DHHC (Asp-His-His-Cys) palmitoyltransferases are eukaryotic integral membrane enzymes that catalyze protein palmitoylation, which is important in a range of physiological processes, including small guanosine triphosphatase (GTPase) signaling, cell adhesion, and neuronal receptor scaffolding. We present crystal structures of two DHHC palmitoyltransferases and a covalent intermediate mimic. The active site resides at the membrane-cytosol interface, which allows the enzyme to catalyze thioester-exchange chemistry by using fatty acyl-coenzyme A and explains why membrane-proximal cysteines are candidates for palmitoylation. The acyl chain binds in a cavity formed by the transmembrane domain. We propose a mechanism for acyl chain-length selectivity in DHHC enzymes on the basis of cavity mutants with preferences for shorter and longer acyl chains.

Phospholamban (PLN) is a type II membrane protein that inhibits the sarcoplasmic reticulum Ca 2þ -ATPase (SERCA), thereby regulating calcium homeostasis in cardiac muscle. In membranes, PLN forms pentamers that have been proposed to function either as a storage for active monomers or as ion channels. Here, we report the T-state structure of pentameric PLN solved by a hybrid solution and solid-state NMR method. In lipid bilayers, PLN adopts a pinwheel topology with a narrow hydrophobic pore, which excludes ion transport. In the T state, the cytoplasmic amphipathic helices (domains Ia) are absorbed into the lipid bilayer with the transmembrane domains arranged in a left-handed coiled-coil configuration, crossing the bilayer with a tilt angle of approximately 11°with respect to the membrane normal. The tilt angle difference between the monomer and pentamer is approximately 13°, showing that intramembrane helix-helix association forces dominate over the hydrophobic mismatch, driving the overall topology of the transmembrane assembly. Our data reveal that both topology and function of PLN are shaped by the interactions with lipids, which fine-tune the regulation of SERCA.hybrid NMR method | PISEMA | calcium regulation | oligomeric protein | dipolar assisted rotational resonance recoupling T he membrane protein complex formed by Ca 2þ -ATPase (SERCA) and phospholamban (PLN) regulates Ca 2þ concentration within the sarcoplasmic reticulum (SR), thereby controlling muscle excitation-contraction coupling (1, 2). PLN is a 52-residue transmembrane (TM) protein highly conserved across mammals (2). Its helix-loop-helix secondary structure is further subdivided into four dynamic domains: domain Ia (1-16), loop (17)(18)(19)(20)(21)(22), and domain II (31-52) (3, 4). The hydrophobic TM domain II is the most conserved and responsible for SERCA inhibition, whereas the cytoplasmic domain harbors two phosphorylation sites that reverse PLN inhibitory function (2). PLN has a direct role in the pathophysiology of the heart muscle, with three lethal mutations linked to dilated cardiomyopathy in humans (R9C-PLN, R14del, and L39-truncated-PLN) (5). In both synthetic and cell membranes, PLN forms pentamers that dissociate into monomers upon interacting with SERCA (1, 6). Although the stoichiometry of the SERCA/PLN complex has been assessed (1, 6), both the role and the structure of the PLN pentamer remain a matter of active debate. Because PLN expression in both atria and ventricles is higher than SERCA, it is likely that oligomerization may participate in SERCA regulation (7). Insights into PLN organization in the membrane have come from biochemical and biophysical data (2,6,8). Initial electrophysiological measurements indicated that PLN formed Ca 2þ channels (9). However, more recent electrochemical studies concluded that PLN does not conduct Cl − or Ca 2þ ions (10).Divergent structural models for the PLN pentamer have been proposed in the literature (8). Although very similar in the secondary structure content, these models differ in t...

The membrane protein complex between the sarcoplasmic reticulum Ca 2+ -ATPase (SERCA) and phospholamban (PLN) controls Ca 2+ transport in cardiomyocytes, thereby modulating cardiac contractility. β-Adrenergic-stimulated phosphorylation of PLN at Ser-16 enhances SERCA activity via an unknown mechanism. Using solid-state nuclear magnetic resonance spectroscopy, we mapped the physical interactions between SERCA and both unphosphorylated and phosphorylated PLN in membrane bilayers. We found that the allosteric regulation of SERCA depends on the conformational equilibrium of PLN, whose cytoplasmic regulatory domain interconverts between three different states: a ground T state (helical and membrane associated), an excited R state (unfolded and membrane detached), and a B state (extended and enzymebound), which is noninhibitory. Phosphorylation at Ser-16 of PLN shifts the populations toward the B state, increasing SERCA activity. We conclude that PLN's conformational equilibrium is central to maintain SERCA's apparent Ca 2+ affinity within a physiological window. This model represents a paradigm shift in our understanding of SERCA regulation by posttranslational phosphorylation and suggests strategies for designing innovative therapeutic approaches to enhance cardiac muscle contractility. -ATPase (SERCA)/phospholamban (PLN) complex regulates Ca 2+ translocation into the sarcoplasmic reticulum (SR) of cardiomyocytes and constitutes the main mechanism of cardiac relaxation (diastole) (1-3). SERCA is a P-type ATPase that translocates two Ca 2+ ions per ATP molecule hydrolyzed in exchange for three H + ( Fig. 1) (4, 5). PLN binds and allosterically inhibits SERCA function, decreasing its apparent affinity for Ca 2+ ions (3, 6). On β-adrenergic stimulation, cAMP-dependent protein kinase A phosphorylates PLN at Ser-16, reversing the inhibition and augmenting cardiac output (3). Disruptions in this regulatory mechanism degenerate into Ca 2+ mishandling and heart failure (3). Several X-ray structures of SERCA have been determined along its enzymatic coordinates, providing atomic details on the structural transitions in the absence of PLN (4, 5). The first image of the SERCA/PLN complex resulted from cryo-EM studies (7), but the low-resolution data prevent an atomic view of PLN structure and architecture within the complex. In addition, mutagenesis and cross-linking data were used to model the complex, suggesting that the inhibitory transmembrane (TM) region of PLN is positioned into a binding groove far from the putative Ca 2+ entry, as well as the ATP binding site, and located between TM helices M2, M4, M6, and M9 of SERCA. The location of PLN's TM domain agrees with a recent crystal structure of the SERCA/PLN complex (8) and is remarkably similar to the one recently identified for a PLN homolog, sarcolipin, in complex with SERCA (9, 10) (Fig. 1).In the SERCA/PLN model, which was further refined using NMR constraints (11), the loop bridging the TM and cytoplasmic domain of PLN adopts an unfolded configuration, stretching tow...

Phospholamban (PLN) is an essential regulator of cardiac muscle contractility. The homopentameric assembly of PLN is the reservoir for active monomers that, upon deoligomerization form 1:1 complexes with the sarco(endo)plasmic reticulum Ca 2؉ -ATPase (SERCA), thus modulating the rate of calcium uptake. In lipid bilayers and micelles, monomeric PLN exists in equilibrium between a bent (or resting) T state and a more dynamic (or active) R state. Here, we report the high-resolution structure and topology of the T state of a monomeric PLN mutant in lipid bilayers, using a hybrid of solution and solid-state NMR restraints together with molecular dynamics simulations in explicit lipid environments. Unlike the previous structural ensemble determined in micelles, this approach gives a complete picture of the PLN monomer structure in a lipid bilayer. This hybrid ensemble exemplifies the tilt, rotation, and depth of membrane insertion, revealing the interaction with the lipids for all protein domains. The N-terminal amphipathic helical domain Ia (residues 1-16) rests on the surface of the lipid membrane with the hydrophobic face of domain Ia embedded in the membrane bilayer interior. The helix comprised of domain Ib (residues 23-30) and transmembrane domain II (residues 31-52) traverses the bilayer with a tilt angle of Ϸ24°. The specific interactions between PLN and lipid membranes may represent an additional regulatory element of its inhibitory function. We propose this hybrid method for the simultaneous determination of structure and topology for membrane proteins with compact folds or proteins whose spatial arrangement is dictated by their specific interactions with lipid bilayers.hybrid method ͉ membrane proteins ͉ oriented solid-state NMR ͉ molecular modeling ͉ PISEMA S tructure and topology are central to membrane protein function (1). Recently determined high-resolution structures reveal the compact folds for several membrane proteins, such as electron and proton-conducting proteins involved in photosynthesis and respiration (http://blanco.biomol.uci.edu/ Membrane Proteins xtal.html). However, a significant population of membrane proteins does not possess a compact tertiary fold, but has its fold space defined through interactions of secondary structure elements (helices, turns, and loops) with the lipid membrane, i.e., the topology (1). This is the case for phospholamban (PLN), a mammalian protein that is essential in the regulation of cardiac muscle contractility (2), and that has recently become a major target for gene therapy to ameliorate cardiomyopathies (3, 4). PLN is located in the sarco(endo)plasmic reticulum (SR) of cardiac myocytes, inhibiting the SR Ca 2ϩ -ATPase (SERCA) by shifting its relative Ca 2ϩ affinity (5). In vitro and in vivo experiments have shown PLN to exist as a homopentamer that deoligomerizes into active monomers that bind SERCA in a 1:1 molar ratio (6). The monomeric form of PLN exists in equilibrium between a dynamically disordered R state and a more restricted T state (7,8) and has 3 ...

LL-37 is the only cathelicidin-derived polypeptide found in humans. Its eclectic function makes this peptide one of the most intriguing chemical defense agents, with crucial roles in moderating inflammation, promoting wound healing, and boosting the human immune system. LL-37 kills both prokaryotic and eukaryotic cells through physical interaction with cell membranes. In order to study its active conformation in membranes, we have reconstituted LL-37 into dodecylphosphocholine (DPC) micelles and determined its three-dimensional structure. We found that, under our experimental conditions, this peptide adopts a helix-break-helix conformation. Both the N- and C-termini are unstructured and solvent exposed. The N-terminal helical domain is more dynamic, while the C-terminal helix is more solvent protected and structured (high density of NOEs, slow H/D exchange). When it interacts with DPC, LL-37 is adsorbed on the surface of the micelle with the hydrophilic face exposed to the water phase and the hydrophobic face buried in the micelle hydrocarbon region. The break between the helices is positioned at K12 and is probably stabilized by a hydrophobic cluster formed by I13, F17, and I20 in addition to a salt bridge between K12 and E16. These results support the proposed nonpore carpet-like mechanism of action, in agreement with the solid-state NMR studies, and pave the way for understanding the function of the mature LL-37 at the atomic level.

Phospholamban (PLN) regulates calcium translocation within cardiac myocytes by shifting sarco(endo)plasmic reticulum Ca 2؉ -ATPase (SERCA) affinity for calcium. Although the monomeric form of PLN (6 kDa) is the principal inhibitory species, recent evidence suggests that the PLN pentamer (30 kDa) also is able to bind SERCA. To date, several membrane architectures of the pentamer have been proposed, with different topological orientations for the cytoplasmic domain: (i) extended from the bilayer normal by 50 -60°; (ii) continuous ␣-helix tilted 28°relative to the bilayer normal; (iii) pinwheel geometry, with the cytoplasmic helix perpendicular to the bilayer normal and in contact with the surface of the bilayer; and (iv) bellflower structure, in which the cytoplasmic domain helix makes Ϸ20°angle with respect to the membrane bilayer normal. Using a variety of cell membrane mimicking systems (i.e., lipid vesicles, oriented lipid bilayers, and detergent micelles) and a combination of multidimensional solution/solidstate NMR and EPR spectroscopies, we tested the different structural models. We conclude that the pinwheel topology is the predominant conformation of pentameric PLN, with the cytoplasmic domain interacting with the membrane surface. We propose that the interaction with the bilayer precedes SERCA binding and may mediate the interactions with other proteins such as protein kinase A and protein phosphatase 1.Ca 2ϩ -ATPase ͉ EPR ͉ membrane protein ͉ solid-state NMR ͉ protein dynamics C alcium translocation into the sarcoplasmic reticulum of cardiac myocytes is controlled by the sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA). Phospholamban (PLN) regulates the activity of SERCA by shifting the apparent calcium affinity for the enzyme. This activity is relieved by phosphorylation of PLN at Ser-16 and/or Thr-17 and high calcium concentration within the cytosol. Wild-type PLN (wt-PLN) forms stable homopentamers in lipid bilayers and in detergent micelles, where each monomer is composed of a helical cytoplasmic domain (residues 1-16), a semiflexible loop (residues 17-21), and a helical transmembrane domain (residues 22-52) (1, 2). Mutagenesis, molecular biology, and in vivo studies revealed that the PLN pentamer depolymerizes into active monomers that bind and inhibit SERCA (3). Similar conclusions were reached by in vitro fluorescence studies (4). Recently, Young and coworkers (5) have reported a cocrystal formed by SERCA and PLN pentamer, suggesting that the pentameric species also is able to bind SERCA. Furthermore, Jones and coworkers (6) hypothesized that the PLN pentamer may act as a chloride ion channel, which is supported by the bellflower structure recently determined by Oxenoid and Chou (2).There are four principal proposed structural models of pentameric wt-PLN, which differ primarily in the topology of the more dynamic cytoplasmic domain. In each of these models (shown in Fig. 1), residues 32-52 are in a coiled helix approximately parallel to the bilayer normal. The first model (extended helix/sheet ...

Summary Rapidly evolving RNA viruses, such as the GII.4 strain of human norovirus (HuNoV), and their vaccines elicit complex serological responses associated with previous exposure. Specific correlates of protection, moreover, remain poorly understood. Here, we report the GII.4-serological antibody repertoire—pre- and post-vaccination—and select several antibody clonotypes for epitope and structural analysis. The humoral response was dominated by GII.4-specific antibodies that blocked ancestral strains or by antibodies that bound to divergent genotypes and did not block viral-entry-ligand interactions. However, one antibody, A1431, showed broad blockade toward tested GII.4 strains and neutralized the pandemic GII.P16-GII.4 Sydney strain. Structural mapping revealed conserved epitopes, which were occluded on the virion or partially exposed, allowing for broad blockade with neutralizing activity. Overall, our results provide high-resolution molecular information on humoral immune responses after HuNoV vaccination and demonstrate that infection-derived and vaccine-elicited antibodies can exhibit broad blockade and neutralization against this prevalent human pathogen.

Phospholamban (PLN) and sarcolipin (SLN) are two single-pass membrane proteins that regulate Ca2+-ATPase (SERCA), an ATP-driven pump that translocates calcium ions into the lumen of the sarcoplasmic reticulum, initiating muscle relaxation. Both proteins bind SERCA through intramembrane interactions, impeding calcium translocation. While phosphorylation of PLN at Ser-16 and/or Thr-17 reestablishes calcium flux, the regulatory mechanism of SLN remains elusive. SERCA has been crystallized in several different states along the enzymatic reaction coordinates, providing remarkable mechanistic information; however, the lack of high-resolution crystals in the presence of PLN and SLN limits the current understanding of the regulatory mechanism. This brief review offers a survey of our hybrid structural approach using solution and solid-state NMR methodologies to understand SERCA regulation from the point of view of PLN and SLN. These results have improved our understanding of the calcium translocation process and are the basis for designing new therapeutic approaches to ameliorate muscle malfunctions.