The various splice variants of the three SERCA-and the two SPCA-pump genes in higher vertebrates encode P-type ATPases of the P 2A group found respectively in the membranes of the endoplasmic reticulum and the secretory pathway. Of these, SERCA2b and SPCA1a represent the housekeeping isoforms. The SERCA2b form is characterized by a luminal carboxy terminus imposing a higher affinity for cytosolic Ca 2þ compared to the other SERCAs. This is mediated by intramembrane and luminal interactions of this extension with the pump. Other known affinity modulators like phospholamban and sarcolipin decrease the affinity for Ca 2þ . The number of proteins reported to interact with SERCA is rapidly growing. Here, we limit the discussion to those for which the interaction site with the ATPase is specified: HAX-1, calumenin, histidine-rich Ca 2þ -binding protein, and indirectly calreticulin, calnexin, and ERp57. The role of the phylogenetically older and structurally simpler SPCAs as transporters of Ca 2þ , but also of Mn 2þ , is also addressed.

Sarco(endo)plasmic reticulum Ca 2؉ ATPase (SERCA) Ca 2؉ transporters pump cytosolic Ca 2؉ into the endoplasmic reticulum, maintaining a Ca 2؉ 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 Ca 2؉ ions. Of all SERCA isoforms, the housekeeping SERCA2b isoform displays the highest Ca 2؉ 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 Ca 2؉ 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 Ca 2؉ -bound E1 conformation and alters Ca 2؉ -transport kinetics, which provides the rationale for the higher apparent Ca 2؉ 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 ␣-and ␤-subunits of Na ؉ ,K ؉ -ATPase. The 2b-tail interaction site may represent a novel target to increase the Ca 2؉ affinity of malfunctioning SERCA2a in the failing heart to improve contractility.endoplasmic reticulum ͉ Ca 2ϩ -ATPase ͉ ion transporter ͉ phospholamban ͉ P-type ATPase C alcium ions in the endoplasmic reticulum (ER) control cellular growth, proliferation, differentiation, and death and are maintained at high concentrations by the ubiquitous ER Ca 2ϩ pump sarco(endo)plasmic reticulum Ca 2ϩ ATPase 2b (SERCA2b) (1). SERCAs cycle between an E1 conformation, with high-affinity Ca 2ϩ -binding sites facing the cytoplasm, and E2 with low-affinity Ca 2ϩ -binding sites facing the lumen of the ER (2, 3). Several 3D crystal structures of SERCA1a, the related fast-twitch skeletal-muscle isoform, provided detailed insights into the mechanism of Ca 2ϩ transport. Conformational changes in the cytosolic domain of the pump are driven by ATP hydrolysis and control the opening and closing of the Ca 2ϩ gates in the transmembrane (TM) domain. The TM region contains 10 TM helices comprising residues that reversibly coordinate Ca 2ϩ for transport (4-7) (reviewed in ref. 3).Unfortunately, much less is known about the structure and mechanism of the ubiquitous SERCA2 isoform (Ϸ84% sequence identity with SERCA1a). Alternative splicing of the SERCA2 messenger yields two variants: SERCA2a, which populates the sarcoplasmic reticulum (SR) of the heart, smooth, and slow-twitch skeletal muscle; and the housekeeping variant SERCA2b, which is present in the ER of all cell types (Fig. S1a) (8). SERCA2b differs from the muscle isoforms SERCA1a or SERCA2a, displaying a 2-fold higher affinity for Ca 2ϩ and lower catalytic turnover rate. These unique functional properties are related to an extended C terminus of 49 residues (2b-tail) containing an 11th TM domain (TM11) and luminal extension (LE) (Fig. S1b) (9-12), b...

Heart failure is the leading cause of death in western countries and is often associated with impaired Ca(2+) handling in the cardiomyocyte. In fact, cardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (ER/SR) Ca(2+) pump SERCA2a, pumping Ca(2+) from the cytosol into the lumen of the ER/SR. This review addresses three important facets that control the SERCA2 activity in the heart. First, we focus on the alternative splicing of the SERCA2 messenger, which is strictly regulated in the developing heart. This splicing controls the formation of three SERCA2 splice variants with different enzymatic properties. Second, we will discuss the role and regulation of SERCA2a activity in the normal and failing heart. The two well-studied Ca(2+) affinity modulators phospholamban and sarcolipin control the activity of SERCA2a within a narrow window. An aberrantly high or low Ca(2+) affinity is often observed in and may even trigger cardiac failure. Correcting SERCA2a activity might therefore constitute a therapeutic approach to improve the contractility of the failing heart. Finally, we address the controversies and unanswered questions of other putative regulators of the cardiac Ca(2+) pump, such as sarcalumenin, HRC, S100A1, Bcl-2, HAX-1, calreticulin, calnexin, ERp57, IRS-1, and -2.

Background:Little is known about the mechanism by which the luminal extension (LE) and 11th transmembrane helix (TM11) of SERCA2b regulate function. Results: Mutant and chimeric SERCA constructs were analyzed kinetically. Conclusion:The LE affects Ca 2ϩ interaction in E1 and E1P, whereas TM11 affects Ca 2ϩ -free E2/E2P states. Significance: Distinct roles of LE and TM11 in controlling the enzyme cycle are revealed.