A Comparative Inventory of Metal Transporters in the Green Alga Chlamydomonas reinhardtii and the Red Alga Cyanidioschizon merolae
Year: 2005
Type: review
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, 1994; Seckbach and Ott, 1994; Matsuzaki et al., 2004). Belonging to the extremophilic Cyanidiales, it shares the same photosynthetic pigments and ecological niche as Cyanidium caldarium and Galdieria sulphuraria, but differs by being smaller, structurally simpler, and dividing only by binary fission (no formation of endospores; Ott and Seckbach, 1994; Gross, 2000; Ciniglia et al., 2004). C. merolae was extensively used in studies of mitochondrial and chloroplast divisions, which can be highly synchronized by light/dark cycles (Kuroiwa, 1998). Its nuclear genome is one of the smallest known in photosynthetic eukaryotes. It has just been completely characterized (Matsuzaki et al., 2004), with the mitochondrial (Ohta et al., 1998) and plastid genomes (Ohta et al., 2003) also being available. According to many phylogenetic studies, red algae and green plants (including green algae) are considered to be sister groups (Cavalier-Smith, 1982; Bhattacharya and Medlin, 1995; Moreira et al., 2000). In particular, red and green plastids are thought to be derived from the same primary endosymbiotic event, the former retaining more cyanobacterial features than the latter (Nozaki et al., 2003; Ohta et al., 2003). Along with fossil evidence (Butterfield, 2000), this supports the idea that red algae are among the most primitive eukaryotes still living today. Red algae are also worth being studied because their plastid seems to be at the origin of the plastids of various algal groups of fundamental importance in marine primary production (diatoms, dinophytes, and brown algae) through a process of secondary endosymbiosis (Moreira and Philippe, 2001; Yoon et al., 2002). We used a high-throughput semiautomated approach to identify metal transporters in the recently released genomic sequences of C. reinhardtii (U.S. Department of Energy Joint Genome Institute [JGI; and C. merolae (Matsuzaki et al., 2004). on both algal all of the and Arabidopsis and as well as resistance-associated protein and of the tolerance of transporters This to and putative metal transporters in C. reinhardtii and C. merolae. among the to the organisms, is in of metal in Arabidopsis, C. and C. merolae of metal in Arabidopsis, C. and C. merolae of the metal transporters in C. reinhardtii to protein number number of the C. reinhardtii protein model subcellular and number of is when both programs and to chloroplast or and to or membrane For more see and and have been and Grossman, et al., et al., of the metal transporters in C. reinhardtii to protein number number of the C. reinhardtii protein model subcellular and number of is when both programs and to chloroplast or and to or membrane For more see and and have been and Grossman, et al., et al., of the metal transporters in C. merolae to protein protein protein model subcellular and number of is when both programs and to chloroplast or and to or membrane For more see and of the metal transporters in C. merolae to protein protein protein model subcellular and number of is when both programs and to chloroplast or and to or membrane For more see and The a of transporters in metal homeostasis and These proteins the of metal like or from the to the of the cell or subcellular proteins putative with the and to be and a and a sequence and and a cation to and and Eide, 2001). The plant of the are of the The is the of the most for which by the model of from an are in and from an are in for from other than C. reinhardtii and C. merolae are in for and Arabidopsis Schizosaccharomyces to and with to and with a of putative The protein of the to yeast and Arabidopsis when It is in the plant membrane and was to be in in this et al., 2003). to have no data are available. phylogenetic analysis that the of this in C. reinhardtii and Arabidopsis through In that it is to that to are found on the same genome to each other the important of to maintain photosynthesis, it is that at one of these proteins is located in the our analysis only these proteins in the or the and groups with and two proteins of function, the is to a few other It worth to and a with that no with other is to act in the of in the Golgi complex of and 2004), and is in the and is essential to maintain homeostasis in this et al., 2004). analysis that is to the as well C. reinhardtii C. merolae to a of the yeast and two mitochondrial proteins to a in iron homeostasis and The ZIP protein in metal homeostasis, generally the of metal like zinc, iron, cadmium, or manganese, from the cell or from a subcellular the are to have with the and being As a a of and sequence and The often contains a and are and to a required for the cation metal the and be the of of the and Eide, 1999; 2000; and Eide, 2001). on sequence and have the ZIP proteins includes from plants and contains and The contains proteins of function present in both and eukaryotes, and the is only found in eukaryotes. We have and in the genomes of C. reinhardtii and C. merolae and, our suggest that these algal are in the or the membrane and of the ZIP The is the most for which by the model of from an are in and from an are in for and in because to in both the and The is from other than C. reinhardtii and C. merolae are in for and few Arabidopsis have been characterized through yeast and to a in cellular uptake, and being at the 2000; and Eide, 2001; Hall and Williams, 2003). In the and two ZIP are in et al., 2000; et al., 2000; et al., in the and are highly expressed to Arabidopsis, in and in et al., et al., 2004). In Arabidopsis, the and proteins are expressed in In to iron, also zinc, manganese, and cadmium, is only for 2000; and Eide, 2001; Hall and Williams, 2003). In Arabidopsis, the iron in of the in this to be to this plant iron are in with the of et and et the in C. reinhardtii of an iron to the iron of cerevisiae also to ZIP with and to has not been and are in the membrane 2004). C. reinhardtii proteins only one is found in C. merolae yeast Arabidopsis and The of and 12 with the is only by analysis not The has been the protein of the a protein of function by genome et al., The yeast protein is the only protein that has been It is in the membrane and in the of et al., 2000). a protein is found in C. merolae but not in C. contains a highly by and as a for human in the which is to this of implicated in human the proteins have recently been to be of transport et al., 2004). It should be that the and proteins are only to the ZIP to in with other their in the to the phylogenetic used not These proteins be on and should be considered as putative our the of a number of putative transporters of the ZIP in the C. reinhardtii genome to yeast and C. merolae. The unicellular C. reinhardtii as many as the complex eukaryotes Arabidopsis and the ZIP in in each of these for Arabidopsis, for and and for C. from genes still present in C. merolae. The proteins are generally to The and proteins have been by functional of a cerevisiae in putative is a a for et al., and has been to transport and the of plant et al., 2000). other proteins have been in the Arabidopsis but have not been characterized et al., 2001). of this is the which is an that can transport and the membrane et al., of the The is the most for which by the model of from an are in and from an are in for from other than C. reinhardtii and C. merolae are in it is to that C. reinhardtii C. merolae to a that this function is in algae. In Arabidopsis, is expressed in and to in and plant et al., This function have with the of land plants. it be by a more of the The proteins a eukaryotic of copper transporters 1998). the of with an and a the of two in the that act as a copper in the and an in in copper transport 2004). In copper uptake, like iron membrane to to the being by the same that are in iron is by the copper transporters and a that is highly for other metal 1998). cerevisiae an protein in the the function of 1998; 2004). are found in but only has been It is a membrane protein in copper in various cell and 2004). et have recently proteins to in Arabidopsis and that and copper in a yeast copper with and in development et al., 2004). thus an important in copper in We have a protein in the genomes of both C. reinhardtii and C. merolae and is more to the Arabidopsis than to the human and yeast is only to the other not It should also be that is than the plant and human be located in the membrane is to be in the membrane P-type ATPases are transporters characterized by the formation of a in the These proteins 8 to 12 and a ATP-binding and P-type ATPases transport a of and and have been to their and The proteins or are in heavy-metal the of which contains a to be in metal cation the in a of organisms, can be two groups with cations or and 2001; Cobbett et al., 2003). of the HMA of P-type The is the most for which by the model of from an are in and from an are in for the in the are by an from other than C. reinhardtii and C. merolae are in for and in the of these in eukaryotes is to photosynthetic Cobbett et that these proteins have from one or from the to be at the origin of the The to genes from the recent of an (Cobbett et al., 2003). The to is in C. merolae and C. that this function have been in both algae. and and with the cation transporters of the and are most copper transporters In particular, and are to yeast plant and human and transporters known to copper to proteins in the network (Cobbett et al., 2003; Hall and Williams, 2003). The protein to the which has been and is to be with the in copper to the et al., 2002). with and is a protein the transport of copper the Arabidopsis have a chloroplast copper of and et al., 2003). a in copper to the However, this protein is to to the membrane In some and green in the of photosynthetic from to be by a or is expressed and by when copper is and 1998; Merchant, 1998). plants only all red algae with and to use the et al., et al., C. merolae also not and plastid genome contains a a (Ohta et al., 2003). The that is not on in C. merolae the of an function in this organism transporters are transporters in a number of physiological processes. This is one of the protein families with and in Arabidopsis, and transporters two for with two highly that the to be The transporters a of each and are to function as or et al., 2003). on transporters can be in and et al., 2001; et al., 2003). of only two and have been implicated in metal transport are transporters as et al., 1998). In the protein the transport of from the the The of the to cadmium, and et al., 1994; et al., et al., 1999; et al., 2003). yeast is also in but is only to et al., et al., 2002). The of in Arabidopsis the tolerance of the plant to and et al., 2003). proteins in Arabidopsis et al., 2001; et al., et al., only has been implicated in detoxification and The is in to and this is with the of in the plant et al., 2003). Moreover, the of a yeast et al., 1998). of the other and characterized and or the 12 are in metal but in the transport of a of (for review, see et al., 2003; et al., 2003). We have found and two in the genome sequences of C. reinhardtii and C. and it is a protein model is to the other of the a we have in to the model on programs and we an model the of the not The of the will be necessary to elucidate the of the of The is the most for which by the model of from an are in and from an are in for from other than C. reinhardtii and C. merolae are in has been by and are transporters located in the mitochondrial or transporters and and are in the of from the mitochondrial to the et al., et al., 1998; et al., 1999; et al., 1999; et al., 2000; et al., 2001). We have recently that a C. reinhardtii of these mitochondrial an essential in by of the the mitochondrial function from et al., The of mitochondrial transporters to act in detoxification be a new among green et al., of to the in the from in the of et al., 2003), that the be by the This was by that is in the of et al., This that the analysis of data be used to The of the fission yeast pombe is in the transport of from the the this is to in the and to et al., the of such a transport function has been in plant and and no has been in the Arabidopsis The Arabidopsis proteins most highly to are the transporters et al., 2001; et al., 2002). We have two and in the genomes of C. reinhardtii and C. and we not protein model to in the sequence data but have found by that it of the of The is the most for which by the model of from an are in and from an are in for from other than C. reinhardtii and C. merolae are in has been by et are known for iron in plants. in all and by ATPases to of iron by membrane and of iron by a of the ZIP and 1994; and 2002). We have that C. reinhardtii C. merolae to of is found in and the of to the of a and 1994; and 2002). In this has recently been et al., 2001). to the and is iron both at and It has been to and or It is to a metal zinc, copper, and in and it seems to be only by iron in et al., et al., 2004). Moreover, the has been to transport iron and by et al., 2004). are not in Arabidopsis, a but are found in the plant The protein has been to transport iron and copper by a of et al., 2004). To our or are not found in algae. we for proteins in the C. reinhardtii and C. merolae genome sequence but not identify This function thus have the of land plants. As as genome sequence data become it be to green algae that to the of plants et al., in this In the of iron as for the plant the of to by iron by a that to and an iron the cell. and are iron and the proteins a complex at the In iron is by the and et genes a and an iron in the C. reinhardtii and in a photosynthetic the of an iron to the iron of cerevisiae (for review, see Hanikenne, 2003). no was found in the C. reinhardtii we have proteins in C. merolae and and are in sequence and are highly in the are to in the membrane like and and we not in C. merolae not in an and sulfur-rich with copper C. merolae have an with the and iron be on copper, in to and are to in the This putative function is not found in yeast and C. reinhardtii and an evolution in C. merolae. such a function have been in other studied unicellular eukaryotes. of the are also known to transport These transporters use the to facilitate transport of cations and iron in are proteins that the of 12 in and a transport in the 8 and and 2001; and 2004). In is required for natural to by and the in the thus metal for a in the of at the of and 2001; and 2004). expressed in is to the transport of a of metal cations (including cadmium, copper, zinc, manganese, and in to iron and 2004). It has been that to and in human et al., 2004). proteins are encoded in the genome of Arabidopsis et al., 2001). of and iron or and and in yeast et al., 2000; et al., 2000). In the genes are iron and the of to an to toxic iron et al., 2000; et al., 2000). of the to et al., 2000). et that is in the membrane and that the protein functions as a metal cadmium, manganese, and from the the homologs, to are found in These proteins transport a of but have been more implicated in manganese, copper, and, more iron homeostasis and 1999; et al., 2000). is in the in the membrane of and in the membrane et al., et al., 2000; and 2001). of the The is the of the two most for which by the model of from an are in and from an are in for from other than C. reinhardtii and C. merolae are in for and in the protein the transport of iron at the of the the for to other et al., 2000). in the The Arabidopsis allows the of proteins in these proteins have not been a in iron homeostasis in plants. We not in C. reinhardtii or cerevisiae not a was in C. merolae protein seems to have been present in the of photosynthetic and organisms. It have been in yeast and C. an iron function be important for C. merolae to cope with an characterized by iron a analysis we have of families or of metal transporters in the unicellular algae C. reinhardtii and C. merolae of the proteins have been in these organisms. The of all in a protein become crucial when of this This allows as to functional multigene families. For example, the and proteins are to each other in Arabidopsis (Cobbett et al., 2003). no and have been more to the function of these transporters in metal homeostasis et al., 2004). and have recently a for some metal transporters in C. when of the genome at the each the we have a number of This our high-throughput approach protein families from to genomes of a few characterized This is especially for families of a of with sequence which is often the among metal families. For example, a of the C. reinhardtii genome with not identify our from the of all in have been used by to C. reinhardtii these are of and than those for C. merolae. the of many short and in C. reinhardtii nuclear genes 1998) is for such the C. merolae genome contains only a number of present in of genes (Matsuzaki et al., 2004). As a some C. reinhardtii proteins be to their or from their or human (e.g. see or The of the protein also has an on the of or than on their in to proteins for the completion of the genome project, a for the Chlamydomonas will be the of to and protein the of the and subcellular of the was used to to et al., it has been that these are not to from et al., 2002). Moreover, all have been for plants or eukaryotes such as and it is known are in C. reinhardtii and C. merolae. et have that the of C. reinhardtii are more to the yeast and plant than those of plant chloroplast The of C. reinhardtii was in the et al., and a in the chloroplast or in the For all these in of should be with In some are when to for the species (e.g. and in and For most of the genes we have and that these genes are As the analysis of these data is to the other genes be in the used for all Moreover, some have been the these genes to the is no as no the sequence was found in these phylogenetic that an for all families and was present in eukaryote with the of the to was generally with the of functions than with the of families. In most it is thus to use unicellular to a fundamental of cellular metal tolerance and are the among the ZIP protein in C. reinhardtii and the in C. merolae as a C. reinhardtii generally to be more complex than C. merolae with to the number of metal transporters As a organism found in and C. reinhardtii and the to Moreover, C. reinhardtii has a life cycle with and for which functions have In that it is to that metal transporters are in the the the are a of algae et al., 2004). it is a highly primitive the that C. merolae a of a complex organism be C. merolae in acidic and and various heavy hot and is to these with a number of metal transporters when to C. The unicellular red alga C. caldarium has been to of iron and in et al., 2003), as well as of zinc, manganese, and copper et al., 2004). In this on metal homeostasis and tolerance in eukaryotes, we present an inventory of metal transporters found in two unicellular with and as well as of the These data through a semiautomated in that be for the the of such an we on the functional and by our this should provide a basis for further molecular and genomic studies of heavy-metal homeostasis and tolerance in photosynthetic organisms. The C. reinhardtii sequence data by the Department of Energy Joint Genome Institute and are for use in this
Source: PLANT PHYSIOLOGY
Language: English
Subfield: Plant Science
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W1995884678