A metallothionein containing a zinc finger within a four-metal cluster protects a bacterium from zinc toxicity

^*Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, United Kingdom; and ^†Department of Biochemistry and Genetics, University of Newcastle, Newcastle NE2 4HH, United Kingdom

Edited by Richard D. Palmiter, University of Washington School of Medicine, Seattle, WA, and approved June 19, 2001 (received for review March 12, 2001)

Abstract

Zinc is essential for many cellular processes, including DNA synthesis, transcription, and translation, but excess can be toxic. A zinc-induced gene, smtA, is required for normal zinc-tolerance in the cyanobacterium Synechococcus PCC 7942. Here we report that the protein SmtA contains a cleft lined with Cys-sulfur and His-imidazole ligands that binds four zinc ions in a Zn₄Cys₉His₂ cluster. The thiolate sulfurs of five Cys ligands provide bridges between the two ZnCys₄ and two ZnCys₃His sites, giving two fused six-membered rings with distorted boat conformations. The inorganic core strongly resembles the Zn₄Cys₁₁ cluster of mammalian metallothionein, despite different amino acid sequences, a different linear order of the ligands, and presence of histidine ligands. Also, SmtA contains elements of secondary structure not found in metallothioneins. One of the two Cys₄-coordinated zinc ions in SmtA readily exchanges with exogenous metal (¹¹¹Cd), whereas the other is inert. The thiolate sulfur ligands bound to zinc in this site are buried within the protein. Regions of β-strand and α-helix surround the inert site to form a zinc finger resembling the zinc fingers in GATA and LIM-domain proteins. Eukaryotic zinc fingers interact specifically with other proteins or DNA and an analogous interaction can therefore be anticipated for prokaryotic zinc fingers. SmtA now provides structural proof for the existence of zinc fingers in prokaryotes, and sequences related to the zinc finger motif can be identified in several bacterial genomes.

Footnotes

↵ ‡ To whom reprint requests should be addressed. E-mail: n.j.robinson{at}ncl.ac.uk or p.j.sadler{at}ed.ac.uk.
This paper was submitted directly (Track II) to the PNAS office.
Data deposition: The atomic coordinates for Zn₄SmtA have been deposited in the Protein Data Bank, www.rcsb.org (PDB ID code 1JJD).
Abbreviations:

ESI,

electrospray ionization;

ICP-AES,

inductively coupled plasma–atomic emission spectrometry;

NOE,

nuclear Overhauser enhancement;

NOESY,

nuclear Overhauser enhancement spectroscopy;

rmsd,

rms deviation;

amu,

atomic mass unit;

MT,

metallothionein;

HSQC,

heteronuclear single quantum coherence