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Transconformations of the SERCA1 Ca-ATPase: A Normal Mode Study

Nathalie Reuter ^*, Konrad Hinsen  and Jean-Jacques Lacapère ^*

^* U410 INSERM. Faculté de médecine Xavier Bichat, Paris Cédex 18, France; and UPR 4301 CNRS, Centre de Biophysique Moléculaire, Orléans Cédex 2, France

Correspondence: Address reprint requests to N. Reuter, E-mail: reuter{at}bichat.inserm.fr or J.-J. Lacapère, E-mail: lacapere{at}bichat.inserm.fr.

The transport of Ca²⁺ by Ca-ATPase across the sarcoplasmic reticulum membrane is accompanied by several transconformations of the protein. Relying on the already established functional importance of low-frequency modes in dynamics of proteins, we report here a normal mode analysis of the Ca²⁺-ATPase based on the crystallographic structures of the E1Ca₂ and E2TG forms. The lowest-frequency modes reveal that the N and A(+Nter) domains undergo the largest amplitude movements. The dynamical domain analysis performed with the DomainFinder program suggests that they behave as rigid bodies, unlike the highly flexible P domain. We highlight two types of movements of the transmembrane helices: i), a concerted movement around an axis perpendicular to the membrane which "twists open" the lumenal side of the protein and ii), an individual translational and rotational mobility which is of lower amplitude for the helices hosting the calcium binding sites. Among all modes calculated for E1Ca, only three are enough to describe the transition to E2TG; the associated movements involve almost exclusively the A and N domains, reflecting the closure of the cytoplasmic headpiece and high displacement of the L7-8 lumenal loop. Subsequently, we discuss the potential contribution of the remaining low-frequency normal modes to the transconformations occurring within the overall calcium transport cycle.

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