Pin1 binds to a subset of mitotic phosphoproteins, many of which are also recognized by MPM-2 . in vitro by unique kinases. Finally, the post-mitotic dephosphorylation of both CC-3 and MPM-2 antigens was prevented when cellular Pin1 activity was blocked by the selective inhibitor juglone. Conclusion These observations show that this mitotic phosphoproteins associated with Pin1 are phosphorylated on multiple sites, suggesting combinatorial regulation of substrate acknowledgement and isomerization. Background One of the biggest challenge in cell biology and malignancy research is still to understand how cells divide and proceed to the equivalent partition of their genetic material in each child cell. The mechanisms underlying mitosis and cytokinesis are tightly controlled and reversible protein phosphorylation plays a major role in this regulation . Early and late mitotic events are dependent on protein phosphorylation by multiple serine and threonine kinases of the NIMA, Polo and Aurora families at the head of which sits the Cdk1/cyclin B complex [1-5]. The nature of most protein substrates of these kinases is still unknown so that the precise roles they play in the regulation of mitosis and cytokinesis remain to be clarified. A few monoclonal antibodies (mAbs) have been raised in different laboratories against mitotic cell extracts and shown to react with subsets of proteins that are phosphorylated upon access into mitosis [6-9]. The most characterized of these antibodies, mAb MPM-2, was selected for its preferential reactivity towards mitotic versus interphase cells  and shown to react with a phospho-epitope present on a set of proteins concentrated in the centrosomes, the kinetochores, the mitotic spindle and the midbody [6,10], reinforcing the idea that this structural rearrangements observed during mitosis are controlled by phosphorylation events. After twenty years of continuous use as a mitotic marker in scores of laboratories -and insistent efforts to characterize the epitope(s) and the kinases involved- it is now clear that many MPM-2 antigens are important mitotic regulators and effectors. They include the AescinIIB Cdc25 phosphatase , the Cdk1-inhibitory Wee1 and Myt1 kinases CD14 [12-14], the NIMA kinase , the microtubule associated-proteins MAP-1 and MAP-4 [16,17], DNA topoisomerase II and , p42mapk , and the Cdc27 component of the anaphase-promoting complex (APC) . Phosphorylation of the MPM-2 antigenic sites is usually thought to be functionally important as the MPM-2 antibody inhibits oocyte maturation upon microinjection and neutralizes mitosis promoting factor activity from M-phase extracts . The biological relevance of MPM-2 phosphoepitopes was further reinforced when Shen and coworkers  showed that this peptidyl-prolyl isomerase (PPIase) Pin1 could bind and regulate many mitotic phosphoproteins also recognized by MPM-2. PPIases are ubiquitous enzymes catalyzing the cis-trans isomerization of the peptide bond preceding a proline residue and are thought to be involved in protein folding, protein assembly, protein trafficking or in the direct regulation of protein activity . Pin1 is unique among prolyl isomerases in that it specifically targets proline residues preceded by a phosphoserine (pS-P) or a phosphothreonine (pT-P) [24-26]. Pin1 is usually a ubiquitously expressed protein that is essential for cell cycle progression in yeast and in mammalian cells . Furthermore, Pin1 has been shown to interact with the essential mitotic kinase NIMA and to suppress its mitosis-promoting activity . It is now believed that Pin1 functions as an essential mitotic regulator since, in addition to NIMA, it binds other MPM-2-reactive proteins with important mitotic functions including Cdc25 [22,28], Myt1, Wee1, Plk1 and Cdc27 . In the recent years, it was postulated that this analysis of the phosphorylated sites recognized by both Pin1 and MPM-2 might be a good starting point for a better understanding of the general role of phosphorylation in the mitotic processes. These efforts, mainly orchestrated by K.P. Lu and collaborators, have led AescinIIB to the elaboration of a novel post-phosphorylation regulatory mechanism, in which the isomerase Pin1 AescinIIB induces conformational changes into targeted proteins that have been first phosphorylated by proline-directed kinases [29,30]. The regulative action of Pin1 may not be confined to mitotic processes. Recent results have shown that DNA damage induces an AescinIIB conversation between Pin1 and p53 depending on specific.
Pin1 binds to a subset of mitotic phosphoproteins, many of which are also recognized by MPM-2