2.A.19 The Ca2+:Cation Antiporter (CaCA) Family
Proteins of the CaCA family are found ubiquitously, having been identified in animals, plants, yeast, archaea and divergent bacteria. They exhibit widely divergent sequences, and several have been shown to have arisen by a tandem intragenic duplication event (Saier et al., 1999). The most conserved portions of this repeat element, α1 and α2, are found in putative TMSs 2-3 and TMSs 7-8 in the model of Iwamoto et al. (1999). These sequences are important for transport function and may form an intramembranous pore/loop-like structure. These carriers function primarily in Ca2+ extrusion (DiPolo and Beauge, 2006). The human Na+:Ca2+ exchangers, when defective, can cause neurodegenerative disorders (Gomez-Villafuertes et al., 2007). Allosteric activation of NCX involves the binding of cytosolic Ca2+ to regulatory domains CBD1 and CBD2 (Giladi and Khananshvili 2013).
The CaCA superfamily is composed of five families: K+-independent Na+/Ca2+ exchangers (NCXs), cation/Ca2+ exchangers (CCXs), YbrG transporters and cation exchangers (CAXs). Phylogenetic and alignment studies indicate that one of the mammalian NCKXs, NCKX6 (2.A.19.4.4), and its related proteins form a unique group, designated cation/Ca2+ exchangers (CCXs) (Cai and Lytton, 2004). Cytoplasmic Ca2+ regulates dimeric Na+/Ca2+ exchangers (NCX) by binding to two adjacent Ca2+-binding domains (CBD1 and CBD2) located in the large intracellular loop between transmembrane segments 5 and 6, and produces structural rearrangements (John et al., 2011). All NCX proteins encoded in the genomes of rice and Arabidopsis have been studied with respect to their phylogeny, domain architecture and expression profiles across different tissues, at various developmental stages and under stress conditions (Singh et al. 2015).
Members of the CaCA family vary in size from 302 amino acyl residues (Methanococcus jannaschii) to 1199 residues (Bos taurus). Even within the animal kingdom, they vary in size from 461 to 1199 residues. The bacterial and archaeal proteins are in general smaller than the eukaryotic proteins (Chung et al., 2001). They have been suggested to traverse the membrane 9 (mammals) or 10 (bacteria) times as α-helical spanners, but some plant homologues (Cax1 and Cax2) exhibit 11 putative TMSs. The E. coli ChaB (YrbG) homologue has been found to have 10 TMSs with both the N- and C-termini localized to the periplasm. Each homologous half of the internally duplicated protein has 5 TMSs with opposite orientation in the membrane (Saaf et al., 2001). This orientation seems to be stabilized by the presence of positively charged residues in the cytoplasmic loops.
The mammalian cardiac muscle homologue probably has 9 TMSs. The N-terminus of this protein is believed to be extracellular, while the C-terminus is intracellular (Iwamoto et al., 1999). A large central loop is not required for transport function and plays a role in regulation. In the preferred 9 TMS model for this mammalian protein, the polypeptide chain loops into the membrane after TMS 2 and after TMS 7. The large central loop separates TMS 5 from TMS 6. TMS 2 and the following loop show sequence similarity to TMS 7 and its loop. TMS 7 may be close to TMSs 2 and 3 in the 3-D structure of the protein (Qui et al., 2001).
The Na+:Ca2+ exchanger plays a central role in cardiac contractility by maintaining Ca2+ homeostasis. Two Ca2+-binding domains, CBD1 and CBD2, located in a large intracellular loop, regulate activity of the exchanger. Ca2+ binding to these regulatory domains activates the transport of Ca2+ across the plasma membrane. The structure of CBD1 shows four Ca2+ ions arranged in a tight planar cluster. The structure of CBD2 in the Ca2+-bound (1.7-Å resolution) and -free (1.4-Å resolution) conformations shows (like CBD1) a classical Ig fold but coordinates only two Ca2+ ions in primary and secondary Ca2+ sites. In the absence of Ca2+, Lys585 stabilizes the structure by coordinating two acidic residues (Asp552 and Glu648), one from each of the Ca2+-binding sites, and prevents protein unfolding (Besserer et al., 2007).
All of the characterized animal proteins catalyze Ca2+:Na+ exchange although some also transport K+. The NCX plasma membrane proteins exchange 3 Na+ for 1 Ca2+ (i.e., 2.A.19.3). Mammalian Na2+/Ca2+ exchangers exist as three isoforms NCX1-3 which are about 70% identical to each other. The NCKX exchangers exchange 1 Ca2+ plus 1 K+ for four Na+ (i.e., 2.A.19.4). The myocyte NCX1.1 splice variant catalyzes Ca2+ extrusion during cardiac relaxation and may catalyze Ca2+ influx during contraction. The E. coli ChaA protein catalyzes Ca2+:H+ antiport but may also catalyze Na+:H+ antiport slowly. All remaining well-characterized members of the family catalyze Ca2+:H+ exchange.
The Na+/Ca2+ exchanger, NCX1 (TC #2.A.19.3.1), is a plasma membrane protein that regulates intracellular Ca2+ levels in cardiac myocytes. Transport activity is regulated by Ca2+, and the primary Ca2+ sensor (CBD1) is located in a large cytoplasmic loop connecting two transmembrane helices. The high-affnity binding of Ca2+ to the CBD1 sensory domain results in conformational changes that stimulate the exchanger to extrude Ca2+. A crystal structure of CBD1 at 2.5Å resolution reveals a novel Ca2+ binding site consisting of four Ca2+ ions arranged in a tight planar cluster. This intricate coordination pattern for a Ca2+ binding cluster is indicative of a highly sensitive Ca2+ sensor and may represent a general platform for Ca2+ sensing Nicoll et al., 2006).
The phylogenetic tree for the CaCA family reveals at least six major branches (Saier et al., 1999). Two clusters consist exclusively of animal proteins, a third contains several bacterial and archaeal proteins, a fourth possesses yeast, plant and blue green bacterial homologues, the fifth contains only the ChaA Ca2+:H+ antiporter of E. coli and the sixth contains only one distant S. cerevisiae homologue of unknown function. Several homologues may be present in a single organism. This fact and the shape of the tree suggest either that isoforms of these proteins arose by gene duplication before the three domains of life split off from each other or that horizontal gene transfer has occurred between these domains (Saier et al., 1999).
Prokaryotic CaCAs contain only the transmembrane domain and is self-sufficient as an active ion transporter, but the eukaryotic NCX proteins possesses in addition, a large intracellular loop that senses intracellular calcium signals and controls the activation of ion transport across the membrane. This provides a necessary layer of regulation for the more complex function. The Ca2+ sensor in the intracellular loop (the Ca2+-binding domain (CBD12)) signals, and allosteric intracellular Ca2+ binding propagates a signal. Another structured domain that is N-terminal to CBD12 in the intracellular loop has two tandem long α-helices, connected by a short linker. It forms a stable cross-over two-helix bundle, resembling an 'awareness ribbon' (Yuan et al. 2018).
Homologues from several cyanobacteria play important roles in salt tolerance (Waditee et al., 2004).
The generalized transport reaction catalyzed by proteins of the CaCA family is:
Ca2+ (in) + [nH+ or nNa+ (out)] ⇌ Ca2+ (out) + [nH+ or nNa+] (in)