Di-Cysteine Residues of the Arabidopsis thaliana HMA4 C-Terminus Are Only Partially Required for Cadmium Transport.

Stanislaus Antony Ceasar,Marc Hanikenne,Zhiguang Xiao,Gilles Lekeux,Patrick Motte,Moreno Galleni

doi:10.3389/fpls.2020.00560

Abstract

Cadmium (Cd) is highly toxic to the environment and humans. Plants are capable of absorbing Cd from the soil and of transporting part of this Cd to their shoot tissues. In Arabidopsis, the plasma membrane Heavy Metal ATPase 4 (HMA4) transporter mediates Cd xylem loading for export to shoots, in addition to zinc (Zn). A recent study showed that di-Cys motifs present in the HMA4 C-terminal extension (AtHMA4c) are essential for high-affinity Zn binding and transport in planta. In this study, we have characterized the role of the AtHMA4c di-Cys motifs in Cd transport in planta and in Cd-binding in vitro. In contrast to the case for Zn, the di-Cys motifs seem to be partly dispensable for Cd transport as evidenced by limited variation in Cd accumulation in shoot tissues of hma2hma4 double mutant plants expressing native or di-Cys mutated variants of AtHMA4. Expression analysis of metal homeostasis marker genes, such as AtIRT1, excluded that maintained Cd accumulation in shoot tissues was the result of increased Cd uptake by roots. In vitro Cd-binding assays further revealed that mutating di-Cys motifs in AtHMA4c had a more limited impact on Cd-binding than it has on Zn-binding. The contributions of the AtHMA4 C-terminal domain to metal transport and binding therefore differ for Zn and Cd. Our data suggest that it is possible to identify HMA4 variants that discriminate Zn and Cd for transport.

Highlights

Cadmium (Cd) is one of the most toxic substances present in the environment (Nriagu, 1988; Clemens et al, 2013) and represents a major health threat to humans, especially upon long-term exposure to low concentrations (Satarug et al, 2003, 2010)
In contrast to the case of Zn, the di-Cys motifs are only partially required for Cd transport to shoot tissues and only partially contribute to the Cd(II)-binding capacity of the
In the AtHMA4CCAA variant, all 13 di-Cys motifs found in the AtHMA4c domain were mutated into di-Ala motifs (Lekeux et al, 2018)

Summary

INTRODUCTION

Cadmium (Cd) is one of the most toxic substances present in the environment (Nriagu, 1988; Clemens et al, 2013) and represents a major health threat to humans, especially upon long-term exposure to low concentrations (Satarug et al, 2003, 2010). Cd emissions into the environment have declined in many industrialized countries, it is still prevalent in emerging countries and low-level contamination of soils results in important contamination of the food chain This calls for a comprehensive understanding of the Cd accumulation mechanisms in plants (Clemens et al, 2013). Typical of PIB-type ATPases, the HMA4 protein possesses eight transmembrane domains that play a key role in ion selectivity (Argüello, 2003; Argüello et al, 2007; Smith et al, 2014; Wang et al, 2014; Lekeux et al, 2019), multiple cytoplasmic catalytic domains, and N- and C-terminal cytosolic extensions rich in putative metal-binding amino acids such as Cys, His, Asp, and Glu (Williams and Mills, 2005; Argüello et al, 2007; Rosenzweig and Argüello, 2012). Our data suggest that the contribution of the AtHMA4 C-terminal domain to metal transport differs for Zn and Cd

MATERIALS AND METHODS

RESULTS AND DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT