Conformation and ion-channeling activity of a 27-residue peptide modeled on the single-transmembrane segment of the IsK (minK) protein

Comparing the structural dynamics of the human KCNE3 in reconstituted micelle and lipid bilayered vesicle environments

KCNE3 is a single transmembrane protein of the KCNE family that modulates the function and trafficking of several voltage-gated potassium channels, including KCNQ1. Structural studies of KCNE3 have been previously conducted in a wide range of model membrane mimics. However, it is important to assess the impact of the membrane mimics used on the observed conformation and dynamics. In this study, we have optimized a method for the reconstitution of the KCNE3 into POPC/POPG lipid bilayer vesicles for electron paramagnetic resonance (EPR) spectroscopy. Our CD spectroscopic data suggested that the degree of regular secondary structure for KCNE3 protein reconstituted into lipid bilayered vesicle is significantly higher than in DPC detergent micelles. Electron paramagnetic resonance (EPR) spectroscopy in combination with site-directed spin labeling (SDSL) was used to probe the structural dynamics of S49C, M59C, L67C, V85C, and S101C mutations of KCNE3 in both DPC micelles and in POPC/POPG lipid bilayered vesicles. Our CW-EPR power saturation data suggested that the site S74C is buried inside the lipid bilayered membrane while the site V85C is located outside the membrane, in contrast to DPC micelle results. These results suggest that the KCNE3 micelle structures need to be refined using data obtained in the lipid bilayered vesicles in order to ascertain the native structure of KCNE3. This work will provide guidelines for detailed structural studies of KCNE3 in a more native membrane environment and comparing the lipid bilayer results to the isotropic bicelle structure and to the KCNQ1-bound cryo-EM structure.

Probing the Dynamics and Structural Topology of the Reconstituted Human KCNQ1 Voltage Sensor Domain (Q1-VSD) in Lipid Bilayers Using Electron Paramagnetic Resonance Spectroscopy

KCNQ1 (Kv7.1 or KvLQT1) is a potassium ion channel protein found in the heart, ear, and other tissues. In complex with the KCNE1 accessory protein, it plays a role during the repolarization phase of the cardiac action potential. Mutations in the channel have been associated with several diseases, including congenital deafness and long QT syndrome. Nuclear magnetic resonance (NMR) structural studies in detergent micelles and a cryo-electron microscopy structure of KCNQ1 from Xenopus laevis have shown that the voltage sensor domain (Q1-VSD) of the channel has four transmembrane helices, S1-S4, being overall structurally similar with other VSDs. In this study, we describe a reliable method for the reconstitution of Q1-VSD into (POPC/POPG) lipid bilayer vesicles. Site-directed spin labeling electron paramagnetic resonance spectroscopy was used to probe the structural dynamics and topology of several residues of Q1-VSD in POPC/POPG lipid bilayer vesicles. Several mutants were probed to determine their location and corresponding immersion depth (in angstroms) with respect to the membrane. The dynamics of the bilayer vesicles upon incorporation of Q1-VSD were studied using ³¹P solid-state NMR spectroscopy by varying the protein:lipid molar ratios confirming the interaction of the protein with the bilayer vesicles. Circular dichroism spectroscopic data showed that the α-helical content of Q1-VSD is higher for the protein reconstituted in vesicles than in previous studies using DPC detergent micelles. This study provides insight into the structural topology and dynamics of Q1-VSD reconstituted in a lipid bilayer environment, forming the basis for more advanced structural and functional studies.

Docking studies and network analyses reveal capacity of compounds from Kandelia rheedii to strengthen cellular immunity by interacting with host proteins during tuberculosis infection

Kandelia rheedii (locally known as Guria or Rasunia), widely found and used in Indian subcontinent, is a well-known herbal cure to tuberculosis. However, neither the mechanism nor the active components of the plant extract responsible for mediating this action has yet been confirmed. Here in this study, molecular interactions of three compounds (emodin, fusaric acid and skyrin) from the plant extract with the host protein targets (casein kinase (CSNK), estrogen receptor (ERBB), dopamine β-hydroxylase (DBH) and glucagon receptor (Gcgr)) has been found. These protein targets are known to be responsible for strengthening cellular immunity against Mycobacteria tuberculosis. The specific interactions of these three compounds with the respective protein targets have been discussed here. The insights from study should further help us designing molecular medicines against tuberculosis.

The KCNE Tango - How KCNE1 Interacts with Kv7.1

The classical tango is a dance characterized by a 2/4 or 4/4 rhythm in which the partners dance in a coordinated way, allowing dynamic contact. There is a surprising similarity between the tango and how KCNE β-subunits "dance" to the fast rhythm of the cell with their partners from the Kv channel family. The five KCNE β-subunits interact with several members of the Kv channels, thereby modifying channel gating via the interaction of their single transmembrane-spanning segment, the extracellular amino terminus, and/or the intracellular carboxy terminus with the Kv α-subunit. Best studied is the molecular basis of interactions between KCNE1 and Kv7.1, which, together, supposedly form the native cardiac I(Ks) channel. Here we review the current knowledge about functional and molecular interactions of KCNE1 with Kv7.1 and try to summarize and interpret the tango of the KCNEs.

Reconstitution of KCNE1 into lipid bilayers: comparing the structural, dynamic, and activity differences in micelle and vesicle environments

KCNE1 (minK), found in the human heart and cochlea, is a transmembrane protein that modulates the voltage-gated potassium KCNQ1 channel. While KCNE1 has previously been the subject of extensive structural studies in lyso-phospholipid detergent micelles, key observations have yet to be confirmed and refined in lipid bilayers. In this study, a reliable method for reconstituting KCNE1 into lipid bilayer vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho(1'-rac-glycerol) (sodium salt) (POPG) was developed. Microinjection of the proteoliposomes into Xenopus oocytes expressing the human KCNQ1 (K(V)7.1) voltage-gated potassium channel led to nativelike modulation of the channel. Circular dichroism spectroscopy demonstrated that the percent helicity of KCNE1 is significantly higher for the protein reconstituted in lipid vesicles than for the previously described structure in 1.0% 1-myristoyl-2-hydroxy-sn-glycero-3-phospho(1'-rac-glycerol) (sodium salt) (LMPG) micelles. SDSL electron paramagnetic resonance spectroscopic techniques were used to probe the local structure and environment of Ser28, Phe54, Phe57, Leu59, and Ser64 of KCNE1 in both POPC/POPG vesicles and LMPG micelles. Spin-labeled KCNE1 cysteine mutants at Phe54, Phe57, Leu59, and Ser64 were found to be located inside POPC/POPG vesicles, whereas Ser28 was found to be located outside the membrane. Ser64 was shown to be water inaccessible in vesicles but found to be water accessible in LMPG micelle solutions. These results suggest that key components of the micelle-derived structure of KCNE1 extend to the structure of this protein in lipid bilayers but also demonstrate the need to refine this structure using data derived from the bilayer-reconstituted protein to more accurately define its native structure. This work establishes the basis for such future studies.

Structural basis of slow activation gating in the cardiac I Ks channel complex

Accessory β-subunits of the KCNE gene family modulate the function of various cation channel α-subunits by the formation of heteromultimers. Among the most dramatic changes of biophysical properties of a voltage-gated channel by KCNEs are the effects of KCNE1 on KCNQ1 channels. KCNQ1 and KCNE1 are believed to form nativeI(Ks) channels. Here, we characterize molecular determinants of KCNE1 interaction with KCNQ1 channels by scanning mutagenesis, double mutant cycle analysis, and molecular dynamics simulations. Our findings suggest that KCNE1 binds to the outer face of the KCNQ1 channel pore domain, modifies interactions between voltage sensor, S4-S5 linker and the pore domain, leading to structural modifications of the selectivity filter and voltage sensor domain. Molecular dynamics simulations suggest a stable interaction of the KCNE1 transmembrane α-helix with the pore domain S5/S6 and part of the voltage sensor domain S4 of KCNQ1 in a putative pre-open channel state. Formation of this state may induce slow activation gating, the pivotal characteristic of native cardiac I(Ks) channels. This new KCNQ1-KCNE1 model may become useful for dynamic modeling of disease-associated mutant I(Ks) channels.

Analysis of the interactions between the C-terminal cytoplasmic domains of KCNQ1 and KCNE1 channel subunits

Ion channel subunits encoded by KCNQ1 and KCNE1 produce the slowly activating K+ current (IKs) that plays a central role in myocardial repolarization. The KCNQ1 alpha-subunit and the KCNE1 beta-subunit assemble with their membrane-spanning segments interacting, resulting in transformation of channel activation kinetics. We recently reported a functional interaction involving C-terminal portions of the two subunits with ensuing regulation of channel deactivation. In the present study, we provide evidence characterizing a physical interaction between the KCNQ1-CT (KCNE1 C-terminus) and the KCNE1-CT (KCNE1 C-terminus). When expressed in cultured cells, the KCNE1-CT co-localized with KCNQ1, co-immunoprecipitated with KCNQ1 and perturbed deactivation kinetics of the KCNQ1 currents. Purified KCNQ1-CT and KCNE1-CT physically interacted in pull-down experiments, indicating a direct association. Deletion analysis of KCNQ1-CT indicated that the KCNE1-CT binds to a KCNQ1 region just after the last transmembrane segment, but N-terminal to the tetramerization domain. SPR (surface plasmon resonance) corroborated the pull-down results, showing that the most proximal region (KCNQ1 amino acids 349-438) contributed most to the bimolecular interaction with a dissociation constant of approximately 4 microM. LQT (long QT) mutants of the KCNE1-CT, D76N and W87F, retained binding to the KCNQ1-CT with comparable affinity, indicating that these disease-causing mutations do not alter channel behaviour by disruption of the association. Several LQT mutations involving the KCNQ1-CT, however, showed various effects on KCNQ1/KCNE1 association. Our results indicate that the KCNQ1-CT and the KCNE1-CT comprise an independent interaction domain that may play a role in IKs channel regulation that is potentially affected in some LQTS (LQT syndrome) mutations.

Membrane protein fragments reveal both secondary and tertiary structure of membrane proteins

Structural data on membrane proteins, while crucial to understanding cellular function, are scarce due to difficulties in applying to membrane proteins the common techniques of structural biology. Fragments of membrane proteins have been shown to reflect, in many cases, the secondary structure of the parent protein with fidelity and are more amenable to study. This chapter provides many examples of how the study of membrane protein fragments has provided new insight into the structure of the parent membrane protein.

KCNQ1 and KCNE1 in the IKs channel complex make state-dependent contacts in their extracellular domains

KCNQ1 and KCNE1 (Q1 and E1) associate to form the slow delayed rectifier I(Ks) channels in the heart. A short stretch of eight amino acids at the extracellular end of S1 in Q1 (positions 140-147) harbors six arrhythmia-associated mutations. Some of these mutations affect the Q1 channel function only when coexpressed with E1, suggesting that this Q1 region may engage in the interaction with E1 critical for the I(Ks) channel function. Identifying the Q1/E1 contact points here may provide new insights into how the I(Ks) channel operates. We focus on Q1 position 145 and E1 positions 40-43. Replacing all native cysteine (Cys) in Q1 and introducing Cys into the above Q1 and E1 positions do not significantly perturb the Q1 channel function or Q1/E1 interactions. Immunoblot experiments on COS-7 cells reveal that Q1 145C can form disulfide bonds with E1 40C and 41C, but not E1 42C or 43C. Correspondingly, voltage clamp experiments in oocytes reveal that Q1 145C coexpressed with E1 40C or E1 41C manifests unique gating behavior and DTT sensitivity. Our data suggest that E1 40C and 41C come close to Q1 145C in the activated and resting states, respectively, to allow disulfide bond formation. These data and those in the literature lead us to propose a structural model for the Q1/E1 channel complex, in which E1 is located between S1, S4, and S6 of three separate Q1 subunits. We propose that E1 is not a passive partner of the Q1 channel, but instead can engage in molecular motions during I(Ks) gating.

Peptide fibrillization

The fibrillization of peptides is relevant to many diseases based on the deposition of amyloids. The formation of fibrils is being intensively studied, especially in terms of nanotechnology applications, where fibrillar peptide hydrogels are used for cell scaffolds, as supports for functional and responsive biomaterials, biosensors, and nanowires. This Review is concerned with fundamental aspects of the self-assembly of peptides into fibrils, and discusses both natural amyloid-forming peptides and synthetic materials, including peptide fragments, copolymers, and amphiphiles.

Preparation, functional characterization, and NMR studies of human KCNE1, a voltage-gated potassium channel accessory subunit associated with deafness and long QT syndrome

KCNE1, also known as minK, is a member of the KCNE family of membrane proteins that modulate the function of KCNQ1 and certain other voltage-gated potassium channels (KV). Mutations in human KCNE1 cause congenital deafness and congenital long QT syndrome, an inherited predisposition to potentially life-threatening cardiac arrhythmias. Although its modulation of KCNQ1 function has been extensively characterized, many questions remain regarding KCNE1's structure and location within the channel complex. In this study, KCNE1 was overexpressed in Escherichia coli and purified. Micellar solutions of the protein were then microinjected into Xenopus oocytes expressing KCNQ1 channels, followed by electrophysiological recordings aimed at testing whether recombinant KCNE1 can co-assemble with the channel. Nativelike modulation of channel properties was observed following injection of KCNE1 in lyso-myristoylphosphatidylglycerol (LMPG) micelles, indicating that KCNE1 is not irreversibly misfolded and that LMPG is able to act as a vehicle for delivering membrane proteins into the membranes of viable cells. 1H-15N TROSY NMR experiments indicated that LMPG micelles are well-suited for structural studies of KCNE1, leading to assignment of its backbone resonances and to relaxation studies. The chemical shift data confirmed that KCNE1's secondary structure includes several alpha-helices and demonstrated that its distal C-terminus is disordered. Surprisingly, for KCNE1 in LMPG micelles, there appears to be a break in alpha-helicity at sites 59-61, near the middle of the transmembrane segment, a feature that is accompanied by increased local backbone mobility. Given that this segment overlaps with sites 57-59, which are known to play a critical role in modulating KCNQ1 channel activation kinetics, this unusual structural feature likely has considerable functional relevance.

Probing the interaction between KCNE2 and KCNQ1 in their transmembrane regions

KCNE1-KCNE5 are single membrane-spanning proteins that associate with voltage-gated potassium channels to diversify their function. Other than the KCNQ1/KCNE1 complex, little is known about how KCNE proteins work. We focus on KCNE2, which associates with KCNQ1 to form K channels critical for gastric acid secretion in parietal cells. We use cysteine (Cys)-scanning mutagenesis to probe the functional role of residues along the KCNE2 transmembrane domain (TMD) in modulating KCNQ1 function. There is an alpha-helical periodicity in how Cys substitutions along the KCNE2 TMD perturb KCNQ1 pore conductance/ion selectivity. However, positions where Cys substitutions perturb KCNQ1 gating kinetics cluster to the extracellular end and cytoplasmic half of the KCNE2 TMD. This is the first systematic perturbation analysis of a KCNE TMD. We propose that the KCNE2 TMD adopts an alpha-helical secondary structure with one face making intimate contact with the KCNQ1 pore domain, while the contacts with the KCNQ1 voltage-sensing domain appear more dynamic.

Serial perturbation of MinK in IKs implies an alpha-helical transmembrane span traversing the channel corpus

I(Ks) channels contain four pore-forming KCNQ1 subunits and two accessory MinK subunits. MinK influences surface expression, voltage-dependence of gating, conduction, and pharmacology to yield the attributes characteristic of native channels in heart. The structure and location of the MinK transmembrane domain (TMD) remains a matter of scrutiny. As perturbation of gating analysis has correctly inferred the peripheral location and alpha-helical nature of TMDs in pore-forming subunits, the method is applied here to human MinK. Tryptophan and Asparagine substitution at 23 consecutive sites yields perturbation with alpha-helical periodicity (residues 44-56) followed by an alternating impact pattern (residues 56-63). Arginine substitution across the span suggests that as few as eight sites are occluded from aqueous solution (residues 50-57). We favor a TMD model that is alpha-helical with the external portion of the span at a lipid-protein boundary and the inner portion within the channel corpus in complex interactions.

KCNE2 is colocalized with KCNQ1 and KCNE1 in cardiac myocytes and may function as a negative modulator of IKs current amplitude in the heart

BACKGROUND

In heterologous expression systems, KCNE1 and KCNE2 each can associate with KCNQ1 and exert apparently opposite effects on its channel function. KCNQ1 and KCNE1 associate to form the slow delayed rectifier I(Ks) channels in the heart. Whether KCNE2 plays any role in I(Ks) function is not clear.

OBJECTIVES

The purpose of this study was to study whether KCNE2 can associate with KCNQ1 in the presence of KCNE1 and modulate its function.

METHODS

Voltage clamp methods were used to study channel function in cardiomyocytes and in oocytes or COS-7 cells and immunocytochemistry/coimmunoprecipitation was used to study protein colocalization/association.

RESULTS

Adult rat ventricular myocytes express functional I(Ks), and KCNE2 is colocalized with KCNQ1 and KCNE1 at surface membrane and t-tubules. A detailed study of KCNQ1 modulation by KCNE2 at different KCNE2 expression levels reveals that, surprisingly, KCNE2 and KCNE1 share the major features in modulating KCNQ1 gating kinetics: slowing of activation, positive shift in the voltage range of activation, and suppression of inactivation. However, KCNE2 reduces KCNQ1 current amplitude whereas KCNE1 increases it, and KCNE2 induces a constitutively active KCNQ1 component whereas KCNE1 does not. Coimmunoprecipitation suggests that KCNQ1, KCNE1, and KCNE2 can form a tripartite complex, indicating that KCNE2 can bind to KCNQ1 in the presence of KCNE1. Coexpressing KCNE2 with KCNQ1 and KCNE1 leads to a decrease in the I(Ks) current amplitude without altering the gating kinetics.

CONCLUSION

Our data suggest that KCNE2 is in close proximity to KCNQ1 and KCNE1 in cardiomyocytes and may participate in dynamic regulation of I(Ks) current amplitude in the heart.

G-protein coupled receptor structure

Because of their central role in regulation of cellular function, structure/function relationships for G-protein coupled receptors (GPCR) are of vital importance, yet only recently have sufficient data been obtained to begin mapping those relationships. GPCRs regulate a wide range of cellular processes, including the senses of taste, smell, and vision, and control a myriad of intracellular signaling systems in response to external stimuli. Many diseases are linked to GPCRs. A critical need exists for structural information to inform studies on mechanism of receptor action and regulation. X-ray crystal structures of only one GPCR, in an inactive state, have been obtained to date. However considerable structural information for a variety of GPCRs has been obtained using non-crystallographic approaches. This review begins with a review of the very earliest GPCR structural information, mostly derived from rhodopsin. Because of the difficulty in crystallizing GPCRs for X-ray crystallography, the extensive published work utilizing alternative approaches to GPCR structure is reviewed, including determination of three-dimensional structure from sparse constraints. The available X-ray crystallographic analyses on bovine rhodopsin are reviewed as the only available high-resolution structures for any GPCR. Structural information available on ligand binding to several receptors is included. The limited information on excited states of receptors is also reviewed. It is concluded that while considerable basic structural information has been obtained, more data are needed to describe the molecular mechanism of activation of a GPCR.

Characterization of an LQT5-related mutation in KCNE1, Y81C: Implications for a role of KCNE1 cytoplasmic domain in IKs channel function

BACKGROUND

Y81C is a new long QT-5 (LQT5)-related KCNE1 mutation, which is located in the post-transmembrane domain (post-TMD) region in close proximity to three other LQT5 mutations (S74L, D76N, and W87R).

OBJECTIVE

We examine the effects of Y81C on the function and drug sensitivity of the slow delayed rectifier channel (I(Ks)) formed by KCNE1 with pore-forming KCNQ1 subunits. We also infer a structural basis for the detrimental effects of Y81C on I(Ks) function.

METHODS

Wild-type (WT) and mutant (harboring Y81C) I(Ks) channels are expressed in oocytes or COS-7 cells. Channel function and KCNQ1 protein expression/subcellular distribution are studied by techniques of electrophysiology, biochemistry, and immunocytochemistry. Ab initio structure predictions of KCNE1 cytoplasmic domain are performed by the Robetta server.

RESULTS

Relative to WT KCNE1, Y81C reduces I(Ks) current amplitude and shifts the voltage range of activation to a more positive range. Y81C does not reduce whole-cell KCNQ1 protein level or interfere with KCNQ1 trafficking to cell surface. Thus, its effects are mediated by altered KCNQ1/KCNE1 interactions in cell surface channels. Importantly, Y81C potentiates the effects of an I(Ks) activator. Preserving the aromatic or hydroxyl side chain at position 81 (Y81F or Y81T) does not prevent the detrimental effects of Y81C. Structure predictions suggest that the post-TMD region of KCNE1 may adopt a helical secondary structure.

CONCLUSION

We propose that the post-TMD region of KCNE1 interacts with the KCNQ1 channel to modulate I(Ks) current amplitude and gating kinetics. Other LQT5 mutations in this region share the Y81C phenotype and probably affect the I(Ks) channel function by a similar mechanism.

Oligomerization of the γ-Aminobutyric Acid Transporter-1 Is Driven by an Interplay of Polar and Hydrophobic Interactions in Transmembrane Helix II

The available evidence indicates that members of the neurotransmitter:sodium symporter family form constitutive oligomers. Their second transmembrane helix (TM2) contains a leucine heptad repeat proposed to be involved in oligomerization. In artificial transmembrane segments, interhelical interactions are stabilized by polar residues. We searched for these hydrogen bond donors in TM2 by mutating the five polar residues in TM2 of the gamma-aminobutyric acid transporter-1 (GAT1). We tested the ability of the resulting mutants to oligomerize by fluorescence microscopy, Foerster resonance energy transfer, and beta-lactamase fragment complementation. Of all generated mutants, only Y86A- (but not Y86F-), E101A-, E101Q-, and E101D-GAT1 were judged by these criteria to be deficient in oligomerization and were retained intracellularly. The observations are consistent with a model where the leucine heptad repeat in TM2 drives a homophilic association that is stabilized by Tyr(86) and Glu(101); Tyr(86) participates in hydrophobic stacking. Glu(101) is in the a-position of the leucine heptad repeat (where positions 1-7 are denoted a-g, and each leucine is in the central d-position). Thus, Glu(101) is in the position predicted for the hydrogen bond donor (i.e. sandwiched between Leu(97) and Leu(104), which are one helical turn above and below Glu(101)). These key residues, namely Tyr(86) and Glu(101), are conserved in related transporters from archaeae to humans; they are therefore likely to support oligomeric assembly in transporter orthologs and possibly other proteins with multiple transmembrane segments.

Membrane-inserted conformation of transmembrane domain 4 of divalent-metal transporter

Divalent-metal transporter 1 (DMT1) is involved in the intestinal iron absorption and in iron transport in the transferrin cycle. It transports metal ions at low pH ( approximately 5.5), but not at high pH (7.4), and the transport is a proton-coupled process. Previously it has been shown that transmembrane domain 4 (TM4) is crucial for the function of this protein. Here we provide the first direct experimental evidence for secondary-structural features and membrane insertions of a 24-residue peptide, corresponding to TM4 of DMT1 (DMTI-TM4), in various membrane-mimicking environments by the combined use of CD and NMR spectroscopies. The peptide mainly adopts an alpha-helical structure in trifluoroethanol, SDS and dodecylphosphocholine micelles, and dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol small unilamellar vesicles. It has been demonstrated from both Halpha secondary shifts and nuclear-Overhauser-enhancement (NOE) connectivities that the peptide is well folded into an alpha-helix from Val(8) to Lys(23) in SDS micelles at pH 4.0, whereas the N-terminus is highly flexible. The alpha-helical content estimated from NMR data is in agreement with that extracted from CD simulations. The highest helicity was observed in the anionic phospholipids [1,2-dimyristoyl- sn -glycero-3-[phospho-rac -(1-glycerol)]], indicating that electrostatic attraction is important for peptide binding and insertion into the membranes. The secondary-structural transition of the peptide occurred at pH 4.3 in the 2,2,2-trifluoroethanol (TFE) water mixed solvent, whereas at a higher pH value (5.6) in SDS micelles, DMT1-TM4 exhibited a more stable structure in SDS micelles than that in TFE in terms of changing the pH and temperature. PAGE did not show high-molecular-mass aggregates in SDS micelles. The position of the peptide relative to SDS micelles was probed by the effects of 5- and 16-doxylstearic acids on the intensities of the peptide proton resonances. The results showed that the majority of the peptide is inserted into the hydrophobic interior of SDS micelles, whereas the C-terminal residues are surface-exposed. The ability of DMT1-TM4 to assume transmembrane features may be crucial for its biological function in vivo.

Structure and dynamics of self-assembling beta-sheet peptide tapes by dynamic light scattering

Oligomeric peptides can be designed which undergo one-dimensional self-assembly in solution to form beta-sheet tapes a single molecule in thickness and micrometers in length.(1) In this paper, we present the first systematic investigation of the size, shape, dynamics, and interactions of beta-sheet tapes formed by a self-assembling 24-residue peptide, K24, in 2-chloroethanol, over a wide range of peptide concentrations c (c: 10(-7)-1 mM), using photon correlation spectroscopy. The tapes behave like semiflexible chains with persistence lengths of several hundred nanometers and much longer contour lengths, even at c approximately 0.1 nM. The polarized q-dependent light-scattering intensity I fits a model of a prolate object with major and minor axes alpha approximately 630 nm and beta approximately 40 nm. This is an unexpected result in view of the previous theoretical predictions that tapelike polymers could form oblate coinlike structures in solution.(2) This experimentally observed behavior is attributed to the pronounced twist and bend of the beta-tapes, which do not allow them to form the coinlike structures, but instead they favor the formation of elongated polymers. At c approximately 10(-2) mM, the tapes are seen to start overlapping and forming networks with unusually large mesh sizes (e.g., ca. 400 nm at 15 mciroM), much larger than those of conventional polymers. With increasing peptide concentration the mesh size decreases and the network becomes a physical gel at c approximately 0.4 mM. These semidilute solutions are characterized by one main relaxation mode associated with the cooperative diffusion of the entangled tape network, and a weaker slower mode, associated with gel cluster formation. The concentration dependence of xi (xi(c) approximately c(-0.34)) is much weaker compared to the expected scaling for Gaussian or swollen chains (xi(c) approximately c(-1) for Gaussian chains, or xi(c) approximately c(-3/4) for swollen ones), but is not inconsistent with the expected scaling for rigid rods. On the basis of the concentration dependencies of the light-scattering intensity I and of the cooperative diffusion coefficient D, the cooperative friction coefficient f(c) is found to display a stronger concentration dependence (f(c) approximately c(1.34)) than in the case of semidilute flexible and semiflexible polymer solutions (f(c) approximately c(0.5)).(3) Thus, we may conclude that the network of entangled tapes approximates in its behavior that of semirigid polymers.

Cryptdin 3 forms anion selective channels in cytoplasmic membranes of human embryonic kidney cells

Cryptdins are antimicrobial peptides secreted by Paneth cells located at the base of intestinal crypts. In addition to their antimicrobial function, cryptdins may also regulate salt and water secretion by intestinal epithelial cells. Recent work with short-circuit current measurements indicated that at least one cryptdin peptide, cryptdin 3, induces apical conductance(s) in Cl(-) secretory, including cystic fibrosis, epithelia. In the present study, we characterized the cryptdin 3-induced anion channel activity in human embryonic kidney (HEK) cells with single-channel patch-clamp techniques. The patch pipette was filled with solution containing different concentrations of cryptdin 3, and, after gigaseal formation, the channel activity was recorded with either cell-attached or inside-out patch modes. We found an anion selective channel with a conductance of 15 pS and open probability of 0.19, regardless of cryptdin 3 concentration. The mean open and closed times varied with the cryptdin 3 concentration. For cryptdin 3 concentrations of 10, 4, 1, and 0.5 microg/ml in the pipette, the corresponding mean open times were 1.2, 7.0, 9.0, and 17.4 ms and the corresponding mean closed times were 1.1, 1.6, 4.2, and 12.5 ms. These results suggest that cryptdin 3 forms anion-selective channels on the cytoplasmic membrane of HEK cells and that the kinetics of one such channel are affected by its interaction with other such channels.

Extensive neurite outgrowth and active synapse formation on self-assembling peptide scaffolds

A new type of self-assembling peptide (sapeptide) scaffolds that serve as substrates for neurite outgrowth and synapse formation is described. These peptide-based scaffolds are amenable to molecular design by using chemical or biotechnological syntheses. They can be tailored to a variety of applications. The sapeptide scaffolds are formed through the spontaneous assembly of ionic self-complementary beta-sheet oligopeptides under physiological conditions, producing a hydrogel material. The scaffolds can support neuronal cell attachment and differentiation as well as extensive neurite outgrowth. Furthermore, they are permissive substrates for functional synapse formation between the attached neurons. That primary rat neurons form active synapses on such scaffold surfaces in situ suggests these scaffolds could be useful for tissue engineering applications. The buoyant sapeptide scaffolds with attached cells in culture can be transported readily from one environment to another. Furthermore, these peptides did not elicit a measurable immune response or tissue inflammation when introduced into animals. These biological materials created through molecular design and self assembly may be developed as a biologically compatible scaffold for tissue repair and tissue engineering.

Structural studies on the effects of the deletion in the red cell anion exchanger (band 3, AE1) associated with South East Asian ovalocytosis

We have carried out a solution-state NMR study of synthetic peptides patterned on the first membrane span of normal human band 3, and the same region of the mutant band 3 present in Southeast Asian ovalocytosis (SAO) which has a nine amino acid deletion. In 1:1 (v/v) chloroform/methanol, the 42 residue normal peptide (R389-K430) consisted of three helical regions. The slow solvent exchange of backbone amide protons revealed the helix from P403 to A416 was more stable than the "cytoplasmic" N-terminal helix from P391 to A400. These helices were separated by a sharp bend at P403, which is probably located at the boundary between the cytoplasmic domain and the first transmembrane span. The SAO deletion (A400-A408) removed the bend at P403, to leave a stable helix from P391 to A416 containing the residuum of the normal first transmembrane helix and with a hydrophobic turn replaced by a polar turn in the SAO peptide. Insertion of fragments of normal band 3 and band 3 SAO into microsomal membranes was investigated using a cell free translation system. A fragment composed of the cytoplasmic domain and the putative first membrane domain of normal band 3 (B3(1)) inserted stably into the membrane. However, the corresponding fragment of band 3 SAO [SAO(1)] did not integrate stably into membranes. Our results suggest that in SAO band 3, the region of the first membrane span of normal band 3 does not integrate properly into the membrane because it lacks a sufficiently long hydrophobic segment, and the deletion also disrupts a conserved structural subdomain at the membrane surface.