A Metallothionein Containing a Zinc Finger within a Four-Metal Cluster Protects a Bacterium from Zinc Toxicity

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 98; no. 17; pp. 9593 - 9598
• Main Authors: Blindauer, Claudia A., Harrison, Mark D., Parkinson, John A., Robinson, Andrea K., Cavet, Jennifer S., Robinson, Nigel J., Sadler, Peter J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 14.08.2001; National Acad Sciences
• Subjects: Amino Acid Sequence; Atoms; Bacteria; Bacterial Proteins - chemistry; Bacterial Proteins - physiology; Binding sites; Biochemistry; Biological Sciences; Cyanobacteria - drug effects; Cyanobacteria - metabolism; Cyanophyta; Cysteine - chemistry; Drug Resistance, Microbial - genetics; GATA protein; GATA transcription factors; Ions; Ligands; Magnetic Resonance Spectroscopy; Metal ions; Metallothionein - chemistry; Metallothionein - physiology; Molecular Sequence Data; Protein Conformation; Protein Structure, Tertiary; Proteins; Protons; Sequence Alignment; Sequence Homology, Amino Acid; smtA gene; SmtA protein; Spectrometry, Mass, Electrospray Ionization; Structure-Activity Relationship; Sulfur; Synechococcus; Toxicity; Zinc; Zinc - analysis; Zinc - pharmacology; Zinc Fingers - physiology
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.171120098

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 Zn4Cys 9His 2cluster. The thiolate sulfurs of five Cys ligands provide bridges between the two ZnCys4and two ZnCys3Hissites, giving two fused six-membered rings with distorted boat conformations. The inorganic core strongly resembles the Zn4Cys 11cluster 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.