CPSF30:A Zinc Finger Protein With An Iron-Sulfur Site

Abstract

Zinc finger proteins (ZF's) are a class of proteins that utilize zinc for structural purposes. These metalloregulatory proteins perform a variety of functions ranging from the modulation of gene expression through interactions with DNA and RNA to mediating protein-protein interactions. ZF'-s are characterized by the presence of one or more domains that contain a combination of four cysteines and/or histidine residues that serve as ligands to coordinate zinc. Coordination of zinc to the ligands cause the ZF domain to fold into a three-- dimensional structure and become functional. Based on the number of cysteines and histidine residues in the ZF domains, it is classified as either a classical or a non-classical ZF. Within the non-classical ZF's, there are 14 distinct classes. One of the classes of non-classical ZF's is the Cys3His class. The first protein of this class to be identified was tristetraprolin (TTP), which contains two Cys3His domains (CCCH). Cleavage Polyadenylation Specificity Factor (CPSF30) is another member of this class of protein. The biological role of CPSF30 is to regulate pre-mRNA processing during polyadenylation. CPSF30 contains five repeats of three cysteines and one histidine residues (CCCH) and is annotated as a zinc finger in protein databases. Our lab has overexpressed a construct of CPSF30 that contains the five CCCH domains and utilized a combination of inductively coupled plasma spectroscopy, X-ray absorption spectroscopy and UV-visible spectroscopy to identify the zinc sites and zinc stoichiometry. We have obtained evidence for zinc coordination, as expected, but with stoichiometry ranging from 2-4 zinc ions, rather than the predicted 5. Instead, the fifth site houses an unexpected 2Fe-2S cluster co-factor. To identify where the 2Fe-2S site is located relative to the Zn sites, I have investigated two mutants - one in which the CCCH domain of the first ZF was mutated to all alanine residues and one in which the CCCH domains of the second ZF was mutated to all alanine's. Data regarding the effects of the mutations on metallation and RNA binding is presented.