Metallothioneins in Plants
The earliest perception restricted the occurrence of metallothioneins to animals, fungi, and certain bacteria and assigned the corresponding functions in plants to the enzymatically synthesized phytochelatins. This picture has now clearly changed, and the existence of plant metallothioneins is generally accepted. Compared to the vertebrate forms, plant metallothioneins show an intriguing sequence diversity, which is contrasted by an enormous lack of information about their possible functions, properties, and structures. The majority of functions proposed is derived from gene expression studies, which are reviewed with a not exhaustive but representative list of examples. However, it becomes evident that so far it is not possible to draw a clear-cut picture that ascribes specific functions to a given metallothionein. Rather, proposed functions overlap or are complementary to each other; sometimes even contradictory results are obtained. Regarding the structures and properties of the plant metallothionein proteins the amount of literature available is, compared to that of other MTs, considerably less comprehensive illustrating the early stages, in which this research area resides. Informations are at hand concerning the metal ion content of the different plant metallothionein species and metal-thiolate cluster stabilities, the number of clusters formed is proposed and predictions regarding the secondary structure are made. Three-dimensional data is so far restricted to one domain of a metallothionein from wheat providing fascinating insights into a cluster arrangement never reported for any metallothionein so far.