Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase 1
The contraction and relaxation of skeletal muscle is regulated by the concomitant rise and fall of the cytoplasmic calcium level. The Ca2+ is released from the sarcoplasmic reticulum by ryanodine receptor and taken back to the SR by SERCA pumps. The idea of a relaxing factor had been put forward for a long time by many researchers seeking for control mechanism of muscle function. The successful candidate was found in a high-speed pellet of rabbit white skeletal muscle capable of hydrolyzing ATP in Ca2+- and Mg2+-dependent manner and requiring the presence of phospholipids. The dependence on phospholipids and incompatibility to detergents implicated that self-forming sealed vesicles accumulated calcium in an ATP-dependent manner (MacLennan 1970). Since this pellet contained remnants of transversal tubules and self-sealed vesicles, it was apparent that its major constituent was the sarcoplasmic reticulum. Based on this, the pump that accumulated calcium into the vesicles and required magnesium was named sarcoplasmic reticulum (SR) Ca2+- and Mg2+-dependent ATPase. The first isolation of this protein has been done from white muscle of rabbit by sixfold purification (MacLennan 1970) well demonstrating its high abundance in the SR. Facilitated by the relatively easy isolation and homology with the other members of the Ca2+ pump family, information collected about this pump of rabbit white muscle has pioneered research on similar isoforms. In the next almost 20 years, this pump and its isoforms were still called SR-calcium ATPases since the acronym SERCA was introduced only later when the third gene coding for such organelle pump was described by Burk et al. (1989).
The Structure and Activity Cycle of SERCA1
The major steps of the catalytic cycle (Fig. 1b) involve the transitions between the two major conformation sates E1 and E2. The E1 conformation state binds two Ca2+ with high-affinity Ca2+ binding sites facing the cytoplasm. The E2 conformation state releases Ca2+ when the binding sites are facing the SR lumen and have low affinity. The ATP and two Ca2+ binds to E1 from the cytosolic side. The ATP is hydrolyzed to ADP and the Ca2+ is occluded by the transmembrane helices in the 2Ca2+E1P conformation. This high energy state changes conformation to E2P while releases ADP, then the binding sites face the lumenal space, open and release the two Ca2+ because of low affinity. The E2P hydrolyzes out the inorganic phosphate and the cycle closes with transformation of E2 to E1.
The characteristics of SERCA1 seem to be depending on species of the source since the pump from bovine muscle has lower catalytic activity compared to that from rabbit (Sacchetto et al. 2012). The authors found that this might be explained by the difference in region connecting M7 and M8 that is longer and more protruding into the SR lumen in bovine than in rabbit.
SERCA1 Inhibitors and Regulators
About a hundred exogenous inhibitors of wide range of structure have been identified for SERCA (reviewed by Michelangeli and East (2011)). Among them, probably the thapsigargin and cyclopiazonic acid are the most frequently used ones. These substances helped better understanding the structure and function of SERCA1.
Endogen SERCA1 activity can be regulated by pH, the Ca2+ in the cytoplasm (less in SR lumen), the ATP/ADP ratio, covalent modifications (e.g., phosphorylation, nitrosylation), and hormones (e.g., thyroxin), but most of the knowledge is accumulated about two small endogenous peptide, phospholamban (PLB) and sarcolipin (SLN) that are homologous with each other. The molecular interaction of these peptides with SERCA1a is analyzed in detail and revealed that they bound to different conformations (Sahoo et al. 2013; Winther et al. 2013). However, based on its coexpression in fast-glycolytic skeletal muscle, preferably sarcolipin seems to have primary importance for SERCA1a but both SLN and PLB are coexpressed, for example, in human vastus lateralis (Fajardo et al. 2013) and can be potentially significant for SERCA1b in myotubes and developing fibers. Only SLN can bind in the presence of Ca and uncoupling ATPase activity from pumping and stimulate heat production (Sahoo et al. 2013). Recently, open reading frames have been found in long (previously annotated noncoding) RNAs coding for a whole family of endogenous peptides like myoregulin (MLN) that is expressed in most skeletal muscles and also in myotube formation where it may regulate SERCA1 (Anderson et al. 2015) including SERCA1b activity.
SERCA1 Expression and Function
The role of SERCA1a is to establish a 10,000-fold concentration gradient across the membrane of sarcoplasmic reticulum (Toyoshima 2009). This function is best known in relaxation of fast glycolytic muscle but it has also thermogenic function when upregulated by thyroid hormone in skeletal muscle and brown adipose tissue (Arruda et al. 2008). SERCA1a is expressed specifically in fast-glycolytic muscle fibers (Brandl et al. 1986) but SERCA1b is the dominant isoform in development of both fast-glycolytic and slow-oxidative muscles (Zádor and Kósa 2015). Certain conditions like overload induce the neonatal splicing without the translation of the protein (Zádor and Kósa 2015) as the expression of SERCA1b is controlled pretranslationally in adult muscle and happens fully only in neonatal and developing muscle (Zádor and Kósa 2015). The role of SERCA1 (probably SERCA1b) is found in relaxation and probably important in store-operated calcium entry (SOCE), a process refilling the calcium depleted SR in developing myotubes, similar to other cells using other SERCAs (reviewed in Zádor and Kósa (2015)). The only functional difference of SERCA1b compared to SERCA1a is the lower tolerance to high intraluminal calcium concentration (Zhao et al. 2015). However, the only structural difference, the eight amino acid tail compared to single glycine at the C-terminal is facing the cytoplasm in each conformation and therefore do not seem to be explanatory for this.
Impact in Pathology
Mutation of SERCA1a has been found associated with Brody disease, an extremely rare disorder (1:10,000,000) characterized by muscular stiffness and low tolerance for exercise (Odermatt et al. 1996). The aberrant splicing of SERCA1 (together with that of the insulin receptor, ryanodine receptor) is associated with myotonic dystrophy (DM), a rare disorder of nonregenerating adult muscle (Kimura et al. 2005). However, the translation of the neonatal pump from the SERCA1b mRNA occurs at variance in the type 1 and type 2 DM (Zhao et al. 2015), although this disease do not show apparent signs of regeneration. The Duchenne muscular dystrophy (DMD) is the most frequently inherited muscle disorder characterized by permanent cycling of degeneration and regeneration of muscle fibers. Although the primary genetic defect is in the X-linked dystrophin gene, DMD is characterized by aberrant calcium metabolism and its symptoms can be alleviated by overexpressing SERCA1 in mouse model (mdx) of the disease (Mázala et al. 2015). Similar to the situation in mdx mice, the ration of SERCA1 to other SERCAs can also change in other pathological conditions like diabetes and denervation (reviewed in Zádor and Kósa (2015)).
Novel splice variants were cloned from human liver cDNA libraries in which exon 11 spliced out with or without exon 4 because of the premature stop in exon 12 results in coding truncated isoforms S1T+4 and S1T-4, respectively. Thus, S1T+4 contains 417, while S1T-4 contains 382 amino acid residues – that means they lack a part of the P domain, the entire N domain, and the transmembrane domains M5–M10 in S1T+4 and in addition, the absence of a part M1 and the entire M2 in S1T-4. The contrast between human SERCA1a and the truncated S1T isoforms is also obvious at the level of molecular weight; the full SERCA1a containing 994 amino acid residues is 109 kDa opposing the 46 kDa S1T+4 and 43 kDa and S1T-4. The truncated S1T isoforms lack calcium pump activity but increase Ca2+ leakage from ER. When expressed in immortalized cell culture, S1T proteins colocalize with SERCA1 (and SERCA2b) in ER and behaves consistent with the hypothesis that the homodimers form cation channel. The truncated SERCA1-s are expressed in human adult and fetal liver and kidney (Chami et al. 2001).
SERCA1a is an important player of calcium metabolism in muscle relaxation of fast-glycolytic muscle fibers and of heat generation in skeletal muscle and brown adipose tissue. Its neonatal isoform, SERCA1b also capable of fulfilling similar functions and in addition, it has a not fully discovered role in muscle development and contribution to SOCE. The crystal structures determined cover almost the entire reaction cycle of SERCA1 but there are still “white spots” in its conformation change. The unknown conformation stages might be enlightened by the action of the range of novel peptide inhibitors. The role of SERCA1b tail is also waiting to be described, either by novel conformations or interactions with other proteins. Last but not least, the structure of SERCA1 from not yet analyzed sources (other than the rabbit white muscle) might contribute with valuable details to a more complete understanding.
- Sahoo SK, Shaikh SA, Sopariwala DH, Bal NC, Periasamy M. Sarcolipin protein interaction with sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) is distinct from phospholamban protein, and only sarcolipin can promote uncoupling of the SERCA pump. J Biol Chem. 2013;288:6881–9.PubMedPubMedCentralCrossRefGoogle Scholar