Effects of Zinc Binding on the Structure and Dynamics of the Intrinsically Disordered Protein Prothymosin α:  Evidence for Metalation as an Entropic Switch^†

Prothymosin α (ProTα) is a small acidic protein that is highly conserved among mammals. The human form has 110 amino acid residues (M.W. 12.1 kDa; pI ∼3.5) and is found to be expressed in a wide variety of tissues. ProTα plays an essential role in cell proliferation and apoptosis, and it is involved in transcriptional regulation of oxidative stress-protecting genes. Despite the multiple biological functions ProTα has, the protein does not adopt a well-defined three-dimensional structure under physiological conditions. Previous studies have shown that the interaction between ProTα and some of its protein targets is significantly enhanced in the presence of zinc ions, suggesting that zinc binding plays a crucial role in the protein's function. In this work, we use nuclear magnetic resonance spectroscopy and electrospray ionization mass spectrometry to characterize the structure and dynamics of ProTα and its complexation with Zn²⁺. We found that zinc binding causes partial folding of the C-terminal half of ProTα, especially the Glu-rich region, while the N-terminal portion of the protein remains largely unstructured. The metalated protein also exhibits a significantly reduced flexibility. ProTα shows a high specificity for Zn²⁺, and the interactions with other divalent cations (Ca²⁺, Mg²⁺) are much weaker. On the basis of the site-specific information obtained here, as well as the results from previous studies, we propose that the conformational and dynamic changes upon zinc binding may act as an entropic switch that greatly facilitates the binding to other proteins.