title: Structural role of countertransport revealed in Ca(2+) pump crystal structure in the absence of Ca(2+).
abstract: Ca(2+)-ATPase of sarcoplasmic reticulum is an ATP-powered Ca(2+) pump but also a H(+) pump in the opposite direction with no demonstrated functional role. Here, we report a 2.4-A-resolution crystal structure of the Ca(2+)-ATPase in the absence of Ca(2+) 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 Ca(2+) binding sites with explicit hydrogens derived by continuum electrostatic calculations show how water and protons fill the space and compensate charge imbalance created by Ca(2+)-release. They suggest that H(+) countertransport is a consequence of a requirement for maintaining structural integrity of the empty Ca(2+)-binding sites. For this reason, cation countertransport is probably mandatory for all P-type ATPases and possibly accompanies transport of water as well.
authors.affiliation: Institute of Molecular and Cellular Biosciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Obara
authors.forename: Naoyuki
authors.initials: N
authors.lastname: Miyashita
authors.forename: Cheng
authors.initials: C
authors.lastname: Xu
authors.forename: Itaru
authors.initials: I
authors.lastname: Toyoshima
authors.forename: Yuji
authors.initials: Y
authors.lastname: Sugita
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Inesi
authors.forename: Chikashi
authors.initials: C
authors.lastname: Toyoshima
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000502:physiology; D055592:Biophysical Phenomena; D001703:Biophysics; D019556:COS Cells; D000252:Calcium-Transporting ATPases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure*; D002645:Chickens; D002522:Chlorocebus aethiops; D003001:Cloning, Molecular; D003461:Crystallography; D017136:Ion Transport / Q000502:physiology; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D019800:Phenol; D011522:Protons; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D055672:Static Electricity; D019284:Thapsigargin; D014867:Water / Q000737:chemistry
chemical_list: D011522:Protons; D014867:Water; D019800:Phenol; D019284:Thapsigargin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases