Na⁺/Ca²⁺ exchangers: three mammalian gene families control Ca²⁺ transport

The experimentally supported model for NCX1 has nine proposed transmembrane-spanning segments (TMSs), a cleaved ‘signal peptide’ (TMS0, with the putative signal peptide cleavage site shown as SPase?) and re-entrant loops, one of which may be partially helical, as part of the conserved α-repeat motifs (dark blue, labelled α1 and α2). Critical amino acids in the α-repeat regions are highlighted in red (Glu¹¹³, Asn¹⁴³, Asp⁸¹⁴ and Asn⁸⁴²). The single N-linked glycosylation site is shown in green (N-CHO). Highlighted regions of the cytosolic loop include the XIP, the two CBDs (CBD1 and CBD2) and the region subject to alternative splicing. Proposed sites involved in regulatory Ca²⁺ binding are also shown.

Cartoon representations of the two CBD structures [59,60] (CBD1 is green, and CBD2 is magenta, with the region subject to alternative splicing, Thr⁵⁷⁰–Ala⁶⁰⁹, in yellow) are shown juxtaposed as they may pack in the native protein. The membrane plane lies at the top, orthogonal to the orientation of the Figure, connected to the CBDs via their labelled N- and C-terminal ends. The orientation is similar to that represented in Figure 2. Red spheres indicate the location of bound Ca²⁺ ions, and the two key amino acids that define the difference in properties between the mutually exclusive alternatively spliced exons A and B are shown as stick representations in blue (Asp⁵⁷⁸ and Lys⁵⁸⁵). Figure prepared with the PDB files 2FWS, 2FWU and 2DPK using PyMOL software (http://www.pymol.org). The 2FWS and 2FWU structures, as well as the location of the four Ca²⁺ ions in CBD1 from 2DPK, were used to prepare a model illustrating a possible packing arrangement, according to [59]. Note that, although based on structural data, this Figure represents a possible composite model and not an actual structure. An interactive three-dimensional version of this Figure is available at http://www.BiochemJ.org/bj/406/0365/bj4060365add1.htm.

(A) The NCX1 cycle. The empty exchanger is shown to bind either one Ca²⁺ ion or three Na⁺ ions, in a mutually exclusive manner. Either binding event allows a conformational change that reorients the accessibility of the ion-binding site across the membrane barrier. Forward-mode Ca²⁺ extrusion in exchange for Na⁺ entry occurs by sequential steps in a clockwise direction. Conversely, reverse-mode Ca²⁺ influx in exchange for Na⁺ exit proceeds around the cycle in an anti-clockwise direction. (B) The NCKX cycle. As in (A), except the empty transporter binds either one Ca²⁺ ion plus one K⁺ ion or four Na⁺ ions. The binding of Ca²⁺ and K⁺ is shown ordered based on unpublished work (F. Visser and J. Lytton) and because kinetic considerations under normal physiological ion concentrations suggest that the cycle is likely to be limited by Ca²⁺ binding to an exchanger with a cytoplasmically oriented site primed with bound K⁺. An animated version of this Figure is available at http://www.BiochemJ.org/bj/406/0365/bj4060365add2.htm.