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Ion Pathways in the Sarcoplasmic Reticulum Ca2+-ATPase

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The sarco(endo)plasmic reticulum Ca2+- ATPase (SERCA) is a transmembrane ion transporter belonging to the PII-type ATPases. It performs the vital task of re-sequestering cytoplasmic Ca2+ to the sarco-endoplasmic reticulum store, thereby also terminating Ca2+-induced signaling such as in muscle contraction. This article focuses on the transport pathways of Ca2+ and H+ ions across the lipid bilayer through SERCA. The ion binding sites of SERCA are accessible from either the cytoplasm or the SR/ER lumen at a time, and the Ca2+ entry and exit channels are both formed mainly by rearrangements of four N-terminal transmembrane (TM) alpha-helices. Recent improvements in the resolution of the crystal structures of rabbit SERCA1a have revealed a hydrated pathway in the Cterminal TM region leading from the ion binding sites to the cytosol. A comparison of different SERCA conformations reveals that this C-terminal pathway is exclusive to Ca2+- free E2-states, suggesting that it may play a functional role in proton release from the ion binding sites. This is in agreement with molecular dynamics (MD) simulations, mutational studies, and in striking analogy to a similar pathway recently described for the related sodium pump. We therefore suggest a model for the ion exchange mechanism in PII-ATPases including not only one, but two cytoplasmic pathways working in concert.
OriginalsprogEngelsk
TidsskriftJournal of Biological Chemistry
Vol/bind288
Sider (fra-til)10759-10765
Antal sider7
ISSN0021-9258
DOI
StatusUdgivet - 12 apr. 2013

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