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Abstract
The human protein p54nrb have been implicated in a variety of nuclear processes including transcription, pre-mRNA processing, nuclear retention of edited RNA and DNA relaxation. We have identified p54 nrb as an antigen of the phosphodependent monoclonal antibodies CC-3 and MPM-2 and shown that this protein is phosphorylated on multiple sites during mitosis. The use of the cyclin-dependent protein kinase inhibitor roscovitine and immunodepletion studies with an anti-cyclin B1 antibody established that Cdk1 was responsible for the phosphorylation of p54nrb. We also show that p54nrb is a target of the peptidylprolyl isomerase Pin1, suggesting that it may be regulated by phosphorylation-dependent conformational changes. In addition, site directed mutagenesis indicated that the interaction of Pin1 with p54nrb was mediated by three threonine residues located in the proline-rich carboxy-terminal extremity of the protein. Our results also showed that Pin1 binding is favored when at least two of the three threonine residues were phosphorylated, suggesting a regulation mechanism based on multisite phosphorylation.