Zinc and RNA Binding Is Linked to the Conformational Flexibility of ZRANB2: A CCCC-Type Zinc Finger Protein

Ran-binding domain-containing protein 2 (ZRANB2) is a zinc finger (ZF) protein that plays a key role in alternative splicing. ZRANB2 is composed of two ZF domains that contain four invariant cysteine residues per domain. ZRANB2 binds RNA targets that contain AGGUAA sequence motifs. Three constructs of ZRANB2, ZRANB2-ZF1 (first ZF domain), ZRANB2-ZF2 (second ZF domain), and ZRANB2-2D (both ZF domains), were isolated in the apo form and shown to bind Zn(II) via UV-visible-monitored competitive titrations with Co(II) as a spectroscopic probe. Zn binding to each construct led to the adoption of a limited secondary structure of each domain, as measured by circular dichroism (CD). Hydrogen-deuterium exchange coupled with mass spectrometry (HDX-MS) of the two-domain construct, ZRANB2-2D, revealed that both ZF domains adopt a more rigid structure upon Zn binding. Zn binding to the first ZF domain resulted in a greater decrease in the conformational dynamics than Zn binding to the second ZF domain. RNA binding to TRA2B pre-mRNA, a physiological splicing target, was measured by fluorescence anisotropy (FA), and high-affinity RNA binding was found to require Zn coordination to both domains. HDX-MS of ZRANB2-2D with TRA2B RNA as well as two optimized RNA sequences that contain a single and double AGGUAA hexamer revealed additional protection from H/D exchange for ZRANB2 in the presence of RNA. Here, greater protection was observed for the second ZF of ZRANB2-2D, suggesting a larger effect on conformational dynamics. A model for zinc-mediated RNA binding of ZRANB2 is proposed.