Membrane Protein Families

	Family Descriptions
	Potassium Transporter Families Transport of metals and alkali cations across plant plasma and organellar membranes is essential for plant growth, development, signal transduction, nutrition and also for use of plants in toxic metal phytoremediation. Alkali cation and metal transporters have been traditionally analyzed in great depth as models for understanding plant membrane transport. The alkali metal potassium (K+) is a major plant macronutrient and K+ is the most abundant cation in plants. Potassium transporters are required for the accumulation of potassium ions (K+) from soil and for their distribution throughout diverse plant tissues, for root and shoot growth, tropisms, cell expansion, enzyme homeostasis, salinity stress, stomatal movements and osmoregulation.		Trk/HKT Transporter Family Trk/HKT transporters are reminiscent of K+ channels in that they possess in a single polypeptide chain four domains resembling P-loops (see inset in Fig 1) (Durell and Guy, 1999). These P-loop-like domains are only weakly conserved to K+ channel P loops. The high-affinity K+ transporters Trk1 (Gaber et al., 1988) and Trk2 (Ramos et al., 1994) from yeast share 49% similarity on the level of amino acids with each other, and 17% and 28%, respectively, with HKT1 from wheat (Schachtman and Schroeder, 1994). Wheat HKT1 was shown to function as a high-affinity Na+ / K+ co-transporter when expressed in yeast and in Xenopus oocytes (Rubio et al., 1995), which correlates to high-affinity Na+ -coupled K+ uptake found in aquatic plants (Maathuis et al., 1996). In wheat Na+ / K+ cotransport is likely to contribute a minor portion to K+ uptake into roots.
	KUP/HAK/KT Transporter Family Bacterial K + uptake permeases named KUPs (Schleyer, 1993) and fungal high-affinity K + transporters named HAKs (Banuelos et al, 1995) form an additional family of K + transporters which was identified independently by several laboratories in plants. The plant genes were named AtKT (Quintero and Blatt, 1997), HAK (Santa-Maria et al., 1997; Rubio et al., 2000), or AtKUP (Fu and Luan, 1998; Kim et al., 1998). Here we name the A. thaliana members of this transporter family AtKUP/HAK/KT.		Cation/H+ Transporter Family Most cations are transported against their electrochemical gradient using proton-coupled transporters rather than primary ion pumps. With proton pumps at the PM and endomembranes of plant cells, we can predict that cation/proton antiporters extrude cations from the cytosol to the The predicted proteins in general have 10-14 transmembrane domains with about 400 to <900 residues. Yet, the substrate specificity, regulation, and membrane localization of these antiporters cannot be predicted with certainty from phylogenetic relationships. Therefore, the functional characterization of these large families is only beginning.
	CNGC Transporter Family (Cyclic Nucleotide-Gated Channels) A family of cyclic nucleotide-gated channels (CNGCs), first discovered in barley (Schuurink et al., 1998), is characterised by the presence at the C-terminus of both cyclic nucleotide and calmodulin binding domains. Membrane-associated domains strongly resemble those of the Shaker super-families, of which the KAT and AKT families are a part. Biochemical studies with a CNGC orthologue from tobacco and CNGC1 from A. thaliana have elegantly demonstrated that the cyclic nucleotide binding domain overlaps with that of calmodulin (Arazi et al., 2000; Köhler and Neuhaus, 2000), thereby suggesting that cyclic nucleotides and calmodulin interact in regulation of channel activity.		CDF metal Transporter Family (Cation Diffusion Facilitator) The Cation Diffusion Facilitator (CDF) family, first identified by Nies and Silver (1995), is a diverse family with members occurring in bacteria, fungi, plants and animal. All of these proteins have six putative transmembrane domains and a signature N-terminal amino acid sequence (Paulsen and Saier, 1997). These proteins also share a characteristic C-terminal cation-efflux domain (Pfam 01545). Eukaryotic family members also contain a histidine rich region between transmembrane domains four and five, which is predicted to be within the cytoplasm (Paulsen and Saier, 1997).
	NRAMP Metal Transporter Family (Natural Resistance-Associated Macrophage Protein) Genes encoding members of the NRAMP family of integral membrane proteins have been identified in bacteria, fungi, plants and animals. The proteins encoded by AtNRAMP genes cluster in two subfamilies: one including AtNRAMP1 and 6 and the other including AtNRAMP2 to 5. In addition, the ethylene insensitivity gene EIN2 which functions in transduction of multiple stress signals contains a NRAMP homologous domain but its homology with other members of the NRAMP family is much lower (Alonso et al., 1999). The functions of AtNRAMP proteins in metal transport have been demonstrated both in the heterologous yeast expression system and in planta (Alonso et al., 1999; Curie et al., 2000; Thomine et al., 2000).		ZIP Metal Transporter Family (Zinc and Iron regulated transporter Proteins) Members of the ZIP (ZRT, IRT-like protein) gene family, a novel metal transporter family first identified in plants, are capable of transporting a variety of cations including Cd, Fe, Mn and Zn (Guerinot, 2000). The family takes its name from the founding members, ZRT1, ZRT2 and IRT1.