Impact of Ser17 Phosphorylation on the Conformational Dynamics of the Oncoprotein MDM2

MDM2 is an important oncoprotein
that downregulates the activity
of the tumor suppressor protein p53 via binding of its N-terminal
domain to the p53 transactivation domain. The first 24 residues of
the MDM2 N-terminal domain form an intrinsically disordered “lid”
region that interconverts on a millisecond time scale between “open”
and “closed” states in unliganded MDM2. While the former
conformational state is expected to facilitate p53 binding, the latter
competes in a pseudo-substrate manner with p53 for its binding site.
Phosphorylation of serine 17 in the MDM2 lid region is thought to
modulate the equilibrium between “open” and “closed”
lid states, but contradictory findings on the favored lid conformational
state upon phosphorylation have been reported. Here, the nature of
the conformational states of MDM2 pSer17 and Ser17Asp variants was
addressed by means of enhanced sampling molecular dynamics simulations.
Detailed analyses of the computed lid conformational ensembles indicate
that both lid variants stabilize a “closed” state, with
respect to wild type. Nevertheless, the nature of the closed-state
conformational ensembles differs significantly between the pSer17
and Ser17Asp variants. Thus, care should be applied in the interpretation
of biochemical experiments that use phosphomimetic variants to model
the effects of phosphorylation on the structure and dynamics of this
disordered protein region.