A Comparative Inventory of Metal Transporters in the Green Alga Chlamydomonas reinhardtii and the Red Alga Cyanidioschizon merolae

Abstract

As in all organisms, metal cations are crucial for nutrition in plants. Several metals, such as copper, iron, zinc, and manganese, act as important cofactors for many enzymes and are essential for both mitochondrial and chloroplast functions. However, when supplied in excess, these essential cations can become toxic, like heavy metals with no generally established function, such as cadmium, lead, or mercury. To maintain micronutrient metal homeostasis and to cope with the deleterious effects of nonessential heavy metals, plants have developed a complex network of metal uptake, chelation, trafficking, and storage processes. Metal transporters are required to maintain metal homeostasis and thus constitute important components of this network (Clemens, 2001; Hall and Williams, 2003). In recent years, a number of membrane transport protein families have been implicated in metal homeostasis in plants. These include the cation diffusion facilitators (CDF), the Zrt-, Irt-like proteins (ZIP), the cation exchangers (CAX), the copper transporters (COPT), the heavy-metal P-type ATPases (HMA), the natural resistance-associated macrophage proteins (NRAMP), and the ATP-binding cassette (ABC) transporters (Williams et al., 2000; Maser et al., 2001; Cobbett et al., 2003; Hall and Williams, 2003). These transporters are encoded by multigene families. For example, 15 ZIP genes, 12 metal tolerance protein (MTP) genes, and 8 HMA genes are present in the Arabidopsis (Arabidopsis thaliana) genome (Cobbett et al., 2003; Delhaize et al., 2003). However, the transport specificities, patterns of expression, or subcellular localizations of metal transport proteins are still largely unknown. To further understand plant metal homeostasis, it will be necessary to elucidate the contribution of each of these transporters to the uptake, trafficking, and storage of essential metals, as well as to the detoxification of toxic heavy metals. In this article, we present an overview of our current knowledge of the metal transport function in metal homeostasis and tolerance in eukaryotes, with a special emphasis on plants. We also provide a timely inventory of putative metal transporters in two unicellular algal models, the green alga Chlamydomonas reinhardtii and the red alga Cyanidioschizon merolae. These new data should facilitate functional genomics and molecular analysis of metal homeostasis and tolerance in photosynthetic organisms. Moreover, the comparison of metal transporters from species belonging to the red and green algae with those of the land plant Arabidopsis, as well as with their human and yeast homologs, allows some light to be shed on the molecular evolution of metal homeostasis and tolerance systems. The use of a simple model organism is often helpful to dissect the functions and the interactions of various transport systems. The yeasts Saccharomyces cerevisiae and, to a lesser extent, Schizosaccharomyces pombe have been successfully used to elucidate the molecular basis of cellular metal homeostasis (Clemens and Simm, 2003; De Freitas et al., 2003; Eide, 2003). However, investigation of the essential chloroplast metal metabolism is not accessible in yeast models. The green alga Chlamydomonas reinhardtii Dangeard is a well-known model of a photosynthetic cell. This unicellular eukaryote is widely used for studies of a number of physiological processes, such as photosynthesis, respiration, nitrogen assimilation, flagella motility, and basal body function (Rochaix et al., 1998; Harris, 2001; Silflow and Lefebvre, 2001). Its short life cycle is easily controlled, making this organism a powerful tool for genetic analysis. The past few years have seen a dramatic development of the molecular technologies applicable to C. reinhardtii (Lefebvre and Silflow, 1999; Grossman, 2000; Fuhrmann, 2002). In the near future, the completion of the Chlamydomonas genome project, which includes molecular mapping, whole-genome sequencing, and extended expressed sequence tag (EST) generation programs (Grossman et al., 2003; Kathir et al., 2003; Shrager et al., 2003), will make C. reinhardtii an even more accessible model. C. reinhardtii has also been used in the past to study metal tolerance (for review, see Hanikenne, 2003) and homeostasis, and especially iron and copper metabolism in the chloroplast (e.g. Merchant, 1998; Moseley et al., 2002a, 2002b). In a recent review, we recommended the use of C. reinhardtii as a photosynthetic eukaryotic model to study metal homeostasis and tolerance (Hanikenne, 2003). The red alga Cyanidioschyzon merolae De Luca, Taddei, and Varano is a blue-green-colored unicellular alga that is found in sulfur-rich acidic hot springs. Its ultrastructure and other cytological and biochemical features suggest that it is one of the most primitive algae. C. merolae has no rigid cell wall and contains one nucleus, one mitochondrion, one chloroplast with a centrally located nucleoid, one Golgi body, and one microbody (Seckbach, 199