A new class of proteins capable of binding transition metals

Ion uptake, transport, and sequestration are essential to meet the nutritio nal requirements for plant growth and development. Furthermore, regulation of these processes is critical for plants to tolerate toxic levels of ions. The examination of isoprenylated proteins encoded by Arabidopsis thaliana and Glycine max cDNAs revealed a unique family of proteins containing putat ive metal-binding motifs (the core sequence is M/LXCXXC). Here, we describe this new class of proteins, which are capable of being isoprenylated and b inding transition metal ions. Members of this family contain consensus isop renylation (CaaX) sites, which we demonstrate are efficiently isoprenylated in vitro. ATFP3, a representative of the Arabidopsis family, was expressed in Escherichia coli and examined for metal-binding activity in vitro. Anal ysis of the interaction of ATFP3 with metal-chelating columns (IMAC) sugges ted that it binds to Cu2+, Ni2+, or Zn2+. To test whether proteins with the se characteristics are present in other plant species, tobacco BY2 cells we re labeled in vivo with [C-14]mevalonate and the resulting mevalonate-label ed proteins were tested for metal-binding activity. Several soluble, isopre nylated proteins which bound copper-IMAC columns were revealed. Consistent with a wide-spread distribution of these proteins in plants, their presence was observed in Arabidopsis, soybean, and tobacco.