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.

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).

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 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).

Homologues from several cyanobacteria have been characterized. They 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).

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Examples: