STRUCTURE OF THE C3HC4 DOMAIN BY H-1-NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY - A NEW STRUCTURAL CLASS OF ZINC-FINGER

A recently identified sequence motif, referred to as “C3HC4” (also “RING finger” and “A Box”) for its distinctive pattern of putative metal-binding residues, has been found in a wide range of proteins. In a previous paper we describedthe expression and purification of fragments encompassing this motif from the Vmw110 (IPC0) protein family. We showedthat the equine herpes virus protein binds zinc ions and adopts a ββαβ fold. We now report the tertiary structure of this domain insolution, as determined by two-dimensional 1H-NMR An amphipathic α-helix lies along one surface of a triple-stranded β-sheet. Four pairs of metal-binding residues sequester two zincs at distinct tetrahedral sites. The first and third pairs bind one metal ion, while the second and fourth pairsbind the other, forming an interleavedwhole. The first and the fourth pairs are contained within two prominent, well-defined loops related by anapproximate dyad symmetry. Conserved residues within the helix, sheet and loops contribute to a compact hydrophobic core.The region comprising the first two β-strands and the α-helix has remarkablestructural similarity with a TFIIIA type of zinc finger, even though the C3HC4 domain appears not tobind specifically to DNA or RNA. Using site-directed mutagenesis we demonstrate that exposed polar side-chains of the C3HC4 α-helix are essential for trans-activationof gene expression by an intact herpes virus regulatory protein. © 1994 Academic Press, Inc.