Ion uptake, transport, and sequestration are essential to meet the nutritional 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 putative metal-binding motifs (the core sequence is M/LXCXXC). Here, we describe this new class of proteins, which are capable of being isoprenylated and binding transition metal ions. Members of this family contain consensus isoprenylation (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. Analysis of the interaction of ATFP3 with metal-chelating columns (IMAC) suggested that it binds to Cu2+, Ni2+, or Zn2+. To test whether proteins with these characteristics are present in other plant species, tobacco BY2 cells were labeled in vivo with [14C]mevalonate and the resulting mevalonate-labeled proteins were tested for metal-binding activity. Several soluble, isoprenylated 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.

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Differential screening of a somatic embryo cDNA library for genes expressed during embryogenesis has disclosed a small RNA which I have named Tiny Up-regulated Protein (TUP). This mRNA is 402 bp in length, is expressed during embryo formation, and reaches its highest accumulation in the flowers and roots of adult carrot plants. The mRNA is barely detectable in leaves. The accumulation of the TUP mRNA in seedlings is increased by auxin and by abscisic acid. The open reading frame encodes a tiny, uniquely structured protein of 6100 Daltons which has a pi of 5.3. Algorithms predicting protein structure suggest that this tiny, unique protein contains a leader sequence targeting it to the endoplasmic reticulum and potentially through the default secretory pathway. Database searches show homology to antibacterial and arabinogalactan proteins. Protein prediction algorithms also suggest that the gene product may be a GPI-anchored protein. Synthesized TUP protein did not affect the growth of bacteria or fungus, and the accumulation of the mRNA decreased in seedlings grown with a fungus. These data suggest that the TUP does not have a role in defense responses in plants. Antisera generated against the synthetic TUP protein reveal