9.A.12 The Copper Transporter-2 (Ctr2) Family

Four proteins, one from Arabidopsis thaliana (CopT1; 169 amino acyl residues), one from humans (hCTR1; 190 residues) and two from Saccharomyces cerevisiae (Ctr2p; 189 residues and Ctr3p; 241 residues) have been cloned, sequenced and expressed in mutant S. cerevisiae. They have three homologues in Schizosaccharomyces pombe (148, 173 and 389 residues), six homologues in the worm, C. elegans (162, 178, 252, 253, 359 and 387 residues), three in Drosophilia melanogastor (174, 220 and 274 residues), one in humans (143 residues) and two in the protozoan Theileria parva (480 and 543 residues). These proteins exhibit 1, 2 or 3 repeat units of about 124 residues each. Thus, CopT1of A. thaliana has one, Ctr3p of S. cerevisiae has 2, and the C. elegans protein K12C11.3 (387 residues) has 3 repeats.

The Ctr6 (148 aa) protein of Schizosaccharomyces pombe is an integral membrane protein that is induced by copper limitation (Bellemare et al., 2002). It can trimerize and harbors a putative copper-binding M-XC-XM-XM motif in its N-terminus that is essential for function. The physiological function of Ctr6 is to mobilize stored copper from the vacuole to the cytosol (Bellemare et al., 2002).

The H. sapiens, A. thaliana, S. cerevisiae and T. parva proteins have 3 putative transmembrane α-helical spanners and display N-terminal hydrophilic sequences homologous to the methionine and histidine-rich Cu+ binding domains of various copper binding proteins. Human CTR1 has its N-terminus extracellular and its C-terminus intracellular (Eisses and Kaplan, 2002). These copper binding proteins include the P-type copper-transporting ATPases (TC #3.A.3). It is not clear that all of these proteins are localized to the plasma membrane, but the majority of the evidence implicates them in Cu+ uptake. The energy coupling mechanism (if any) has not been investigated , and it is not known if they function by a channel or a carrier type mechanism. However, human high affinity (~3 μM) Ctr1, a homomultimer, takes up Cu+ across the plasma membrane by an energy-independent mechanism that is stimulated by extracellular acidic pH and high K+ concentrations (Lee et al., 2002). The mouse Ctr1 transporter is essential for copper homeostasis and embryonic development (Andrews, 2001; Kuo et al., 2001; Lee et al., 2001). The Arabidopsis CopT1 protein plays a role in root elongation and pollen development, revealing a role for copper acquisition in these processes (Sancenón et al., 2004).

In S. pombe, two proteins, Ctr4 and Ctr5, together comprise a heteromeric Cu2+ uptake transporter. Both proteins exhibit regions of strong sequence similarity with Ctr3 of S. cerevisiae. They exhibit 2 and 3 putative TMSs, respectively, and are coregulated by Cu2+ and the Cuf1 transcription factor. They have been shown to physically interact to yield the active transporter.

The Ctr2 family is of the same topology and exhibits very limited sequence similarity with members of the Ctr1 family (TC #9.A.11) (Puig et al., 2002). This sequence similarity is maximal in TMS 3 but is insufficient to establish homology. PSI-BLAST with several interactions also reveals common motifs, again suggesting a common evolutionary origin for these two families.

The generalized transport reaction presumably catalyzed by proteins of the Ctr2 family is:

Cu+ (out) → Cu+ (in).

 

References:

Andrews, N.C. (2001). Mining copper transport genes. Proc. Natl. Acad. Sci. USA 98: 6543-6545.
Bellemare, D.R., L. Shaner, K.A. Morano, J. Beaudoin, R. Langlois, and S. Labbé. (2002). Ctr6, a vacuolar membrane copper transporter in Schizosaccharomyces pombe. J. Biol. Chem. 277: 46676-46686.
Eide, D. and M. L. Guerinot. (1997). Metal ion uptake in eukaryotes. ASM News 63: 199-205.
Eisses, J.F. and J.H. Kaplan. (2002). Molecular characterization of hCTR1, the human copper uptake protein. J. Biol. Chem. 277: 29162-29171.
Harris, E.D. (2000). Cellular copper transport and metabolism. Annu. Rev. Nutr. 20: 291-310.
Kampfenkel, K., S. Kushnir, E. Babiychuk, D. Inzé, and M.V. Montagu. (1995). Molecular characterization of a putative Arabidopsis thaliana copper transporter and its yeast homologue. J. Biol. Chem. 270: 28479-28486.
Kuo, Y.-M., B. Zhou, D. Cosco, and J. Gitschier. (2001). The copper transporter CTR1 provides an essential function in mammalian embryonic development. Proc. Natl. Acad. Sci. USA 98: 6836-6841.
Lee, J., J.R. Prohaska, and D.J. Thiele. (2001). Essential role for mammalian copper transporter Ctr1 in copper homeostasis and embryonic development. Proc. Natl. Acad. Sci. USA 98: 6842-6847.
Lee, J., M.M.O. Peña, Y. Nose, and D.J. Thiele. (2002). Biochemical characterization of the human copper transporter Ctr1. J. Biol. Chem. 277: 4380-4387.
Puig, S., J. Lee, M. Lau, and D.J. Thiele. (2002). Biochemical and genetic analyses of yeast and human high affinity copper transporters suggest a conserved mechanism for copper uptake. J. Biol. Chem. 277: 26021-26030.
Sancenón, V., S. Puig, I. Nateu-Andrés, E. Dorcey, D.J. Thiele, and L. Peñarrubia. (2004). The Arabidopsis copper transporter COPT1 functions in root elongation and pollen development. J. Biol. Chem. 279: 15348-15355.
Zhou, B. and J. Gitschier. (1997). hCTR1: a human gene for copper uptake identified by complementation in yeast. Proc. Natl. Acad. Sci. USA 94: 7481-7486.
Zhou, H. and D.J. Thiele. (2001). Identification of a novel high affinity copper transport complex in the fission yeast Schizosaccharomyces pombe. J. Biol. Chem. 276: 20529-20535.
 

Examples:

TC#NameOrganismal TypeExample
9.A.12.1.1Copper uptake transporter Plants CopT1 of Arabidopsis thaliana
 
9.A.12.1.2Copper uptake transporter Animals hCTR1 of Homo sapiens
 
9.A.12.1.3Copper uptake transporter Yeast Ctr2p of Saccharomyces cerevisiae
 
9.A.12.1.4Copper uptake transporter Yeast Ctr3p of Saccharomyces cerevisiae
 
9.A.12.1.5The heterodimeric copper uptake transporter, Ctr4/Ctr5YeastCtr4/Ctr5 of Schizosaccharomyces pombe
Ctr4
Ctr5
 
9.A.12.1.6Vacuolar, trimeric copper release proteinYeastCtr6 of Schizosaccharomyces pombe