The p53 tumour suppressor protein is tightly controlled by the E3 ubiquitin 
ligase, mouse double minute 2 (MDM2), but maintains MDM2 expression as part of a 
negative feedback loop. We have identified the immunophilin, 25kDa FK506-binding 
protein (FKBP25), previously shown to be regulated by p53-mediated repression, 
as an MDM2-interacting partner. We show that FKBP25 stimulates 
auto-ubiquitylation and proteasomal degradation of MDM2, leading to the 
induction of p53. Depletion of FKBP25 by siRNA leads to increased levels of MDM2 
and a corresponding reduction in p53 and p21 levels. These data are consistent 
with the idea that FKBP25 contributes to regulation of the p53-MDM2 negative 
feedback loop.