Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Sarcolipin

  • Sushant Singh
  • Sanjaya K. Sahoo
  • Muthu Periasamy
Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101787

Synonyms

 SLN

Historical Background

Sarcolipin (SLN) was first identified by David MacLennan in 1974 as a “proteolipid” in purified rabbit skeletal muscle sarcoplasmic reticulum (SR) preparations (MacLennan 1974). In 1992, Wawrzynow et al. determined its sequence, confirmed its molecular weight as 3,733 Da, and named it sarcolipin (SLN) with reference to its nature and origin (Wawrzynow et al. 1992). SLN protein sequence is highly conserved from fish to man. For nearly three decades, the exact function of SLN remained unknown. Early studies suggested that it is very similar to phospholamban (PLB), an important regulator of the SERCA pump in cardiac muscle (Fig. 1). Using reconstituted synthetic SLN and SERCA, Lee et al. first proposed the idea that SLN binding to SERCA could promote uncoupling of SERCA ATP hydrolysis from Ca2+ transport (Mall et al. 2006). More recent studies from our laboratory have shown that SLN binding to SERCA causes uncoupling of SR Ca2+ transport from ATP hydrolysis, and SLN is involved in muscle thermogenesis and metabolism.
Sarcolipin, Fig. 1

Mouse SLN and PLB sequence comparison. Amino acid analysis between SLN and PLB in mouse. SLN is a 31 amino acids long protein with a 7 amino acid cytosolic region and a 5 amino acid C-terminus. In comparison, PLB is 52 amino acids long with an extended cytosolic domain of 30 amino acid residues. SLN and PLB differ in their respective N-terminal and C-terminal amino acid sequences; however, they are conserved in the transmembrane region. It is important to note that the major differences between PLB and SLN occur at their respective N- and C-terminuse ends

SLN Expression in Cardiac and Skeletal Muscle

SLN is primarily expressed in striated muscle, and its expression is developmentally regulated in cardiac and skeletal muscle tissues. In rodents, SLN is expressed at high level in the embryonic and neonatal skeletal muscle but decreases in adult fast-twitch glycolytic skeletal muscle but continues to be expressed abundantly in slow-twitch oxidative muscle (Fig. 2). Interestingly, SLN expression is abundant in the atria compared to the ventricle in cardiac muscle, and its expression is altered in various disease states (Babu et al. 2007b).
Sarcolipin, Fig. 2

SLN expression pattern in different muscle. Western blot analysis of sarcolipin protein, showing differential expression pattern in muscle tissue. The α-actin was used as a loading control

Structural Features of SLN

SLN is encoded by a single gene, composed of two exons and the gene structure is highly conserved. SLN is distinct from PLB; it is composed of 31 amino acids with unique N- and C-terminal sequence (MacLennan and Kranias 2003; Sahoo et al. 2013; Shaikh et al. 2016). Compared to PLB, SLN has a short N-terminal region composed of aa 1–7, whereas residues 27–31 form the unique C-terminus that protrudes inside the lumen of the SR. The transmembrane (TM) residues 8–26 form the α-helical region and reside in the SR membrane. Mutagenesis studies have shown that the C-terminal tyrosine residues are critically important for localization and interaction of SLN with SERCA. Gramolini et al. have also shown that tyrosine residues in the C-terminal region are essential for the stability and retention of SLN in the ER membrane, in the absence of SERCA (Gramolini et al. 2004). The N-terminus of SLN is comparatively short with seven residues and is less conserved across species (Shaikh et al. 2016) (Fig. 3).
Sarcolipin, Fig. 3

SLN sequence comparison across different species. The protein sequence of SLN is highly conserved across different species, from fish to man. Protein sequence analysis across different species reveals its highly conserved C-terminus region, i.e., RSYQY, as well as the TM region

SLN Binding to SERCA Promotes Uncoupling of SERCA from Ca2+ Transport

The mechanism of SLN binding and interaction with SERCA is still an emerging area of research. SLN interaction with SERCA has been recently studied using in vitro functional assays. Studies from Tony Lee’s group first suggested SLN binding to SERCA could promote uncoupling of SERCA ATP hydrolysis from its Ca2+ transport function (Mall et al. 2006). They showed that at saturating protein ratios of SLN/SERCA, SLN did not affect ATP hydrolysis but decreased Ca2+ uptake. Interestingly, the heat released by SERCA is increased in the presence of SLN. Based on this, it was suggested that SLN binding to SERCA prevents the release of Ca2+ into the lumen and promotes slippage of Ca2+ back to the cytosol. The energy released from the resulting ATP hydrolysis would thus be released as heat, without any Ca2+ transport. These initial in vitro studies predicted that SLN could play a role in muscle thermogenesis and temperature homeostasis.

Recent studies from our laboratory have shown that SLN binds to SERCA even at high Ca2+ (Sahoo et al. 2013, 2015; Toyoshima et al. 2013; Winther et al. 2013; Asahi et al. 2002, 2003). The ability of SLN to bind to different SERCA kinetic state intermediates showed that SLN can remain bound to SERCA throughout the kinetic cycle. This is distinct from PLB which binds only to Ca2+ free SERCA states (i.e., E2 form of SERCA) and is unable to bind with phospho-intermediates of SERCA. This was one of the important findings that highlights how SLN/SERCA interaction differs from PLB (Sahoo et al. 2013). We further showed that SLN decreases the Vmax of SERCA Ca2+-uptake without affecting the SERCA affinity for Ca2+. However, the total amount of ATP hydrolyzed was not affected in the presence of SLN (Fig. 4). These results suggest that the presence of SLN uncouples the hydrolysis of ATP from the Ca2+uptake (Sahoo et al. 2013, 2015). Two independent groups led by Nissen and Toyoshima have been successful in generating SERCA /SLN cocrystals; these crystals showed that SLN is localized to the TM groove on SERCA further confirming our SLN/SERCA interaction studies (Sahoo et al. 2013; Toyoshima et al. 2013; Winther et al. 2013).
Sarcolipin, Fig. 4

SLN uncouples SERCA pump. (a) SERCA ATPase activity is not changed between SERCA with and without SLN. (b) SERCA calcium ATPase activity Vmax is decreased in the presence of SLN in comparison to SERCA alone

SLN Plays a Role in Muscle Thermogenesis and Metabolism

The physiological relevance of SLN in muscle was studied using genetically manipulated mice (Bal et al. 2012; Maurya et al. 2015). Ablation of SLN does not affect muscle growth and/or function since the WT and null could not be distinguished from each other (Babu et al. 2007a). However, when these SLN ablated mice were challenged with acute cold, they failed to maintain their body temperature, whereas PLB-null mice was able to maintain their body temperature (Bal et al. 2012). Transgenic reintroduction of SLN in the null background restored body temperature (Bal et al. 2012). This confirmed that SLN plays an essential role in muscle thermogenesis. In addition, SLN null mice gained more body weight when challenged with high-fat diet in comparison to WT mice. However, skeletal muscle-specific overexpression of SLN in mice can resist high-fat diet induced obesity and enhanced basal metabolic rate (Maurya et al. 2015). Collectively, these studies have shown that SLN is an important regulator of muscle thermogenesis and basal metabolic rate.

Summary

SLN is a 31-aa SR membrane protein, that colocalizes with the SERCA pump. It is primarily expressed in cardiac and skeletal muscle tissues and its expression is regulated in a developmental and tissue-specific manner. SLN binds to the SERCA pump in a Ca2+ sensitive manner but interacts with SERCA even at high Ca2+. Interestingly, SLN binding does not inhibit ATP hydrolysis but promotes uncoupling of SERCA from Ca2+ transport resulting in futile cycling. By this mechanism, SLN can increase ATP hydrolysis and heat production. Current data suggest that SLN plays an important role in muscle thermogenesis and metabolism. These and other studies together suggest that SLN is a novel target to enhance energy expenditure in muscle and control metabolism.

References

  1. Asahi M, Kurzydlowski K, Tada M, MacLennan DH. Sarcolipin inhibits polymerization of phospholamban to induce superinhibition of sarco (endo) plasmic reticulum Ca2+-ATPases (SERCAs). J Biol Chem. 2002;277(30):26725–8.PubMedCrossRefGoogle Scholar
  2. Asahi M, Sugita Y, Kurzydlowski K, De Leon S, Tada M, Toyoshima C, et al. Sarcolipin regulates sarco (endo) plasmic reticulum Ca2+-ATPase (SERCA) by binding to transmembrane helices alone or in association with phospholamban. Proc Natl Acad Sci USA. 2003;100(9):5040–5.PubMedPubMedCentralCrossRefGoogle Scholar
  3. Babu GJ, Bhupathy P, Carnes CA, Billman GE, Periasamy M. Differential expression of sarcolipin protein during muscle development and cardiac pathophysiology. J Mol Cell Cardiol. 2007a;43(2):215–22.PubMedPubMedCentralCrossRefGoogle Scholar
  4. Babu GJ, Bhupathy P, Timofeyev V, Petrashevskaya NN, Reiser PJ, Chiamvimonvat N, et al. Ablation of sarcolipin enhances sarcoplasmic reticulum calcium transport and atrial contractility. Proc Natl Acad Sci USA. 2007b;104(45):17867–72.PubMedPubMedCentralCrossRefGoogle Scholar
  5. Bal NC, Maurya SK, Sopariwala DH, Sahoo SK, Gupta SC, Shaikh SA, et al. Sarcolipin is a newly identified regulator of muscle-based thermogenesis in mammals. Nat Med. 2012;18(10):1575–9.PubMedPubMedCentralCrossRefGoogle Scholar
  6. Gramolini AO, Kislinger T, Asahi M, Li W, Emili A, MacLennan DH. Sarcolipin retention in the endoplasmic reticulum depends on its C-terminal RSYQY sequence and its interaction with sarco (endo) plasmic Ca2+-ATPases. Proc Natl Acad Sci USA. 2004;101(48):16807–12.PubMedPubMedCentralCrossRefGoogle Scholar
  7. MacLennan DH. Isolation of proteins of the sarcoplasmic reticulum. Methods Enzymol. 1974;32:291.PubMedCrossRefGoogle Scholar
  8. MacLennan DH, Kranias EG. Phospholamban: a crucial regulator of cardiac contractility. Nat Rev Mol Cell Biol. 2003;4(7):566–77.PubMedCrossRefGoogle Scholar
  9. Mall S, Broadbridge R, Harrison SL, Gore MG, Lee AG, East JM. The presence of sarcolipin results in increased heat production by Ca2+-ATPase. J Biol Chem. 2006;281(48):36597–602.PubMedCrossRefGoogle Scholar
  10. Maurya SK, Bal NC, Sopariwala DH, Pant M, Rowland LA, Shaikh SA, et al. Sarcolipin is a key determinant of the basal metabolic rate, and its overexpression enhances energy expenditure and resistance against diet-induced obesity. J Biol Chem. 2015;290(17):10840–9.PubMedPubMedCentralCrossRefGoogle Scholar
  11. 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(10):6881–9.PubMedPubMedCentralCrossRefGoogle Scholar
  12. Sahoo SK, Shaikh SA, Sopariwala DH, Bal NC, Bruhn DS, Kopec W, et al. The N terminus of sarcolipin plays an important role in uncoupling sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) ATP hydrolysis from Ca2+ transport. J Biol Chem. 2015;290(22):14057–67.PubMedPubMedCentralCrossRefGoogle Scholar
  13. Shaikh SA, Sahoo SK, Periasamy M. Phospholamban and sarcolipin: are they functionally redundant or distinct regulators of the sarco (endo) plasmic reticulum calcium ATPase? J Mol Cell Cardiol. 2016;91:81–91.PubMedCrossRefGoogle Scholar
  14. Toyoshima C, Iwasawa S, Ogawa H, Hirata A, Tsueda J, Inesi G. Crystal structures of the calcium pump and sarcolipin in the Mg2+-bound E1 state. Nature. 2013;495(7440):260–4.PubMedCrossRefGoogle Scholar
  15. Wawrzynow A, Theibert JL, Murphy C, Jona I, Martonosi A, Collins JH. Sarcolipin, the “proteolipid” of skeletal muscle sarcoplasmic reticulum, is a unique, amphipathic, 31-residue peptide. Arch Biochem Biophys. 1992;298(2):620–3.PubMedCrossRefGoogle Scholar
  16. Winther A-ML, Bublitz M, Karlsen JL, Møller JV, Hansen JB, Nissen P, et al. The sarcolipin-bound calcium pump stabilizes calcium sites exposed to the cytoplasm. Nature. 2013;495(7440):265–9.PubMedCrossRefGoogle Scholar

Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Sushant Singh
    • 1
  • Sanjaya K. Sahoo
    • 1
  • Muthu Periasamy
    • 1
  1. 1.Center for Metabolic Origins of Disease, Cardiovascular Metabolism ProgramSanford Burnham Prebys Medical Discovery InstituteOrlandoUSA