Human full-length Securin is a natively unfolded protein

Human Securin, also called PTTG1 (pituitary tumor transforming gene I product), is an estrogen-regulated proto-oncogene with multifunctional properties. We characterized human full-length Securin using a variety of biophysical techniques, such as nuclear magnetic resonance, circular dichroism, and size-exclusion chromatography. Under physiological conditions, Securin is devoid of tertiary and secondary structure except for a small amount of poly-(L-proline) type II helix and its hydrodynamic characteristics suggest it behaves as an extended polypeptide. These results suggest that Securin is unstructured in solution and so belongs to the family of natively unfolded proteins. In addition, to gain structural and quantitative insight, we investigated the binding of Securin to p53. Analytical ultracentrifugation and fluorescence anisotropy studies revealed no evidence of any direct interaction between unmodified recombinant Securin and p53 in vitro.