Aromatic Anchor Research Articles

Computational Design and Experimental Validation of an Orthogonal X-bond/H-bond System to Improve the Binding Affinity and Recognition Specificity between Anesthetic Protein Kinase Cι and its Pseudosubstrate Peptide

Protein kinase C isoform [Formula: see text] (PKC[Formula: see text]) is an essential regulator of diverse neurobiological signaling pathways, which has been implicated in general anesthetic action and a variety of nervous neoplasms. Traditional efforts have been primarily addressed on the development of small-molecule inhibitors to target the catalytic or regulatory domain of this kinase. Instead, we herein reported a rational chemical modification of Par3 pseudosubstrate peptide to target the kinase domain with improved affinity and specificity. Structural and energetic analysis suggested that the Par3 peptide can be divided into two binding-independent sections; they are separately anchored to the substrate-binding pocket of PKC[Formula: see text] with two aromatic anchor residues Phe818 and Phe823. The side-chain phenyl group of these two aromatic residues was systematically substituted by different halogen moieties at [Formula: see text]-, [Formula: see text]- and [Formula: see text]-positions of the phenyl group. It is revealed that the [Formula: see text]-bromination of Par3 Phe818 and the [Formula: see text]-iodinization of Par3 Phe823 residue can separately form two geometrically and energetically satisfactory orthogonal halogen (X)-bond/hydrogen (H)-bond [ortX/H] motifs across Par3 complex interface with PKC[Formula: see text]; they co-define a so-called ortX/H system and were demonstrated to improve both the binding affinity and recognition specificity of the kinase–peptide interaction by integrating in silico analysis and in vitro assay. The peptide affinity promotes 23.5-fold and 6.6-fold upon the formation of ortX/H motif-I and motif-II, respectively, which further increases to 43.5-fold by forming the complete ortX/H system, indicating cooperativity between the two motifs in the system. In addition, we also demonstrated that the ortX/H system can considerably enhance the peptide selectivity for its cognate PKC[Formula: see text] and noncogante isoforms PKC[Formula: see text] and PKC[Formula: see text].

Planar aromatic anchors control the electrical conductance of gold|molecule|graphene junctions.

The synthesis of a family of alkanethiol molecules with planar aromatic head groups, designed to anchor molecules effectively to graphene electrodes, is reported. Characterisation of self-assembled monolayers of these molecules on a gold surface via conductive atomic force microscopy shows that when an aromatic head group is present, the conductance G graphene obtained using a graphene coated probe is higher than the conductance G Pt obtained using a platinum (Pt) probe. For Pt probe and graphene probe junctions, the tunnelling decay constant of benzyl ether derivatives with an alkanethiol molecular backbone is determined as β = 5.6 nm-1 and 3.5 nm-1, respectively. The conductance ratio G graphene/G Pt increases as the number of rings present in the aromatic head unit, n, increases. However, as the number of rings increases, the conductance path length increases because the planar head groups lie at an angle to the plane of the electrodes. This means that overall conductance decreases as n increases. Density functional theory-based charge transport calculations support these experimental findings. This study confirms that planar aromatic head groups can function as effective anchoring units for graphene electrodes in large area molecular junctions. However, the results also indicate that the size and geometry of these head groups must be considered in order to produce effective molecular designs.

Chaperoning of the histone octamer by the acidic domain of DNA repair factor APLF.

Nucleosome assembly requires the coordinated deposition of histone complexes H3-H4 and H2A-H2B to form a histone octamer on DNA. In the current paradigm, specific histone chaperones guide the deposition of first H3-H4 and then H2A-H2B. Here, we show that the acidic domain of DNA repair factor APLF (APLFAD) can assemble the histone octamer in a single step and deposit it on DNA to form nucleosomes. The crystal structure of the APLFAD-histone octamer complex shows that APLFAD tethers the histones in their nucleosomal conformation. Mutations of key aromatic anchor residues in APLFAD affect chaperone activity in vitro and in cells. Together, we propose that chaperoning of the histone octamer is a mechanism for histone chaperone function at sites where chromatin is temporarily disrupted.

Structural studies of geranylgeranylglyceryl phosphate synthase, a prenyltransferase found in thermophilic Euryarchaeota.

Archaea are uniquely adapted to thrive in harsh environments, and one of these adaptations involves the archaeal membrane lipids, which are characterized by their isoprenoid alkyl chains connected via ether linkages to glycerol 1-phosphate. The membrane lipids of the thermophilic and acidophilic euryarchaeota Thermoplasma volcanium are exclusively glycerol dibiphytanyl glycerol tetraethers. The first committed step in the biosynthetic pathway of these archaeal lipids is the formation of the ether linkage between glycerol 1-phosphate and geranylgeranyl diphosphate, and is catalyzed by the enzyme geranylgeranylglyceryl phosphate synthase (GGGPS). The 1.72 Å resolution crystal structure of GGGPS from T. volcanium (TvGGGPS) in complex with glycerol and sulfate is reported here. The crystal structure reveals TvGGGPS to be a dimer, which is consistent with the absence of the aromatic anchor residue in helix α5a that is required for hexamerization in other GGGPS homologs; the hexameric quaternary structure in GGGPS is thought to provide thermostability. A phylogenetic analysis of the Euryarchaeota and a parallel ancestral state reconstruction investigated the relationship between optimal growth temperature and the ancestral sequences. The presence of an aromatic anchor residue is not explained by temperature as an ecological parameter. An examination of the active site of the TvGGGPS dimer revealed that it may be able to accommodate longer isoprenoid substrates, supporting an alternative pathway of isoprenoid membrane-lipid synthesis.

The Role of Water in the Stability of Wild-type and Mutant Insulin Dimers.

Insulin dimerization and aggregation play important roles in the endogenous delivery of the hormone. One of the important residues at the insulin dimer interface is PheB24, which is an invariant aromatic anchor that packs toward its own monomer inside a hydrophobic cavity formed by ValB12, LeuB15, TyrB16, CysB19, and TyrB26. Using molecular dynamics and free-energy simulations within explicit solvent, the structural and dynamical consequences of mutations of Phe at position B24 to glycine (Gly), alanine (Ala), and d-Ala and the des-PheB25 variant are quantified. Consistent with experiments, it is found that the Gly and Ala modifications lead to insulin dimers with reduced stability by 4 and 5 kcal/mol from thermodynamic integration and 4 and 8 kcal/mol from results using molecular mechanics-generalized Born surface area, respectively. Given the experimental difficulties to quantify the thermodynamic stability of modified insulin dimers, such computations provide a valuable complement. Interestingly, the Gly mutant exists as a strongly and a weakly interacting dimer. Analysis of the molecular dynamics simulations shows that this can be explained by water molecules that replace direct monomer-monomer H-bonding contacts at the dimerization interface involving residues B24 to B26. It is concluded that such solvent molecules play an essential role and must be included in future insulin dimerization studies.

UV‐Patterning of Anti‐Biofouling Zwitterionic Copolymer Layer with an Aromatic Anchor Group

A thin layer (around 2 nm in thickness) composed of a random copolymer of zwitterionic carboxymethyl betaine (CMB) and p‐trimethoxysilylstyrene (STMS) (9:1) is constructed on a glass substrate or a silicon wafer. The copolymer layer is highly resistant against nonspecific adsorption of bovine serum albumin (BSA). However, on UV irradiation at 193 nm, the layer becomes hydrophobic and BSA is significantly adsorbed on the substrate. Upon UV irradiation through a photomask, a patterning of the fluorophore‐labeled protein with a resolution of about 1 µm can be clearly observed. On the other hand, the copolymer layer of CMB and 3‐methacryloxypropyltrimethoxysilane (MPTMS) (9:1) without an aromatic group exhibit less distinct pattern. Further, the poly(CMB‐r‐STMS) layer decomposes more quickly compared with the poly(CMB‐r‐MPTMS) layer at low irradiation dose. The zwitterionic polymer layer with an aromatic anchor group will be used at the modification of substrates applicable to sensing devices. image

Modulate Organic‐Metal Oxide Heterojunction via [1,6] Azafulleroid for Highly Efficient Organic Solar Cells

By creating an effective π-orbital hybridization between the fullerene cage and the aromatic anchor (addend), the azafulleroid interfacial modifiers exhibit enhanced electronic coupling to the underneath metal oxides. High power conversion efficiency of 10.3% can be achieved in organic solar cells using open-cage phenyl C61 butyric acid methyl ester (PCBM)-modified zinc oxide layer.

Electron and heat transport in porphyrin-based single-molecule transistors with electro-burnt graphene electrodes.

We have studied the charge and thermal transport properties of a porphyrin-based single-molecule transistor with electro-burnt graphene electrodes (EBG) using the nonequilibrium Green’s function method and density functional theory. The porphyrin-based molecule is bound to the EBG electrodes by planar aromatic anchor groups. Due to the efficient π–π overlap between the anchor groups and graphene and the location of frontier orbitals relative to the EBG Fermi energy, we predict HOMO-dominated transport. An on–off ratio as high as 150 is predicted for the device, which could be utilized with small gate voltages in the range of ±0.1 V. A positive thermopower of +280 μV/K is predicted for the device at the theoretical Fermi energy. The sign of the thermopower could be changed by tuning the Fermi energy. By gating the junction and changing the Fermi energy by +10 meV, this can be further enhanced to +475 μV/K. Although the electrodes and molecule are symmetric, the junction itself can be asymmetric due to different binding configurations at the electrodes. This can lead to rectification in the current–voltage characteristic of the junction.

Aromatic Anchor at an Invariant Hormone-Receptor Interface: FUNCTION OF INSULIN RESIDUE B24 WITH APPLICATION TO PROTEIN DESIGN

Crystallographic studies of insulin bound to fragments of the insulin receptor have recently defined the topography of the primary hormone-receptor interface. Here, we have investigated the role of Phe(B24), an invariant aromatic anchor at this interface and site of a human mutation causing diabetes mellitus. An extensive set of B24 substitutions has been constructed and tested for effects on receptor binding. Although aromaticity has long been considered a key requirement at this position, Met(B24) was found to confer essentially native affinity and bioactivity. Molecular modeling suggests that this linear side chain can serve as an alternative hydrophobic anchor at the hormone-receptor interface. These findings motivated further substitution of Phe(B24) by cyclohexanylalanine (Cha), which contains a nonplanar aliphatic ring. Contrary to expectations, [Cha(B24)]insulin likewise exhibited high activity. Furthermore, its resistance to fibrillation and the rapid rate of hexamer disassembly, properties of potential therapeutic advantage, were enhanced. The crystal structure of the Cha(B24) analog, determined as an R6 zinc-stabilized hexamer at a resolution of 1.5 Å, closely resembles that of wild-type insulin. The nonplanar aliphatic ring exhibits two chair conformations with partial occupancies, each recapitulating the role of Phe(B24) at the dimer interface. Together, these studies have defined structural requirements of an anchor residue within the B24-binding pocket of the insulin receptor; similar molecular principles are likely to pertain to insulin-related growth factors. Our results highlight in particular the utility of nonaromatic side chains as probes of the B24 pocket and suggest that the nonstandard Cha side chain may have therapeutic utility.

Differential regulation of staphylococcal virulence by the sensor kinase SaeS in response to neutrophil-derived stimuli.

Two-component systems (TCSs) are highly conserved across bacteria and are used to rapidly sense and respond to changing environmental conditions. The human pathogen Staphylococcus aureus uses the S. aureus exoprotein expression (sae) TCS to sense host signals and activate transcription of virulence factors essential to pathogenesis. Despite its importance, the mechanism by which the histidine kinase SaeS recognizes specific host stimuli is unknown. After mutagenizing the predicted extracellular loop of SaeS, we discovered one methionine residue (M31) was essential for the ability of S. aureus to transcribe sae target genes, including hla, lukAB/lukGH, and hlgA. This single M31A mutation also significantly reduced cytotoxicity in human neutrophils to levels observed in cells following interaction with ΔsaeS. Another important discovery was that mutation of two aromatic anchor residues (W32A and F33A) disrupted the normal basal signaling of SaeS in the absence of inducing signals, yet both mutant kinases had appropriate activation of effector genes following exposure to neutrophils. Although the transcriptional profile of aromatic mutation W32A was consistent with that of WT in response to human α-defensin 1, mutant kinase F33A did not properly transcribe the γ-toxin genes in response to this stimulus. Taken together, our results provide molecular evidence for how SaeS recognizes host signals and triggers activation of select virulence factors to facilitate evasion of innate immunity. These findings have important implications for signal transduction in prokaryotes and eukaryotes due to conservation of aromatic anchor residues across both of these domains and the important role they play in sensor protein structure and function.

A comprehensive analysis of the geranylgeranylglyceryl phosphate synthase enzyme family identifies novel members and reveals mechanisms of substrate specificity and quaternary structure organization

Geranylgeranylglyceryl phosphate synthase (GGGPS) family enzymes catalyse the formation of an ether bond between glycerol-1-phosphate and polyprenyl diphosphates. They are essential for the biosynthesis of archaeal membrane lipids, but also occur in bacterial species, albeit with unknown physiological function. It has been known that there exist two phylogenetic groups (I and II) of GGGPS family enzymes, but a comprehensive study has been missing. We therefore visualized the variability within the family by applying a sequence similarity network, and biochemically characterized 17 representative GGGPS family enzymes regarding their catalytic activities and substrate specificities. Moreover, we present the first crystal structures of group II archaeal and bacterial enzymes. Our analysis revealed that the previously uncharacterized bacterial enzymes from group II have GGGPS activity like the archaeal enzymes and differ from the bacterial group I enzymes that are heptaprenylglyceryl phosphate synthases. The length of the isoprenoid substrate is determined in group II GGGPS enzymes by 'limiter residues' that are different from those in group I enzymes, as shown by site-directed mutagenesis. Most of the group II enzymes form hexamers. We could disrupt these hexamers to stable and catalytically active dimers by mutating a single amino acid that acts as an 'aromatic anchor'.

Determinants of Preferential Binding of Apo Calmodulin to the IQ Motif of Neuronal Sodium Channel NaV1.2

The neuronal voltage-gated sodium channel (NaV1.2) is regulated by calmodulin (CaM), a highly conserved, ubiquitous eukaryotic protein that mediates many calcium-triggered signaling events. Fast inactivation of the channel depends on CaM-mediated feedback transduction of calcium flux during the repolarization phase of an action potential. CaM binds to an intracellular loop (the III-IV linker) and an IQ motif [IQRAYRRYLLK] in the cytosolic C-terminal tail of the channel. The NaV1.2 IQ motif binds only to the C-domain of CaM with high affinity to both its calcium-free (apo) and calcium-saturated states. However, the IQ motif binds more favorably to apo CaM than to calcium-saturated CaM. To determine the molecular basis for this calcium-dependent difference in association, mutational perturbations of residues in the NaV1.2 IQ motif were designed to disrupt close contacts observed in our solution (NMR) structure of the semi-open C-domain of apo-CaM bound to the IQ motif (2KXW.pdb). The contributions of these residues to binding energetics were determined by monitoring CaM-induced disruption of FRET in biosensors containing wild-type or mutant sequences of the IQ motif bracketed by auto-fluorescent proteins YFP and CFP. All mutations lowered affinity for calcium-saturated CaM, but they had uniformly more deleterious effects on the binding of apo CaM. Furthermore, the decrease in affinity for apo CaM caused by loss of the Ile-Gln pair was 30-fold greater than that observed for loss of the Tyr-Tyr pair. Thus, the energy of interaction between the NaV1.2 IQ motif and semi-open apo CaM is not accounted for primarily by the classical “aromatic anchors” that dominate interactions of calcium-saturated CaM with its target sequences in kinases, receptors and other channels. Support: NIH R01 GM57001, Carver Charitable Trust Grant 01-224.

Residues at the Cytoplasmic End of Transmembrane Helix 2 Determine the Signal Output of the TarEc Chemoreceptor

Baseline signal output and communication between the periplasmic and cytoplasmic domains of the Escherichia coli aspartate chemoreceptor Tar(Ec) are both strongly influenced by residues at the C-terminus of transmembrane helix 2 (TM2). In particular, the cytoplasmic aromatic anchor, composed of residues Trp-209 and Tyr-210 in wild-type Tar(Ec), is important for determining the CheA kinase-stimulating activity of the receptor and its ability to respond to chemoeffector-induced stimuli. Here, we have studied the effect on Tar(Ec) function of the six-residue sequence at positions 207-212. Moving various combinations of aromatic residues among these positions generates substantial changes in receptor activity. Trp has the largest effect on function, both in maintaining normal activity and in altering activity when it is moved. Tyr has a weaker effect, and Phe has the weakest; however, all three aromatic residues can alter signal output when they are placed in novel positions. We also find that Gly-211 plays an important role in receptor function, perhaps because of the flexibility it introduces into the TM2-HAMP domain connector. The conservation of this Gly residue in the high-abundance chemoreceptors of E. coli and Salmonella enterica suggests that it may be important for the nuanced, bidirectional transmembrane signaling that occurs in these proteins.

A Pt(ii)–Dip complex stabilizes parallel c-myc G-quadruplex

A new G-quadruplex (GQ) stabilizer, [Pt(Dip)2](PF6)2 (Dip: 4,7-diphenyl-1,10-phenanthroline), is prepared by the microwave irradiation method. The complex can highly stabilize G-quadruplex, but has negligible interactions with duplex DNA. Aromatic anchors on the polypyridyl ligands bestow the stabilizer with a high binding preference towards parallel GQ.

The Residue Composition of the Aromatic Anchor of the Second Transmembrane Helix Determines the Signaling Properties of the Aspartate/Maltose Chemoreceptor Tar of Escherichia coli

Repositioning of the tandem aromatic residues (Trp-209 and Tyr-210) at the cytoplasmic end of the second transmembrane helix (TM2) modulates the signal output of the aspartate/maltose chemoreceptor of Escherichia coli (Tar(Ec)). Here, we directly assessed the effect of the residue composition of the aromatic anchor by studying the function of a library of Tar(Ec) variants that possess all possible combinations of Ala, Phe, Tyr, and Trp at positions 209 and 210. We identified three important properties of the aromatic anchor. First, a Trp residue at position 209 was required to maintain clockwise (CW) signal output in the absence of adaptive methylation, but adaptive methylation restored the ability of all of the mutant receptors to generate CW rotation. Second, when the aromatic anchor was replaced with tandem Ala residues, signaling was less compromised than when an Ala residue occupied position 209 and an aromatic residue occupied position 210. Finally, when Trp was present at position 209, the identity of the residue at position 210 had little effect on baseline signal output or aspartate chemotaxis, although maltose taxis was significantly affected by some substitutions at position 210. All of the mutant receptors we constructed supported some level of aspartate and maltose taxis in semisolid agar swim plates, but those without Trp at position 209 were overmethylated in their baseline signaling state. These results show the importance of the cytoplasmic aromatic anchor of TM2 in maintaining the baseline Tar(Ec) signal output and responsiveness to attractant signaling.

Importance of Aromatic Anchor Residue Identity and Location for the Tilt and Dynamics of Transmembrane Peptides

Aromatic or polar amino acid residues often flank each side of a membrane-spanning alpha helix, whether in an integral membrane protein or a model peptide. The aromatic residues Trp, Tyr and to a lesser extent Phe tend to partition to the membrane-water interface and act as “anchors” to stabilize the transmembrane orientation. The synthetic model peptide, GWALP23 (acetyl-GGALW5(LA)6LW19LAGA-[ethanol]amide), has proven valuable for experimentation, with only one Trp “anchor” near each end of the transmembrane sequence. Indeed, with relatively minimal complications from the peptide dynamics, the average tilt of GWALP23 has been shown to vary systematically in lipid bilayer membranes of different thickness (see J. Biol. Chem. 285, 31723). We have employed 2H-alanines and solid-state NMR spectroscopy to investigate the consequences of moving or replacing W5 or W19 in GWALP23 with selected Tyr, Phe or Trp residues at the same or nearby locations. We find that GWALP23 peptides having Y5, F5 or W5 exhibit essentially the same average tilt in bilayer membranes of DOPC, DMPC or DLPC; with somewhat increased dynamics for the F5 peptide. When “double” anchors are present in Y4,5GWALP23 or F4,5GWALP23, the peptides appear less responsive to the bilayer thickness, as the dynamics become dramatically more extensive. Moving W19 to position 18, a 100° radial change, alters the direction of the helix tilt, as expected. We conclude that, in the absence of other functional groups, the aromatic residues determine the preferred orientations and dynamics of transmembrane peptides. Increased dynamics are observed when the ring hydrogen bonding is removed (Phe), or when two aromatic anchors are present on one side of the core transmembrane sequence.

Response of GWALP Transmembrane Peptides to Changes in the Tryptophan Anchor Positions

While the interfacial partitioning of charged or aromatic anchor residues may determine the preferred orientations of transmembrane peptide helices, the dependence of helix orientation on anchor residue position is not well understood. When anchor residue locations are changed systematically, some adaptations of the peptide-lipid interactions may be required to compensate for the altered interfacial interactions. Recently, we have developed a novel transmembrane peptide, termed GW(5,19)ALP23 (acetyl-GGALW(5)LALALALALALALW(19)LAGA-ethanolamide), which proves to be a well-behaved sequence for an orderly investigation of protein-lipid interactions. Its roughly symmetric nature allows for shifting the anchoring Trp residues by one Leu-Ala pair inward (GW(7,17)ALP23) or outward (GW(3,21)ALP23), thus providing fine adjustments of the formal distance between the tryptophan residues. With no other obvious anchoring features present, we postulate that the inter-Trp distance may be crucial for aspects of the peptide-lipid interaction. Importantly, the amino acid composition is identical for each of the resulting related GWALP23 sequences, and the radial separation between the pairs of Trp residues on each side of the transmembrane α-helix remains similar. Here we address the adaptation of the aforementioned peptides to the varying Trp locations by means of solid-state (2)H nuclear magnetic resonance experiments in varying lipid bilayer membrane environments. All of the GW(x,y)ALP23 sequence isomers adopt transmembrane orientations in DOPC, DMPC, and DLPC environments, even when the Trp residues are quite closely spaced, in GW(7,17)ALP23. Furthermore, the dynamics for each peptide isomer are less extensive than for peptides possessing additional interfacial Trp residues. The helical secondary structure is maintained more strongly within the Trp-flanked core region than outside of the Trp boundaries. Deuterium-labeled tryptophan indole rings in the GW(x,y)ALP23 peptides provide additional insights into the behavior of the Trp side chains. A Trp side chain near the C-terminus adopts a different orientation and undergoes somewhat faster dynamics than a corresponding Trp side chain located an equivalent distance from the N-terminus. In contrast, as the inter-Trp distance changes, the variations among the average orientations of the Trp indole rings at either terminus are systematic yet fairly small. We conclude that subtle adjustments to the peptide tilt, and to the N- and C-terminal Trp side chain torsion angles, permit the GW(x,y)ALP23 peptides to maintain preferred transmembrane orientations while adapting to lipid bilayers with differing hydrophobic thicknesses.

The Potential of Perylene Bisimide Derivatives for the Solubilization of Carbon Nanotubes and Graphene

Carbon nanotubes and graphene are outstanding materials of the 21st century with a broad spectrum of applications. However, major challenges are faced such as the intrinsically low solubility of both sp2 carbon allotropes. To overcome this hurdle the potential of noncovalent functionalization is summarized with a special focus on the establishment of the perylene bisimide unit as aromatic anchor to the graphitic surface. Rational surfactant design is unmasked as the key to solubilization of the carbon allotropes, while at the same time tailoring their surface properties, or even electronic properties in a fully reversible fashion.

Charged or Aromatic Anchor Residue Dependence of Transmembrane Peptide Tilt

The membrane-spanning segments of integral membrane proteins often are flanked by aromatic or charged amino acid residues, which may "anchor" the transmembrane orientation. Single spanning transmembrane peptides such as those of the WALP family, acetyl-GWW(LA)(n)LWWA-amide, furthermore adopt a moderate average tilt within lipid bilayer membranes. To understand the anchor residue dependence of the tilt, we introduce Leu-Ala "spacers" between paired anchors and in some cases replace the outer tryptophans. The resulting peptides, acetyl-GX(2)ALW(LA)(6)LWLAX(22)A-amide, have Trp, Lys, Arg, or Gly in the two X positions. The apparent average orientations of the core helical sequences were determined in oriented phosphatidylcholine bilayer membranes of varying thickness using solid-state (2)H NMR spectroscopy. When X is Lys, Arg, or Gly, the direction of the tilt is essentially constant in different lipids and presumably is dictated by the tryptophans (Trp(5) and Trp(19)) that flank the inner helical core. The Leu-Ala spacers are no longer helical. The magnitude of the apparent helix tilt furthermore scales nicely with the bilayer thickness except when X is Trp. When X is Trp, the direction of tilt is less well defined in each phosphatidylcholine bilayer and varies up to 70° among 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1,2-dimyristoyl-sn-glycero-3-phosphocholine, and 1,2-dilauroyl-sn-glycero-3-phosphocholine bilayer membranes. Indeed, the X = Trp case parallels earlier observations in which WALP family peptides having multiple Trp anchors show little dependence of the apparent tilt magnitude on bilayer thickness. The results shed new light on the interactions of arginine, lysine, tryptophan, and even glycine at lipid bilayer membrane interfaces.

Calmodulin Is a Functional Regulator of Cav1.4 L-type Ca2+ Channels

Cav1.4 channels are unique among the high voltage-activated Ca2+ channel family because they completely lack Ca2+-dependent inactivation and display very slow voltage-dependent inactivation. Both properties are of crucial importance in ribbon synapses of retinal photoreceptors and bipolar cells, where sustained Ca2+ influx through Cav1.4 channels is required to couple slow graded changes of the membrane potential with tonic glutamate release. Loss of Cav1.4 function causes severe impairment of retinal circuitry function and has been linked to night blindness in humans and mice. Recently, an inhibitory domain (ICDI: inhibitor of Ca2+-dependent inactivation) in the C-terminal tail of Cav1.4 has been discovered that eliminates Ca2+-dependent inactivation by binding to upstream regulatory motifs within the proximal C terminus. The mechanism underlying the action of ICDI is unclear. It was proposed that ICDI competitively displaces the Ca2+ sensor calmodulin. Alternatively, the ICDI domain and calmodulin may bind to different portions of the C terminus and act independently of each other. In the present study, we used fluorescence resonance energy transfer experiments with genetically engineered cyan fluorescent protein variants to address this issue. Our data indicate that calmodulin is preassociated with the C terminus of Cav1.4 but may be tethered in a different steric orientation as compared with other Ca2+ channels. We also find that calmodulin is important for Cav1.4 function because it increases current density and slows down voltage-dependent inactivation. Our data show that the ICDI domain selectively abolishes Ca2+-dependent inactivation, whereas it does not interfere with other calmodulin effects.