RNase A is inhibited by the cysteine-rich protein thionein but not by the metal-containing form metallothionein.

Metallothionein (MT) is a cysteine-rich protein that displays a high redox activity, acts like an acute phase protein, and forms metal-thiolate clusters. However, little is known about how the presence of metals in MT or their absence in Thionein (T) modify the structure of the protein and if such differential conformations alter the protein-protein interactions that MT/T establishes with other proteins. Ribonucleases (RNases) are a group of enzymes that hydrolyze different classes of RNA and due to the relevance of their function, they are highly regulated by specific inhibitors. Earlier results showed a decrease in RNase activity during an inflammatory process that appears to be a consequence of a de novo synthesis of an unidentified RNase inhibitor (RI) (Brambila et al in Exp Mol Pathol 71:125–131, https://doi.org/10.1006/exmp.2001.2383 , 2001). Therefore, we performed experimental studies to evaluate the ability of MT/T to inhibit RNase A activity in vitro. Computational methods were utilized to characterize the tertiary structure of MT/T in the presence and absence of zinc ions and to evaluate the stability of a complex with RNase A. Our results show that only in the absence of metals, T, the homodimeric protein has an inhibitory effect on RNase A activity via a mechanism that involves the formation of a complex between RNase A and T by specific interactions. Also, the involvement of residues, including cysteines that coordinate the cations, that participate in intermolecular interactions in the metal-bound form, aids in explaining the metal dependent inhibitory properties in the metal-free protein. This work suggests an additional role of T in the regulation of enzyme activity and protein synthesis.