 tumor suppressor using different
mechanisms. While MDM2 is responsible for controlling p53 degradation, MDMX regulates p53 DNA
binding and transcriptional activities. MDMX overexpression occurs in a subset of human tumors leading
to inactivation of p53. Efficient p53 response to stress and DNA damage involves phosphorylation of
MDMX followed by release of p53. Successful targeting of the p53 pathway for cancer therapy requires
understanding how MDMX regulates p53 function. MDMX contains an ordered p53 binding domain and
RING domain, and two long disordered regions that regulate p53 and CK1α binding in a phosphorylation-
dependent manner. Our work identified intra- and intermolecular interactions essential for MDMX
functions: (1) MDMX/p53 interaction inhibits p53 DNA binding. (2) MDMX/MDM2 interaction promotes
MDMX degradation. (3) MDMX/14-3-3 interaction promotes MDMX nuclear translocation. (4)
MDMX/CK1α interaction promotes MDMX/p53 binding and inhibition of p53. (5) MDMX intramolecular
interaction regulates p53 binding. Importantly, these interactions are regulated by two phosphorylation
sites (S289, S367) located in intrinsically disordered regions (IDR) of MDMX. We hypothesize that the
coordinated control is mediated by intramolecular interactions between ordered and disordered regions of
MDMX. The IDRs regulate the switching of intra/inter-molecular contacts in a phosphorylation-dependent
manner. We propose integrated molecular biology and biophysics approach to elucidate the mechanism
of MDMX regulation: (1) Determine how the length and flexibility of MDMX IDR1 regulates auto-
inhibition of p53 binding. (2) Investigate how CK1α phosphorylation switches MDMX to an open
conformation that inhibits p53 DNA binding. (3) Determine how Chk2 phosphorylation of IDR2
S367 regulates multiple MDMX interactions. These experiments will significantly advance the
understanding of how stress signals activate p53 by targeting disordered regions of MDMX, which is
essential to develop MDMX inhibitors for cancer therapy.

Grant Number: 5R01CA141244-14
NIH Institute/Center: NIH
Principal Investigator: JIANDONG CHEN