Structural Insight into the Ion-Exchange Mechanism of the Sodium/Calcium Exchanger

Abstract

Sodium/calcium (Na(+)/Ca(2+)) exchangers (NCX) are membrane transporters that play an essential role in maintaining the homeostasis of cytosolic Ca(2+) for cell signaling. We demonstrated the Na(+)/Ca(2+)-exchange function of an NCX from Methanococcus jannaschii (NCX_Mj) and report its 1.9 angstrom crystal structure in an outward-facing conformation. Containing 10 transmembrane helices, the two halves of NCX_Mj share a similar structure with opposite orientation. Four ion-binding sites cluster at the center of the protein: one specific for Ca(2+) and three that likely bind Na(+). Two passageways allow for Na(+) and Ca(2+) access to the central ion-binding sites from the extracellular side. Based on the symmetry of NCX_Mj and its ability to catalyze bidirectional ion-exchange reactions, we propose a structure model for the inward-facing NCX_Mj.

Newly characterized Golgi-localized family of proteins is involved in calcium and pH homeostasis in yeast and human cells

“…In the vicinity of this acidic residue, there are numerous serine and threonine residues, the side chain oxygen of which could help create a hydrophilic microenvironment around the Ca 2+ transport pathway. These motifs also contain glycine and alanine residues that may provide the flexibility needed for potential conformational changes (30,31). All of these features, without exception, are found in Gdt1p/TMEM165, and by analogy, we can call their two typical ExGD(KR)(TS) motifs α-1 and α-2, respectively.…”

Sodium recognition by the Na + /Ca 2+ exchanger in the outward-facing conformation

“…In a recent breakthrough, the NCX exchanger from the archaeobacterium Methanocaldococcus jannaschii was isolated and functionally reconstituted, and its crystal structure determined at 1.9-Å resolution (22). The structure of NCX_Mj revealed an intriguing architecture consisting of two inverted topological repeats of five transmembrane helices each (Fig.…”

“…The sites referred to as S ext and S int appeared to be Na + sites, based on chemical and geometric considerations. The fourth site, S mid , was tentatively interpreted also as a Na + site, although in this case the ion-coordination number and coordination geometry would be atypical (22).…”

“…If one assumes that the exchange of Na + and Ca 2+ mediated by NCX_Mj proceeds according to a "ping-pong" mechanism, in analogy with other NCXs (13), binding of Na + and Ca 2+ ought to be mutually exclusive (22), even if the exchange stoichiometry is 3:1 rather than 4:1. Thus, the detection of concurrent electron density signals for three Na + and one Ca 2+ must reflect the coexistence of protein molecules with either of these two ion configurations in the crystal lattice.…”

Na+/Ca2+exchangers regulate the migration and proliferation of human gastric myofibroblasts

“…The plasma membrane protein NCX is a ubiquitously expressed protein with three isoforms that all can function in forward (also known as Ca 2ϩ exit) mode, or in reverse Ca 2ϩ entry mode, depending on the membrane potential and the chemical gradient of Na ϩ and Ca 2ϩ (25,26).…”