Structural basis for the high Ca2+ affinity of the ubiquitous SERCA2b Ca2+ pump

doi:10.1073/pnas.0906797106

Structural basis for the high Ca²⁺ affinity of the ubiquitous SERCA2b Ca²⁺ pump

^aLaboratory of Ca
²⁺-Transport ATPases, Department of Molecular Cell Biology,
^bDivision of Biochemistry, Molecular, and Structural Biology, Department of Chemistry,
^cLaboratory of Biosignaling and Therapeutics, Department of Molecular Cell Biology, and
^dLaboratory of Medicinal Chemistry, Department of Pharmaceutical Sciences, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium

Edited by David H. MacLennan, University of Toronto, Toronto, ON, Canada, and approved September 21, 2009 (received for review June 19, 2009)

Abstract

Sarco(endo)plasmic reticulum Ca²⁺ ATPase (SERCA) Ca²⁺ transporters pump cytosolic Ca²⁺ into the endoplasmic reticulum, maintaining a Ca²⁺ gradient that controls vital cell functions ranging from proliferation to death. To meet the physiological demand of the cell, SERCA activity is regulated by adjusting the affinity for Ca²⁺ ions. Of all SERCA isoforms, the housekeeping SERCA2b isoform displays the highest Ca²⁺ affinity because of a unique C-terminal extension (2b-tail). Here, an extensive structure–function analysis of SERCA2b mutants and SERCA1a2b chimera revealed how the 2b-tail controls Ca²⁺ affinity. Its transmembrane (TM) segment (TM11) and luminal extension functionally cooperate and interact with TM7/TM10 and luminal loops of SERCA2b, respectively. This stabilizes the Ca²⁺-bound E1 conformation and alters Ca²⁺-transport kinetics, which provides the rationale for the higher apparent Ca²⁺ affinity. Based on our NMR structure of TM11 and guided by mutagenesis results, a structural model was developed for SERCA2b that supports the proposed 2b-tail mechanism and is reminiscent of the interaction between the α- and β-subunits of Na⁺,K⁺-ATPase. The 2b-tail interaction site may represent a novel target to increase the Ca²⁺ affinity of malfunctioning SERCA2a in the failing heart to improve contractility.

Footnotes

¹To whom correspondence should be addressed. E-mail: frank.wuytack{at}med.kuleuven.be
Author contributions: I.V., L.R., F.W., and P.V. designed research; I.V., M.T., M.D.M., H.C., E.L., L.R., and P.V. performed research; I.V., M.T., and P.V. analyzed data; and P.V. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The NMR chemical shifts have been deposited in the BioMagResBank, www.bmrb.wisc.edu (accession no. 20102).
This article contains supporting information online at www.pnas.org/cgi/content/full/0906797106/DCSupplemental.