Plant metallothioneins and functional analysis of a barley metallothionein promoter

Most living organisms express a number of small cysteine-rich metalloproteins with strong binding capacity of transition metals, which are known as metallothioneins (MTs). Plant MTs are differentially expressed in various plant organs, as well as developmentally regulated and inducible in response to a wide variety of environmental stimuli and stressors. However, there is still limited knowledge regarding their physiological functions in plants. Here we present some fundamental steps towards a functional characterisation of a barley (Hordeum vulgare) type 2 metallothionein promoter in transgenic Arabidopsis thaliana

A combined zinc/cadmium sensor and zinc/cadmium export regulator in a heavy metal pump

Heavy metal pumps (P1B-ATPases) are important for cellular heavy metal homeostasis. AtHMA4, an Arabidopsis thaliana heavy metal pump of importance for plant Zn2+ nutrition, has an extended C-terminal domain containing 13 cysteine pairs and a terminal stretch of 11 histidines. Using a novel size-exclusion chromatography, inductively coupled plasma mass spectrometry approach we report that the C-terminal domain of AtHMA4 is a high affinity Zn2+ and Cd2+ chelator with capacity to bind 10 Zn2+ ions per C terminus. When AtHMA4 is expressed in a Zn2+-sensitive zrc1 cot1 yeast strain, sequential removal of the histidine stretch and the cysteine pairs confers a gradual increase in Zn2+ and Cd2+ tolerance and lowered Zn2+ and Cd2+ content of transformed yeast cells. We conclude that the C-terminal domain of AtHMA4 serves a dual role as Zn2+ and Cd2+ chelator (sensor) and as a regulator of the efficiency of Zn2+ and Cd2+ export. The identification of a post-translational handle on Zn2+ and Cd2+ transport efficiency opens new perspectives for regulation of Zn2+ nutrition and tolerance in eukaryotes. <br/