Cyclin A-CDK phosphorylates Sp1 and enhances Sp1-mediated transcription

Cyclin A‐mediated activation of cyclin‐dependent kinases (CDKs) is essential for cell cycle transversal. Cyclin A activity is regulated on several levels and cyclin A elevation in a number of cancers suggests a role in tumorigenesis. In the present study, we used a modified DNA binding site selection and PCR amplification procedure to identify DNA binding proteins that are potential substrates of cyclin A–CDK. One of the sequences identified is the Sp1 transcription factor binding site. Co‐immunoprecipitation experiments show that cyclin A and Sp1 can interact physically. In vitro and in vivo phosphorylation studies indicate that cyclin A–CDK complexes can phosphorylate Sp1. The phosphorylation site is located in the N‐terminal region of the protein. Cells overexpressing cyclin A have elevated levels of Sp1 DNA binding activity, suggesting that cyclin A–CDK‐mediated phosphorylation augments Sp1 DNA binding properties. In co‐transfection studies, cyclin A expression stimulated transcription from an Sp1‐regulated promoter. Mutation of the phosphorylation site abrogated cyclin A–CDK‐dependent phosphorylation, augmentation of Sp1 transactivation function and DNA binding activity.