MAB3776 Sigma-AldrichAnti-MDM2 Antibody, clone SMP14

The DNA damage response to infection with minute virus of mice (MVM) leads to activated p53; however, p21 levels are reduced via a proteasome-mediated mechanism. This loss was sustained, as virus replicated in infected cells held at the G(2)/M border. Addition of the cyclin-dependent kinase (CDK) inhibitor roscovitine after S-phase entry reduced MVM replication, suggesting that CDK activity was critical for continued viral replication and virus-induced reduction of p21 may thus be necessary to prevent inhibition of CDK.

The product of the MDM2 gene interacts with and regulates a number of proteins, in particular the tumor suppressor p53. The MDM2 protein is likely to be extensively modified in vivo, and such modification may regulate its functions in cells. We identified a potential cyclin-dependent kinase (CDK) site in murine MDM2, and found the protein to be efficiently phosphorylated in vitro by cyclin A-containing complexes (cyclin A-CDK2 and cyclin A-CDK1), but MDM2 was either weakly or not phosphorylated by other cyclin-containing complexes. Moreover, a peptide containing a putative MDM2 cyclin recognition motif specifically inhibited phosphorylation by cyclin A-CDK2. The site of cyclin A-CDK2 phosphorylation was identified as Thr-216 by two-dimensional phosphopeptide mapping and mutational analysis. Phosphorylation of MDM2 at Thr-216 both weakens its interaction with p53 and modestly augments its binding to p19(ARF). Interestingly, an MDM2-specific monoclonal antibody, SMP14, cannot recognize MDM2 phosphorylated at Thr-216. Changes in SMP14 reactivity of MDM2 in staged cell extracts indicate that phosphorylation of MDM2 at Thr-216 in vivo is most prevalent at the onset of S phase when cyclin A first becomes detectable.

The function of p53 is modulated by binding to a number of cellular and viral proteins, such as MDM2 and SV40 large T antigen. An initial immunochemical characterization of the p53-MDM2 complex in a rat fibroblast cell line (Clone 6) suggested that the anti-p53 monoclonal antibody Bp53-19 failed to immunoprecipitate the complex, and only recognized a fraction of the available p53 protein. Following the recent identification of the Bp53-19 epitope at the N-terminal end of p53, in the vicinity of where MDM2 protein was known to bind, we investigated the possibility that Bp53-19 might identify a region of p53 that interacts with MDM2 protein. MDM2 was found to bind with great specificity to short synthetic peptides derived from the N-terminus of p53. Several p53 synthetic peptides libraries, and an alanine substitution series at the optimal binding site, were used to establish the MDM2 binding site, in fine detail, to the sequence TFSGLW (aa 18-23) in mouse and TFSDLW in man (aa 18-23). The key residues required for MDM2 binding are almost identical to those required for the monoclonal antibody Bp53-19 to bind and this region of p53 is recognised by many other anti-p53 antibodies.