Open AccessStructural role of countertransport revealed in Ca2+pump crystal structure in the absence of Ca2+
Author(s) -
Koji Obara,
Naoyuki Miyashita,
Cheng Xu,
Itaru Toyoshima,
Yuji Sugita,
Giuseppe Inesi,
Chikashi Toyoshima
Publication year - 2005
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.0506222102
Subject(s) - chemistry , crystallography , crystal structure , thapsigargin , p type atpase , atpase , transmembrane protein , endoplasmic reticulum , binding site , biophysics , biochemistry , enzyme , biology , receptor
Ca2+ -ATPase of sarcoplasmic reticulum is an ATP-powered Ca2+ pump but also a H+ pump in the opposite direction with no demonstrated functional role. Here, we report a 2.4-Å-resolution crystal structure of the Ca2+ -ATPase in the absence of Ca2+ stabilized by two inhibitors, dibutyldihydroxybenzene, which bridges two transmembrane helices, and thapsigargin, also bound in the membrane region. Now visualized are water and several phospholipid molecules, one of which occupies a cleft between two transmembrane helices. Atomic models of the Ca2+ binding sites with explicit hydrogens derived by continuum electrostatic calculations show how water and protons fill the space and compensate charge imbalance created by Ca2+ -release. They suggest that H+ countertransport is a consequence of a requirement for maintaining structural integrity of the empty Ca2+ -binding sites. For this reason, cation countertransport is probably mandatory for all P-type ATPases and possibly accompanies transport of water as well.