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+ 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.
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