The plant MT1 metallothioneins are stabilized by binding cadmiums and are required for cadmium tolerance and accumulation.
The small Arabidopsis genome contains nine metallothionein-like (MT) sequences with classic, cysteine-rich domains separated by spacer sequences, quite unlike the small conserved MT families found vertebrate genomes. Phylogenetic analysis revealed four ancient and divergent classes of plant MTs that predate the monocot-dicot divergence. A distinct cysteine spacing pattern suggested differential metal ion specificity for each class. The in vivo stability of representatives of the four classes of plant MT proteins and a mouse MT2 control expressed in E. coli were enhanced by cadmium (Cd). Particular MTs were also stabilized by arsenic (As), copper (Cu), and or zinc (Zn). To understand why plants have such a diversity of MT sequences, the Arabidopsis MT1 class, comprised of three genes, MT1a, MT1b, and MT1c, was characterized in more detail in plants. MT1 family transcripts were knocked down to less than 5-10% of wild-type levels in Arabidopsis by expression of a RNA interference (RNAi) construct. The MT1 knockdown plant lines were all hypersensitive to Cd and accumulated several fold lower levels of As, Cd, and Zn than wildtype, while Cu and Fe levels were unaffected. The ancient class of MT1 protein sequences may be preserved in plant genomes, because it has distinct metal-binding properties, confers tolerance to cadmium, and can assist with zinc homeostasis.
Zimeri
Anne Marie
AM
Department of Genetics, University of Georgia, Athens, GA, 30602, USA.|Toxicology & Mycology Research Unit, USDA-ARS, Russell Research Center, Athens, GA, 30605, USA.
Dhankher
Om Parkash
OP
Department of Genetics, University of Georgia, Athens, GA, 30602, USA.|Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, MA, 01002, USA.
McCaig
Bonnie
B
D.O.E. Plant Research Laboratories, Michigan State University, East Lansing, MI, 48824, USA.
Meagher
Richard B
RB
Department of Genetics, University of Georgia, Athens, GA, 30602, USA. meagher@uga.edu and zimeri@uga.edu.
D000818:Animals; D017360:Arabidopsis / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D002104:Cadmium / Q000378:metabolism* / Q000494:pharmacology*; D003001:Cloning, Molecular; D004926:Escherichia coli / Q000235:genetics; D018506:Gene Expression Regulation, Plant / Q000235:genetics; D008668:Metallothionein / Q000096:biosynthesis / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D005810:Multigene Family; D010641:Phenotype; D010802:Phylogeny; D034622:RNA Interference; D012689:Sequence Homology, Nucleic Acid
D029681:Arabidopsis Proteins; C491078:MT1A protein, Arabidopsis; C506761:MT1b protein, Arabidopsis; C506762:MT1c protein, Arabidopsis; D002104:Cadmium; D008668:Metallothionein