This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Plant metallothioneins (MTs) are a ubiquitous class of proteins with a specific but mostly still unknown role in metal ion homeostasis, detoxification, and plant development. Plant MTs are low molecular mass, cysteine-rich proteins with an extremely high metal ion content (up to 7 mass equivalents). In contrast to their mammalian counterparts, the information available about plant MTs are rather limited and research activities in this area only started to increase recently [e.g. 1]. It was also only this year that our group determined the first three dimensional structure of a plant MT, specifically of the larger domain of the seed-specific wheat Zn6Ec-1 protein. While we have now concise data about the number of metal ions per protein at hand for a number of MTs from the four different plant MT subgroups, the nature of the coordinating ligands is not always clear. While metal ions are predominantly bound in tetrahedral tetrathiolate coordination environments, occasionally highly conserved His residues are found, which might play a role as well. We also have increasing evidence for the existence of additional metal ion binding sites with lower affinity. It is very well feasible that Cys residues have only a minor role in this new coordination mode.