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Probing ion binding sites in the Na+/Ca2+ exchanger
Munekazu Shigekawa1, Takahiro Iwamoto, Akira Uehara
1Department of Molecular Physiology, National Cardiovascular Center Research Institute, Suita, Osaka 565-8565, Japan. shigekaw@ri.ncvc.go.jp
Annals of the New York Academy of Sciences
|December 28, 2002
Summary
The Na(+)/Ca(2+) exchanger (NCX) membrane domain has alpha repeats forming ion pathways. Mutations in these repeats affect ion transport, highlighting their role in ion selection and translocation.
Area of Science:
- Biochemistry
- Molecular Biology
- Membrane Transport
Background:
- The Na(+)/Ca(2+) exchanger (NCX) is crucial for cellular calcium homeostasis.
- Its membrane domain contains conserved alpha-1 and alpha-2 repeat sequences.
- Understanding the structure-function relationship of these repeats is key to NCX mechanism.
Purpose of the Study:
- To determine the topological arrangement of residues within the NCX alpha repeats.
- To investigate the functional significance of these residues in ion transport.
- To elucidate the role of alpha repeats in forming the ion translocation pathway.
Main Methods:
- Substituted cysteine accessibility scanning to probe residue topology.
- Kinetic analysis of transport activity in site-directed NCX1 mutants.
- Evaluation of ion and drug interactions with mutated NCX1 variants.
Main Results:
- The alpha-1 repeat forms an extracellular reentrant loop, while the alpha-2 repeat forms a complex cytoplasmic reentrant loop.
- Mutations in the alpha-1 loop alter interactions with Ca(2+)(o), Ni(2+), and Co(2+).
- Mutations in the alpha-2 loop affect interactions with Li(+), KB-R7943, Ca(2+)(o), and Ni(2+).
Conclusions:
- The alpha-repeat regions are integral to the NCX ion translocation pathway.
- The alpha-1 repeat loop is critical for selective ion binding and transport.
- These findings provide insights into the molecular mechanism of Na(+)/Ca(2+) exchange.