p21Wafl/Cip1 ,best known as a broad-specificity inhibitor of cyclin/cyclin-dependent kinase complexes,can interact with various target proteins,and this ability relies on its structural plasticity.Therefore,studies on the structural properties of p21 are very important to understand its structure-function relationship.However,detailed studies on its secondary strcture and biophysical properties have been comparatively sparse.A human p21 gene was cloned into the temperature expression vector pBV220 and transformed into Escherichia coli strain JM109.Recombinant protein was expressed as a non-fusion protein and purified by gel filtration and anion exchange chromatography.The purified protein was verified by Western blot and the functional activity was recognized by pull-down assay.Furthermore,circular dichroisrn,fluorescence spectroscopy,and fluorescence quenching methods were used to characterize the conformational properties of the purified protein.The results indicate that it was largely unstructured under the native solution conditions,and its tryptophan residues were exposed and located in a positively charged microenvironment.This study lays a good foundation for further study of p21 binding to its different partners.