Channels formed by the transmembrane helix of phospholamban: a simulation study

Binding of a monoclonal antibody to the phospholamban cytoplasmic domain interferes with the channel activity of phospholamban reconstituted in a tethered bilayer lipid membrane

Phospholamban (PLN), a membrane protein present in the sarcoplasmic reticulum of cardiac myocytes, is a crucial regulator of cardiac function. It is known that PLN appears as a monomer and as a pentamer. However, the role of the PLN pentamer and its ability to generate an ion channel are a matter of debate. To address this issue we employed an experimental approach that combines electrochemical impedance spectroscopy and surface plasmon resonance measurements. In particular, we investigated the channel activity of wild-type PLN reconstituted in a tethered bilayer lipid membrane (tBLM) on a gold surface. Our results indicate that reconstituted PLN can generate ion-conducting channels in a tBLM. Experiments with a PLN monoclonal antibody support an oriented incorporation of PLN in the tBLM. We show that the binding of the antibody to the PLN cytoplasmic domain interferes with PLN channel activity.

Structure-function relation of phospholamban: modulation of channel activity as a potential regulator of SERCA activity

Phospholamban (PLN) is a small integral membrane protein, which binds and inhibits in a yet unknown fashion the Ca²⁺-ATPase (SERCA) in the sarcoplasmic reticulum. When reconstituted in planar lipid bilayers PLN exhibits ion channel activity with a low unitary conductance. From the effect of non-electrolyte polymers on this unitary conductance we estimate a narrow pore with a diameter of ca. 2.2 Å for this channel. This value is similar to that reported for the central pore in the structure of the PLN pentamer. Hence the PLN pentamer, which is in equilibrium with the monomer, is the most likely channel forming structure. Reconstituted PLN mutants, which either stabilize (K27A and R9C) or destabilize (I47A) the PLN pentamer and also phosphorylated PLN still generate the same unitary conductance of the wt/non-phosphorylated PLN. However the open probability of the phosphorylated PLN and of the R9C mutant is significantly lower than that of the respective wt/non-phosphorylated control. In the context of data on PLN/SERCA interaction and on Ca²⁺ accumulation in the sarcoplasmic reticulum the present results are consistent with the view that PLN channel activity could participate in the balancing of charge during Ca²⁺ uptake. A reduced total conductance of the K⁺ transporting PLN by phosphorylation or by the R9C mutation may stimulate Ca²⁺ uptake in the same way as an inhibition of K⁺ channels in the SR membrane. The R9C-PLN mutation, a putative cause of dilated cardiomyopathy, might hence affect SERCA activity also via its inherent low open probability.

Structure, dynamics, and ion conductance of the phospholamban pentamer

A 52-residue membrane protein, phospholamban (PLN) is an inhibitor of an adenosine-5'-triphosphate-driven calcium pump, the Ca2+-ATPase. Although the inhibition of Ca2+-ATPase involves PLN monomers, in a lipid bilayer membrane, PLN monomers form stable pentamers of unknown biological function. The recent NMR structure of a PLN pentamer depicts cytoplasmic helices extending normal to the bilayer in what is known as the bellflower conformation. The structure shows transmembrane helices forming a hydrophobic pore 4 A in diameter, which is reminiscent of earlier reports of possible ion conductance through PLN pentamers. However, recent FRET measurements suggested an alternative structure for the PLN pentamer, known as the pinwheel model, which features a narrower transmembrane pore and cytoplasmic helices that lie against the bilayer. Here, we report on structural dynamics and conductance properties of the PLN pentamers from all-atom (AA) and coarse-grained (CG) molecular dynamics simulations. Our AA simulations of the bellflower model demonstrate that in a lipid bilayer membrane or a detergent micelle, the cytoplasmic helices undergo large structural fluctuations, whereas the transmembrane pore shrinks and becomes asymmetric. Similar asymmetry of the transmembrane region was observed in the AA simulations of the pinwheel model; the cytoplasmic helices remained in contact with the bilayer. Using the CG approach, structural dynamics of both models were investigated on a microsecond timescale. The cytoplasmic helices of the CG bellflower model were observed to fall against the bilayer, whereas in the CG pinwheel model the conformation of the cytoplasmic helices remained stable. Using steered molecular dynamics simulations, we investigated the feasibility of ion conductance through the pore of the bellflower model. The resulting approximate potentials of mean force indicate that the PLN pentamer is unlikely to function as an ion channel.

On the function of pentameric phospholamban: ion channel or storage form?

Phospholamban (PLN) is an integral membrane protein that inhibits the sarcoplasmic reticulum Ca(2+)-ATPase, thereby regulating muscle contractility. We report a combined electrochemical and theoretical study demonstrating that the pentameric PLN does not possess channel activity for conducting chloride or calcium ions across the lipid membrane. This suggests that the pentameric configuration of PLN primarily serves as a storage form for the regulatory function of muscle relaxation by the PLN monomer.

Structural constraints on the transmembrane and juxtamembrane regions of the phospholamban pentamer in membrane bilayers: Gln29 and Leu52

The Ca2+-ATPase of cardiac muscle cells transports Ca2+ ions against a concentration gradient into the sarcoplasmic reticulum and is regulated by phospholamban, a 52-residue integral membrane protein. It is known that phospholamban inhibits the Ca2+ pump during muscle contraction and that inhibition is removed by phosphorylation of the protein during muscle relaxation. Phospholamban forms a pentameric complex with a central pore. The solid-state magic angle spinning (MAS) NMR measurements presented here address the structure of the phospholamban pentamer in the region of Gln22-Gln29. Rotational echo double resonance (REDOR) NMR measurements show that the side chain amide groups of Gln29 are in close proximity, consistent with a hydrogen-bonded network within the central pore. 13C MAS NMR measurements are also presented on phospholamban that is 1-13C-labeled at Leu52, the last residue of the protein. pH titration of the C-terminal carboxyl group suggests that it forms a ring of negative charge on the lumenal side of the sarcoplasmic reticulum membrane. The structural constraints on the phospholamban pentamer described in this study are discussed in the context of a multifaceted mechanism for Ca2+ regulation that may involve phospholamban as both an inhibitor of the Ca2+ ATPase and as an ion channel.

Model of a putative pore: the pentameric alpha-helical bundle of SARS coronavirus E protein in lipid bilayers

The coronavirus responsible for the severe acute respiratory syndrome contains a small envelope protein, E, with putative involvement in host apoptosis and virus morphogenesis. To perform these functions, it has been suggested that protein E can form a membrane destabilizing transmembrane (TM) hairpin, or homooligomerize to form a TM pore. Indeed, in a recent study we reported that the α-helical putative transmembrane domain of E protein (ETM) forms several SDS-resistant TM interactions: a dimer, a trimer, and two pentameric forms. Further, these interactions were found to be evolutionarily conserved. Herein, we have studied multiple isotopically labeled ETM peptides reconstituted in model lipid bilayers, using the orientational parameters derived from infrared dichroic data. We show that the topology of ETM is consistent with a regular TM α-helix. Further, the orientational parameters obtained unequivocally correspond to a homopentameric model, by comparison with previous predictions. We have independently confirmed that the full polypeptide of E protein can also aggregate as pentamers after expression in Escherichia coli. This interaction must be stabilized, at least partially, at the TM domain. The model we report for this pentameric α-helical bundle may explain some of the permabilizing properties of protein E, and should be the basis of mutagenesis efforts in future functional studies.

Structure prediction of small transmembrane helix bundles

In this work, we will introduce a novel computational approach to predict the structures of small helical hetero-oligomeric transmembrane bundles. The approach is based on the generation and evaluation of a large library of randomly generated helix bundle conformations. This library will be evaluated by energy-dependent distributions of the structural parameters of the conformations. The approach enables us to model a subunit of cytochrome c oxidase (occ), consisting of four TM helices, to an accuracy of 1.7A and the transducer protein of the sensory Rhodopsin II-transducer complex to an accuracy of 2.3A when including two transducer-contacting Rhodopsin helices. As the approach does not afford a unique solution for each protein, experimental data would be needed to discriminate the possible models. In addition to predicting the structure of helix bundles, one can also gain insight into possible higher-energy conformations or flexible regions of the protein.

Models and simulations of ion channels and related membrane proteins

The past year has seen major advances in our understanding of ion channels, resulting from molecular dynamics simulations and modelling studies. Simulations of gramicidin have revealed that proton conduction along a water wire is limited by the dynamics of water reorientation. Plausible models are now available for a number of other channels, including alamethicin, the influenza A virus M2 protein, and the pore domains of the nicotinic acetylcholine receptor and Kv channels. Molecular dynamics simulations and continuum calculations have revealed some of the subtleties of the interactions between transmembrane helices and their lipid bilayer environment.