The regions of securin and cyclin B proteins recognized by the ubiquitination machinery are natively unfolded

Abstract
The proteins securin and cyclin B are destroyed in mitosis by the ubiquitin/proteasome system. This destruction is important to mitotic progression. The N‐terminal regions of these proteins contain the sequence features recognized by the ubiquitination system. We have demonstrated using circular dichroism and 1‐D and 2‐D nuclear magnetic resonance that these rather substantial regions are natively unfolded. Based on these findings, we propose a model that helps to explain previously enigmatic observations.