A calcium pump made visible

Phospholamban and sarcolipin: Are they functionally redundant or distinct regulators of the Sarco(Endo)Plasmic Reticulum Calcium ATPase?

In muscle, the Sarco(Endo)plasmic Reticulum Calcium ATPase (SERCA) activity is regulated by two distinct proteins, PLB and SLN, which are highly conserved throughout vertebrate evolution. PLB is predominantly expressed in the cardiac muscle, while SLN is abundant in skeletal muscle. SLN is also found in the cardiac atria and to a lesser extent in the ventricle. PLB regulation of SERCA is central to cardiac function, both at rest and during extreme physiological demand. Compared to PLB, the physiological relevance of SLN remained a mystery until recently and some even thought it was redundant in function. Studies on SLN suggest that it is an uncoupler of the SERCA pump activity and can increase ATP hydrolysis resulting in heat production. Using genetically engineered mouse models for SLN and PLB, we showed that SLN, not PLB, is required for muscle-based thermogenesis. However, the mechanism of how SLN binding to SERCA results in uncoupling SERCA Ca(2+) transport from its ATPase activity remains unclear. In this review, we discuss recent advances in understanding how PLB and SLN differ in their interaction with SERCA. We will also explore whether structural differences in the cytosolic domain of PLB and SLN are the basis for their unique function and physiological roles in cardiac and skeletal muscle.

The role of skeletal-muscle-based thermogenic mechanisms in vertebrate endothermy

Thermogenesis is one of the most important homeostatic mechanisms that evolved during vertebrate evolution. Despite its importance for the survival of the organism, the mechanistic details behind various thermogenic processes remain incompletely understood. Although heat production from muscle has long been recognized as a thermogenic mechanism, whether muscle can produce heat independently of contraction remains controversial. Studies in birds and mammals suggest that skeletal muscle can be an important site of non-shivering thermogenesis (NST) and can be recruited during cold adaptation, although unequivocal evidence is lacking. Much research on thermogenesis during the last two decades has been focused on brown adipose tissue (BAT). These studies clearly implicate BAT as an important site of NST in mammals, in particular in newborns and rodents. However, BAT is either absent, as in birds and pigs, or is only a minor component, as in adult large mammals including humans, bringing into question the BAT-centric view of thermogenesis. This review focuses on the evolution and emergence of various thermogenic mechanisms in vertebrates from fish to man. A careful analysis of the existing data reveals that muscle was the earliest facultative thermogenic organ to emerge in vertebrates, long before the appearance of BAT in eutherian mammals. Additionally, these studies suggest that muscle-based thermogenesis is the dominant mechanism of heat production in many species including birds, marsupials, and certain mammals where BAT-mediated thermogenesis is absent or limited. We discuss the relevance of our recent findings showing that uncoupling of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) by sarcolipin (SLN), resulting in futile cycling and increased heat production, could be the basis for NST in skeletal muscle. The overall goal of this review is to highlight the role of skeletal muscle as a thermogenic organ and provide a balanced view of thermogenesis in vertebrates.

S-palmitoylation and s-oleoylation of rabbit and pig sarcolipin

Sarcolipin (SLN) is a regulatory peptide present in sarcoplasmic reticulum (SR) from skeletal muscle of animals. We find that native rabbit SLN is modified by a fatty acid anchor on Cys-9 with a palmitic acid in about 60% and, surprisingly, an oleic acid in the remaining 40%. SLN used for co-crystallization with SERCA1a (Winther, A. M., Bublitz, M., Karlsen, J. L., Moller, J. V., Hansen, J. B., Nissen, P., and Buch-Pedersen, M. J. (2013) Nature 495, 265-2691; Ref. 1) is also palmitoylated/oleoylated, but is not visible in crystal structures, probably due to disorder. Treatment with 1 m hydroxylamine for 1 h removes the fatty acids from a majority of the SLN pool. This treatment did not modify the SERCA1a affinity for Ca(2+) but increased the Ca(2+)-dependent ATPase activity of SR membranes indicating that the S-acylation of SLN or of other proteins is required for this effect on SERCA1a. Pig SLN is also fully palmitoylated/oleoylated on its Cys-9 residue, but in a reverse ratio of about 40/60. An alignment of 67 SLN sequences from the protein databases shows that 19 of them contain a cysteine and the rest a phenylalanine at position 9. Based on a cladogram, we postulate that the mutation from phenylalanine to cysteine in some species is the result of an evolutionary convergence. We suggest that, besides phosphorylation, S-acylation/deacylation also regulates SLN activity.

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

Sarco(endo)plasmic reticulum Ca(2+)ATPase (SERCA) pump activity is modulated by phospholamban (PLB) and sarcolipin (SLN) in cardiac and skeletal muscle. Recent data suggest that SLN could play a role in muscle thermogenesis by promoting uncoupling of the SERCA pump (Lee, A.G. (2002) Curr. Opin. Struct. Biol. 12, 547-554 and Bal, N. C., Maurya, S. K., Sopariwala, D. H., Sahoo, S. K., Gupta, S. C., Shaikh, S. A., Pant, M., Rowland, L. A., Bombardier, E., Goonasekera, S. A., Tupling, A. R., Molkentin, J. D., and Periasamy, M. (2012) Nat. Med. 18, 1575-1579), but the mechanistic details are unknown. To better define how binding of SLN to SERCA promotes uncoupling of SERCA, we compared SLN and SERCA1 interaction with that of PLB in detail. The homo-bifunctional cross-linker (1,6-bismaleimidohexane) was employed to detect dynamic protein interaction during the SERCA cycle. Our studies reveal that SLN differs significantly from PLB: 1) SLN primarily affects the Vmax of SERCA-mediated Ca(2+) uptake but not the pump affinity for Ca(2+); 2) SLN can bind to SERCA in the presence of high Ca(2+), but PLB can only interact to the ATP-bound Ca(2+)-free E2 state; and 3) unlike PLB, SLN interacts with SERCA throughout the kinetic cycle and promotes uncoupling of the SERCA pump. Using SERCA transmembrane mutants, we additionally show that PLB and SLN can bind to the same groove but interact with a different set of residues on SERCA. These data collectively suggest that SLN is functionally distinct from PLB; its ability to interact with SERCA in the presence of Ca(2+) causes uncoupling of the SERCA pump and increased heat production.

A model for modulation of neuronal synchronization by D4 dopamine receptor-mediated phospholipid methylation

We describe a new molecular mechanism of dopamine-induced membrane protein modulation that can tune neuronal oscillation frequency to attention-related gamma rhythm. This mechanism is based on the unique ability of D4 dopamine receptors (D4R) to carry out phospholipid methylation (PLM) that may affect the kinetics of ion channels. We show that by deceasing the inertia of the delayed rectifier potassium channel, a transition to 40 Hz oscillations can be achieved. Decreased potassium channel inertia shortens spike duration and decreases the interspike interval via its influence on the calcium-dependent potassium current. This mechanism leads to a transition to attention-related gamma oscillations in a pyramidal cell-interneuron network. The higher frequency and better synchronization is observed with PLM affecting pyramidal neurons only, and recurrent excitation between pyramidal neurons is important for synchronization. Thus dopamine-stimulated methylation of membrane phospholipids may be an important mechanism for modulating firing activity, while impaired methylation can contribute to disorders of attention.

Inhibition of plasma membrane Ca2+-ATPase by heparin is modulated by potassium

Heparin is related to several protein receptors that control Ca2+ homeostasis. Here, we studied the effects of heparin on the plasma membrane Ca2+-ATPase from erythrocytes. Both ATP hydrolysis and Ca2+ uptake were inhibited by heparin without modification of the steady-state level of phosphoenzyme formed by ATP. Calmodulin did neither modify the inhibition nor the binding of heparin. Inhibition by heparin was counteracted by K+ but not by Li+. This effect was extended to other sulfated polysaccharides with high number of sulfate residues. Hydrolysis of p-nitrophenylphosphate was equally inhibited by heparin. No evidence for enzyme uncoupling was observed: Ca2+ uptake and ATP hydrolysis remained tightly associated at any level of heparin, and heparin did not increase the passive Ca2+ efflux of inside-out vesicles. Vanadate blocked this efflux, indicating that the main point of Ca2+ escape from these vesicles was linked to the Ca2+ pump. It is discussed that sulfated polysaccharides may physiologically increase the steady-state level of Ca2+ in the cytosol by inhibiting the Ca2+ pumps in a K+ (and tissue) regulated way. It is suggested that heparin regulates the plasma membrane Ca2+-ATPase by binding to the E2 conformer.

Self-association of isolated large cytoplasmic domain of plasma membrane H+ -ATPase from Saccharomyces cerevisiae: role of the phosphorylation domain in a general dimeric model for P-ATPases

Large cytoplasmic domain (LCD) plasma membrane H+ -ATPase from S. cerevisiae was expressed as two fusion polypeptides in E. coli: a DNA sequence coding for Leu353-Ileu674 (LCDh), comprising both nucleotide (N) and phosphorylation (P) domains, and a DNA sequence coding for Leu353-Thr543 (LCDDeltah, lacking the C-terminus of P domain), were inserted in expression vectors pDEST-17, yielding the respective recombinant plasmids. Overexpressed fusion polypeptides were solubilized with 6 M urea and purified on affinity columns, and urea was removed by dialysis. Their predicted secondary structure contents were confirmed by CD spectra. In addition, both recombinant polypeptides exhibited high-affinity 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP) binding (Kd = 1.9 microM and 2.9 microM for LCDh and LCDDeltah, respectively), suggesting that they have native-like folding. The gel filtration profile (HPLC) of purified LCDh showed two main peaks, with molecular weights of 95 kDa and 39 kDa, compatible with dimeric and monomeric forms, respectively. However, a single elution peak was observed for purified LCDDeltah, with an estimated molecular weight of 29 kDa, as expected for a monomer. Together, these data suggest that LCDh exist in monomer-dimer equilibrium, and that the C-terminus of P domain is necessary for self-association. We propose that such association is due to interaction between vicinal P domains, which may be of functional relevance for H+ -ATPase in native membranes. We discuss a general dimeric model for P-ATPases with interacting P domains, based on published crystallography and cryo-electron microscopy evidence.

Origins of cross-orientation suppression in the visual cortex

The response of a neuron in striate cortex to an optimally oriented stimulus is suppressed by a superimposed orthogonal stimulus. The neural mechanism underlying this cross-orientation suppression (COS) may arise from intracortical or subcortical processes or from both. Recent studies of the temporal frequency and adaptation properties of COS suggest that depression at thalamo-cortical synapses may be the principal mechanism. To examine the possible role of synaptic depression in relation to COS, we measured the recovery time course of COS. We find it too rapid to be explained by synaptic depression. We also studied potential subcortical processes by measuring single cell contrast response functions for a population of LGN neurons. In general, contrast saturation is a consistent property of LGN neurons. Combined with rectifying nonlinearities in the LGN and spike threshold nonlinearities in visual cortex, contrast saturation in the LGN can account for most of the COS that is observed in the visual cortex.

Defining the intramembrane binding mechanism of sarcolipin to calcium ATPase using solution NMR spectroscopy

Sarcolipin (SLN) is an integral membrane protein that is expressed in both skeletal and cardiac muscle, where it inhibits SERCA (calcium ATPase) by lowering its apparent Ca2+ affinity in a manner similar to that of its homologue phospholamban (PLN). We use solution NMR to map the structural changes occurring within SLN upon interaction with the regulatory target, SERCA, co-reconstituting the two proteins in dodecylphosphocholine (DPC) detergent micelles, a system that preserves the native structure of SLN and the activity of SERCA, with the goal of comparing these interactions with those of the previously studied PLN-SERCA complex. Our analysis of the structural dynamics of SLN in DPC micelles shows this polypeptide to be partitioned into four subdomains: a short unstructured N terminus (residues 1-6), a short dynamic helix (residues 7-14), a more rigid helix (residues 15-26), and an unstructured C terminus (residues 27-31). Upon addition of SERCA, the different domains behave according to their dynamics, molding onto the surface of the enzyme. Remarkably, each domain of SLN behaves in a manner similar to that of the corresponding domains in PLN, supporting the hypothesis that both SLN and PLN bind SERCA in the same groove and with similar mechanisms.

Mechanism of a molecular valve in the halorhodopsin chloride pump

Halorhodopsin is a light-driven chloride anion pump in which the trans-->cis photoisomerization of a retinal chromophore triggers a photocycle resulting in the translocation of chloride across the plasma membrane. The mechanism of chloride transfer past the cis retinal is determined here by computing multiple pathways for this process. The calculations reveal two conditions of the valve mechanism. First, a lumen absent in the ground state structure is transiently opened by chloride passage. Second, this activated opening, which is achieved by flexible deformation of the surrounding protein, is shown to significantly raise the chloride translocation barrier between photocycles, thus preventing chloride backflow. Unlike macroscopic valve designs, the protein allows differential ion flows in the pumping and resting states that are tuned to match the physiological timescales of the cell, thus creating a "kinetic" valve.

Analysis of functional motions in Brownian molecular machines with an efficient block normal mode approach: myosin-II and Ca2+ -ATPase

The structural flexibilities of two molecular machines, myosin and Ca(2+)-ATPase, have been analyzed with normal mode analysis and discussed in the context of their energy conversion functions. The normal mode analysis with physical intermolecular interactions was made possible by an improved implementation of the block normal mode (BNM) approach. The BNM results clearly illustrated that the large-scale conformational transitions implicated in the functional cycles of the two motor systems can be largely captured with a small number of low-frequency normal modes. Therefore, the results support the idea that structural flexibility is an essential part of the construction principle of molecular motors through evolution. Such a feature is expected to be more prevalent in motor proteins than in simpler systems (e.g., signal transduction proteins) because in the former, large-scale conformational transitions often have to occur before the chemical events (e.g., ATP hydrolysis in myosin and ATP binding/phosphorylation in Ca(2+)-ATPase). This highlights the importance of Brownian motions associated with the protein domains that are involved in the functional transitions; in this sense, Brownian molecular machines is an appropriate description of molecular motors, although the normal mode results do not address the origin of the ratchet effect. The results also suggest that it might be more appropriate to describe functional transitions in some molecular motors as intrinsic elastic motions modulating local structural changes in the active site, which in turn gets stabilized by the subsequent chemical events, in contrast with the conventional idea of local changes somehow getting amplified into larger-scale motions. In the case of myosin, for example, we favor the idea that Brownian motions associated with the flexible converter propagates to the Switch I/II region, where the salt-bridge formation gets stabilized by ATP hydrolysis, in contrast with the textbook notion that ATP hydrolysis drives the converter motion. Another useful aspect of the BNM results is that selected low-frequency normal modes have been identified to form a set of collective coordinates that can be used to characterize the progress of a significant fraction of large-scale conformational transitions. Therefore, the present normal mode analysis has provided a stepping-stone toward more elaborate microscopic simulations for addressing critical issues in free energy conversions in molecular machines, such as the coupling and the causal relationship between collective motions and essential local changes at the catalytic active site where ATP hydrolysis occurs.

The ATPases: a new family for a family-based drug design approach

The rapid discovery of new drugs is greatly facilitated when a family of related proteins is targeted with a similar approach in chemistry. Few protein families have so far been investigated using this kind of 'family-based' approach. Therefore, to increase the size of our Pharmacopeia and to cure human diseases more efficiently, new druggable protein families must be identified. It is shown in this review that ATPases are very good candidates for a family-based approach. The human proteome contains many ATPases, which are involved in several diseases. All the ATPases contain a nucleotide-binding site, and it is therefore possible to target all of them with a single strategy in chemistry; the design of competitive ATP inhibitors. Moreover, because a similar approach has been conducted with the protein kinases, the compound libraries and the knowledge developed in the kinase field can be directly applied to the ATPases.