9.A.43 The Cadmium Tolerance Efflux Pump (CTEP) Family Cadmium (Cd) causes the generation of reactive oxygen species (ROS), which in turn causes cell damage. Kim et al (2008) isolated a novel gene from a wheat root cDNA library, which conferred Cd²⁺-specific tolerance when expressed in Saccharomyces cerevisiae. The gene, called TaTM20, for Triticum aestivum transmembrane 20, encodes a hydrophobic polypeptide of 889 amino acids, containing 20 transmembrane domains arranged as a 5-fold internal repeating unit of 4 transmembrane domains each. Expression of TaTM20 in yeast cells stimulated Cd²⁺ efflux resulting in a decrease in the content of yeast intracellular Cd²⁺. TaTM20-induced Cd²⁺ tolerance was maintained in yeast even under conditions of reduced glutathione (GSH). Thus, TaTM20 enhances Cd²⁺ tolerance in yeast through the stimulation of Cd²⁺ efflux from the cell, partially independent of GSH. Treatment of wheat seedlings with Cd²⁺ induced their expression of TaTM20, decreasing subsequent root Cd²⁺ accumulation and suggesting a possible role for TaTM20 in Cd²⁺ tolerance in wheat (Kim et al. 2008). The proteins of this family seem to be exclusively from plants, but many of them do not have the 4 TMS repeats. They instead each have a single 4 TMS unit, usually at their C-termini. It is possible that these proteins are related to the bacterial protein in TC family 9.B.257, but this has not been examined carefully. The multi-pass transmembrane protein ACCELERATED CELL DEATH 6 (ACD6) is an immune regulator in Arabidopsis thaliana. Chen et al. 2023 identified two loci, MODULATOR OF HYPERACTIVE ACD6 1 (MHA1) and its paralog MHA1-LIKE (MHA1L), that code for approximately 7 kDa proteins, which differentially interact with specific ACD6 variants (see TC family 8.A.234). MHA1L enhances the accumulation of an ACD6 complex, thereby increasing the activity of the ACD6 standard allele for regulating plant growth and defenses. The intracellular ankyrin repeats of ACD6 are structurally similar to those found in mammalian ion channels. Increased ACD6 activity was linked to enhanced calcium channel-mediated influx, with MHA1L as a direct regulator of ACD6, indicating that peptide-regulated ion channels are not restricted to animals (Chen et al. 2023). Thus, some (but not all of the) evidence suggests that CTEPs are channel proteins.
This family belongs to the Ankyrin Repeat Domain-containing (Ank) Superfamily.
References:
Chen, J., L. Li, J.H. Kim, B. Neuhäuser, M. Wang, M. Thelen, R. Hilleary, Y. Chi, L. Wei, K. Venkataramani, M. Exposito-Alonso, C. Liu, J. Keck, A.C. Barragan, R. Schwab, U. Lutz, Z.M. Pei, S.Y. He, U. Ludewig, D. Weigel, and W. Zhu. (2023). Small proteins modulate ion-channel-like ACD6 to regulate immunity in Arabidopsis thaliana. Mol. Cell 83: 4386-4397.e9.
Jahrmann, T., M. Bastida, M. Pineda, E. Gasol, M.D. Ludevid, M. Palacín, and P. Puigdomènech. (2005). Studies on the function of TM20, a transmembrane protein present in cereal embryos. Planta 222: 80-90.
Kim, Y.Y., D.Y. Kim, D. Shim, W.Y. Song, J. Lee, J.I. Schroeder, S. Kim, N. Moran, and Y. Lee. (2008). Expression of the novel wheat gene TM20 confers enhanced cadmium tolerance to bakers' yeast. J. Biol. Chem. 283: 15893-15902.
Examples:
TC#	Name	Organismal Type	Example
9.A.43.1.1	Cd²⁺ tolerance factor, TaTM20, a hydrophobic polypeptide of 889 amino acids, containing 20 transmembrane domains arranged as a 5-fold internal repeating unit of 4 transmembrane domains each (Kim et al., 2008). The 4 TMS repeat has been referred to as the pfam13962 or PGG domain. The PGG domain is named for the conserved sequence motif found at the start of the domain.The protein appears to function as a Cd²⁺ exporter, possibly using the pmf and functioning as a secondary carrier (see family description).	Plants	*Cd²⁺ tolerance factor of Triticum aestivum* (Q2MJU2)**

9.A.43.1.10	Uncharacterized protein of 464 aas and 4 C-terminal TMSs (residues 300 - 450). The N-terminal domain is an ankyrin domain, showing extensive similarity to members of TC families 1.A.4 (TRP-CC) and 8.A.28 (Ankyrin domain proteins). The C-terminal TMSs are in a 1 + 3 TMS arrangement as is true of other members of the TC# 9.A.43 (CTEP) family.		UP of Acer yangbiense

9.A.43.1.11	Uncharacterized protein of 219 aas and 4 C-terminal TMSs.		UP of Panicum miliaceum

9.A.43.1.12	Uncharacterized ankyrin repeat family protein of 574 aas and 4 C-terminal TMSs. Other TMSs may be present in the large N-terminal ankyrin domain. This protein, and many others, share domains with 9.A.43 and 8.A.28, due to the ankyrin domains.		Ankyrin repeat protein of Arabidopsis thaliana

9.A.43.1.13	Uncharacterized protein of 222 aas and 4 C-terminal TMSs.		UP of Brachypodium distachyon

9.A.43.1.14	The accelerated cell death 6, ACD6, of 670 aas and 5 or 6 C-terminal TMSs. It is an activator of the defense response against virulent pathogens, including bacteria, fungi and oomycetes, that acts in a positive feedback loop with the defense signal salicylic acid (SA) (Lu et al. 2009). It regulates the salicylic acid (SA) signaling pathway leading to cell death and modulating cell fate (e.g. cell enlargement and/or cell division) (Lu et al. 2003). In response to SA signaling, it triggers the accumulation of the LRR receptor-like serine/threonine-protein kinase, (FLS2) at the plasma membrane, thus priming defenses (Zhang et al. 2014). Irt is involved in SA-dependent freezing signaling and tolerance (Miura and Ohta 2010). Information exchange between the ankyrin and transmembrane domains may be involved in activating defense signaling (Lu et al. 2005). It is an immune regulator in Arabidopsis (Chen et al. 2023). Two loci, MODULATOR OF HYPERACTIVE ACD6 1 (MHA1) and its paralog MHA1-LIKE (MHA1L), code for approximately 7 kDa proteins, which differentially interact with specific ACD6 variants. MHA1L enhances the accumulation of an ACD6 complex, thereby increasing the activity of the ACD6 standard allele for regulating plant growth and defenses. The intracellular ankyrin repeats of ACD6 are structurally similar to those found in mammalian ion channels. Several lines of evidence link increased ACD6 activity to enhanced calcium influx, with MHA1L as a direct regulator of ACD6, indicating that peptide-regulated ion channels are not restricted to animals (Chen et al. 2023).		ACD6 of Arabidopsis thaliana

9.A.43.1.15	Uncharacterized ankyrin domain-containing protein of 210 aas and 4 TMSs. Overlapping regions of homology with other members of this family as well as with TC# 9.B.257 seems likely but have not been demonstrated.		UP of Arachis duranensis

9.A.43.1.2	Uncharacterized protein of 175 aas and 4 TMSs.		UP of Glycine max (Soybean) (Glycine hispida)

9.A.43.1.3	Uncharacterized protein of 459 aas and 4 C-terminal TMSs.		UP of Solanum lycopersicum (Tomato) (Lycopersicon esculentum)

9.A.43.1.4	Uncharacterized protein of 179 aas and 4 TMSs.		UP of Populus trichocarpa (Western balsam poplar) (Populus balsamifera subsp. trichocarpa)

9.A.43.1.5	Embryogeneis transmembrane protein of 275 aas and 4 TMSs		*E-TMP of Zea mays* (Maize)**

9.A.43.1.6	Embryogenesis transmembrane protein of 1276 aas and 20 TMSs, E-TMP or TM20. It is present in differentiating provascular cells in rice and maize embryos, and has a polarized distribution within the cell in the more differentiated stages of development, and modifieds transport properties when injected into frog oocytes (Jahrmann et al. 2005).		*E-TMP of Oryza sativa subsp. japonica* (Rice)**

9.A.43.1.7	Embryogenesis transmembrane protein of 275 aas and 6 TMSs		*E-TMP of Oryza sativa subsp. japonica* (Rice)**

9.A.43.1.8	Uncharacterized protein of 227 aas and 5 TMSs.		UP of Triticum urartu (Red wild einkorn) (Crithodium urartu)

9.A.43.1.9	Embryogenesis transmembrane protein Dek34 or TM20 of 1,389 aas and 21 TMSs in a 1 + 4 + 4 + 4 + 4 + 4 TMS arrangement, where each 4 TMS repeat unit has a 1 + 3 TMS arrangement. TM20 is present in differentiating provascular cells in maize embryos. The protein has a polarized distribution within the cell in the more differentiated stages of development. Xenopus laevis oocytes microinjected with TM20 appear to show modified transport activities across the plasma membrane (Jahrmann et al. 2005).		*TM20 of Zea mays* (Maize)**