The Mechanism of Ca2+ Transport by Sarco(Endo)plasmic Reticulum Ca2+-ATPases

Abstract

Function Ca2ϩ pumps, together with Ca 2ϩ release channels, form ubiquitous Ca 2ϩ regulatory systems in muscle and non-muscle cells. The sarco(endo)plasmic reticulum Ca 2ϩ -ATPases (SERCA) 1 and the plasma membrane Ca 2ϩ -ATPases have the highest affinity for Ca 2ϩ removal from the cytoplasm and, together, set resting cytoplasmic Ca 2ϩ concentrations. Three differentially expressed genes encode SERCA proteins (1). SERCA1a and -1b are expressed in fast-twitch skeletal muscle, but loss of SERCA1 function in Brody disease is sufficiently compensated to preserve life (2). SERCA2a is the cardiac/slow-twitch isoform, whereas SERCA2b, with a C-terminal extension, is expressed in smooth muscle and non-muscle tissues. It is almost certainly an essential gene. SERCA3 is expressed in a limited set of non-muscle tissues, including endothelial, epithelial, and lymphocytic cells and platelets, and its knockout is not lethal (3).SERCA enzymes are typical of the class of P-type ATPases, which form a phosphoprotein intermediate and undergo conformational changes during the course of ATP hydrolysis (4, 5). Some of the conformational states can be stabilized, either by adjustment of reaction conditions or through mutagenesis, and characterized as intermediates in the overall reaction cycle (Fig. 1A). The phosphorylated intermediate, E 1 P(Ca) 2 , can phosphorylate ADP, whereas E 2 P can only react with water. The formation of E 1 P requires that two high affinity Ca 2ϩ binding sites be occupied. The enzyme is then phosphorylated by ATP and, concomitantly, the two Ca 2ϩ ions are occluded and can no longer exchange with cytoplasmic Ca 2ϩ . The rate-limiting transition to E 2 P is accompanied by loss of Ca 2ϩ into the lumen, the affinity having fallen by 3 orders of magnitude. Hydrolysis of E 2 P and regeneration of the high affinity Ca 2ϩ binding sites (E 1 (Ca) 2 ) complete the reversible cycle. High lumenal Ca 2ϩ drives the formation of E 1 P from phosphate (P i ), and its effect on the level of E 1 P led Jencks (5, 6) to postulate a second set of Ca 2ϩ

Impaired Intracellular Calcium Mobilization and NFATc1 Availability in Tolerant Anti-Insulin B Cells

“…In the absence of Ca 2ϩ , the ATPase activity of SERCA is low, leading to a slow leak of Ca 2ϩ from intracellular stores (33,34). In addition, inhibition of SERCA activity by the drug thapsigargin results in an increase in [Ca 2ϩ ] i that reflects Ca 2ϩ release from ER stores.…”

Protective effects of Hsp70 on the structure and function of SERCA2a expressed in HEK-293 cells during heat stress

“…SARCO(ENDO)PLASMIC reticulum Ca 2ϩ -ATPases (SERCAs) are typical of the class of P-type ATPases that bind specific ions to be transported, hydrolyze ATP to form a phosphoprotein intermediate, and undergo conformational changes resulting in ion translocation (29). SERCA pumps are 110-kDa integral membrane proteins that consist of 10 transmembrane helexes (M1-M10), three cytoplasmic domains (A, actuator; N, nucleotide-binding; P, phosphorylation), and small luminal loops (48).…”

Mass spectrometry in aging research

“…SERCA plays a major role in muscle relaxation through the ATP-dependent transport of cytosolic calcium into the lumen of the sarcoplasmic reticulum (Møller, Juul, & le Maire, 1996;MacLennan, Rice, & Green, 1997). In general, calcium transport into the SR or ER constitutes an important part in the maintenance of intracellular calcium homeostasis together with calcium transport across the plasma membrane and into the mitochondria (Trump & Berezesky, 1995).…”

“…The ca. 110 kDa SERCA is classified as a P-type ATPase, which forms a covalent aspartyl-phosphoryl enzyme intermediate (at Asp 351 ) during the calcium transport cycle through g-phosphoryl transfer from ATP (Møller, Juul, & le Maire, 1996;MacLennan, Rice, & Green, 1997). It is expressed in three major isoforms: the fast-twitch skeletal isoform (SERCA1), the cardiac/slow-twitch isoform (SERCA2a), and its alternatively spliced gene product, the smooth/non-muscle isoform (SERCA2b), and an additional nonmuscle isoform (SERCA3) (Lytton et al, 1992;Wu & Lytton, 1993).…”