Cadmium coordination to the zinc binding domains of the non-classical zinc finger protein Tristetraprolin affects RNA binding selectivity.

Tristetraprolin [(TTP), also known as NUP475 and TIS11] is a non-classical zinc finger protein that regulates inflammatory response via a protein-RNA interaction. Specifically, TTP recognizes AU-rich RNA sequences located on the 3' untranslated region of messenger RNA associated with cytokines. Recently, TTP was shown to be upregulated when cells were exposed to cadmium. Other types of zinc finger proteins have been shown to bind Cd(II), thus the Cd(II) binding properties of TTP were pursued. Metal binding titrations using Co(II) as a spectroscopic probe for Cd(II) were performed. Cd(II) was found to coordinate to the two Cys(3)His structural zinc sites of TTP (TTP-2D) with an upper-limit dissociation constant, K(d), of 3.5±0.1 nM. Upon reconstitution of TTP-2D with Cd(II), the protein recognized target RNA, UUUAUUUAUUU, with a dissociation constant, K(d), of 2.4±0.2 nM. The Cd(II)TTP-2D/RNA binding event was more sensitive to base mutations than the Zn(II)TTP-2D/RNA binding event. A single base mutation within the UUUAUUUAUUU oligomer decreased the Cd(II)TTP-2D binding affinity 50-fold and a double mutation decreased the affinity 1000-2000 fold. In comparison, only 2-fold and 15-25 fold changes for Zn(II)TTP-2D binding to the identical RNA sequences are measured.