A novel CPx-ATPase from the cadmium hyperaccumulator Thlaspi caerulescens

Thlaspi caerulescens exhibits a unique capacity for cadmium tolerance and accumulation. We investigated the molecular basis of this exceptional Cd2+ tolerance by screening for T. caerulescens genes, which alleviate Cd2+ toxicity upon expression in Saccharomyces cerevisiae. This allowed for the isolation of a cDNA encoding a peptide with homology to the C-terminal part of a heavy metal ATPase. The corresponding TcHMA4 full-length sequence was isolated from T. caerulescens and compared to its homolog from Arabidopsis thaliana (AtHMA4). Expression of TcHMA4 and AtHMA4 cDNAs conferred Cd sensitivity in yeast, while expression of TcHMA4-C and AtHMA4-C cDNAs encoding the C-termini of, respectively, TcHMA4 and AtHMA4 conferred Cd tolerance. Moreover, heterologous expression in yeast suggested a higher Cd binding capacity of TcHMA4-C compared to AtHMA4-C. In planta, both HMA4 genes were expressed at a higher level in roots than in shoots. However, TcHMA4 shows a much higher constitutive expression than AtHMA4. Our data indicate that HMA4 could be involved in Cd2+ transport and possibly in the Cd hyperaccumulation character.