Ion-Channel Formation Assisted by Electrostatic Interhelical Interactions in Covalently Dimerized Amphiphilic Helical Peptides

Systematic Design and Validation of Ion Channel Stabilization of Amphipathic α-Helical Peptides Incorporating Tryptophan Residues

Aromatic interactions such as π-π interaction and cation-π interaction are present in membrane proteins and play important roles in both structure and function. To systematically investigate the effect of aromatic residues on the structural stability and ion permeability of peptide-formed ion channels, we designed several peptides with one or two tryptophan (Trp) residues incorporated at different positions in amphipathic α-helical peptides. Circular dichroism (CD) studies revealed the preferable position of Trp residues for self-association in these designed peptides. Systematically designed di-substituted peptides with two Trps at each helix termini demonstrated intermolecular Trp-Trp interactions caused by aggregation. In the presence of liposomes, Trp on the hydrophilic face of the peptide enhanced interaction with the lipid membrane to increase the amphipathic α-helical contents. Appropriate incorporation and positioning of Trp enabled peptides to form more stable channels and had notable effects with Trp di-substituted peptides. The ion channel forming capability of a series of these peptides showed that the cation-π interactions between Trp and Lys residues in adjacent transmembrane helices contribute to remarkable stabilization of the channel structure.

Guided reconstitution of membrane protein fragments

Structural analysis by NMR of G protein-coupled receptors (GPCRs) has proven to be extremely challenging. To reduce the number of peaks in the NMR spectra by segmentally labeling a GPCR, we have developed a Guided Reconstitution method that includes the use of charged residues and Cys activation to drive heterodimeric disulfide bond formation. Three different cysteine-activating reagents: 5-5'-dithiobis(2-nitrobenzoic acid) [DTNB], 2,2'-dithiobis(5-nitropyridine) [DTNP], and 4,4'-dipyridyl disulfide [4-PDS] were analyzed to determine their efficiency in heterodimer formation at different pHs. Short peptides representing the N-terminal (NT) and C-terminal (CT) regions of the first extracellular loop (EL1) of Ste2p, the Saccharomyces cerevisiae alpha-factor mating receptor, were activated using these reagents and the efficiencies of activation and rates of heterodimerization were analyzed. Activation of NT peptides with DTNP and 4-PDS resulted in about 60% yield, but heterodimerization was rapid and nearly quantitative. Double transmembrane domain protein fragments were biosynthesized and used in Guided Reconstitution reactions. A 102-residue fragment, 2TM-tail [Ste2p(G31-I120C)], was heterodimerized with CT-EL1-tail(DTNP) at pH 4.6 with a yield of ∼75%. A 132-residue fragment, 2TMlong-tail [Ste2p(M1-I120C)], was expressed in both unlabeled and (15)N-labeled forms and used with a peptide comprising the third transmembrane domain, to generate a 180-residue segmentally labeled 3TM protein that was found to be segmentally labeled using [(15)N,(1)H]-HSQC analysis. Our data indicate that the Guided Reconstitution method would be applicable to the segmental labeling of a membrane protein with 3 transmembrane domains and may prove useful in the preparation of an intact reconstituted GPCR for use in biophysical analysis and structure determination.

Phosphorylation of Grb14 BPS domain by GSK-3 correlates with complex forming of Grb14 and insulin receptor

Growth factor receptor-bound protein 14 (Grb14) interacts with insulin receptor (IR) through the between PH and SH2 (BPS) domain. Grb14-IR complex formation is initiated by insulin stimulation, and the binding event results in the inhibition of insulin signalling. Thus, Grb14 is regarded as an endogenous suppressor of insulin signal transduction; however, there are no studies describing the mechanism whereby Grb14-IR complex formation is suppressed in the absence of insulin stimulation. In the present study, multiple phosphorylation motifs for glycogen synthase kinase 3 (GSK-3) were identified within the Grb14 BPS domain (Ser(358), Ser(362) and Ser(366) of human Grb14). Pharmacological inhibition as well as knockdown of GSK-3 facilitated complex formation between Grb14 and IR, implicating GSK-3 activity in regulating Grb14-IR binding. In situ proximity ligation assay and in vitro kinase assays of phosphopeptides suggested that serine residues in the BPS domain would be substrates for GSK-3. The kinase assays also indicated phosphoserine 370 (in human Grb14) was required for the phosphorylation of Ser(358), Ser(362) and Ser(366) by GSK-3. Grb14-IR binding was also facilitated by replacement of the serines with Ala. We also observed that Ser(366) of endogenous Grb14 in Hep G2 cell was phosphorylated and the phosphorylation was influenced by treatments with insulin, as well as the GSK-3 inhibitor.

Protein phosphatase 2A dephosphorylates phosphoserines in nucleocytoplasmic shuttling and secretion of high mobility group box 1

High mobility group box 1 (HMGB1), a non-histone chromosomal protein, is a proinflammatory cytokine. There are two known pathways for the release of HMGB1 into the extracellular milieu-passive and active. The passive pathway is attributable to cell death from damage or necrosis, and the active pathway is secretion from immunocompetent cells activated by proinflammatory stimuli. Recent studies have shown that post-translational modifications of HMGB1, including phosphorylation, are involved in the relocation of HMGB1 to the cytoplasm and subsequent secretion. With regard to the HMGB1 phosphorylation, Youn and Shin [Nucleocytoplasmic shuttling of HMGB1 is regulated by phosphorylation that redirects it toward secretion. J Immunol 2006;177:7889-97] reported that treatment of the murine macrophage RAW264.7 with okadaic acid resulted in nucleocytoplasmic translocation and secretion of HMGB1. Herein, we demonstrate the physical interaction between HMGB1 and protein phosphatase 2A (PP2A) in the RAW264.7. The results of in vitro phosphatase assay further indicate that PP2A dephosphorylates specific phosphoserine residues within one of the two nuclear localization signals (NLSs) of HMGB1. The cytoplasmic relocation of HMGB1 through PP2A inhibition was markedly suppressed by replacement of the Ser residues within the NLS with Ala. These consequences imply that PP2A correlates in the nucleocytoplasmic shuttling of HMGB1.

Caveolin-1 is a competitive inhibitor of heme oxygenase-1 (HO-1) with heme: identification of a minimum sequence in caveolin-1 for binding to HO-1

Heme oxygenase (HO) catalyzes the O(2)-dependent degradation of heme to biliverdin IXα, carbon monoxide (CO), and free ferrous iron through a multistep mechanism. Electrons required for HO catalysis in mammals are provided by NADPH-cytochrome P450 reductase. Recently, Kim et al. reported for the first time that HO, especially inducible HO-1, appears in caveolae and showed that caveolin-1, a principal isoform of the caveolin family, physically interacts with HO-1 [ Jung , N. H. et al. ( 2003 ) IUBMB Life 55 , 525 - 532 ; Kim , H. P. et al. ( 2004 ) FASEB J. 18 , 1080 - 1089 ]. In the present study, we confirmed by immunoprecipitation experiments that rat HO-1 and rat caveolin-1 (residues 1-101) directly interact with each other and that the HO-1 activity is inhibited by caveolin-1 (1-101). The 82-101 residues of caveolin-1 (CAV(82-101)), called the caveolin scaffolding domain, play essential roles in caveolin-related protein-protein interactions. The HO-1 activity is also inhibited by CAV(82-101) in a competitive manner with hemin, and a hemin titration experiment showed that CAV(82-101) interferes with hemin binding to HO-1. The enzyme kinetics and surface plasmon resonance experiments gave comparable K(i) and K(D) values of 5.2 and 1.0 μM for CAV(82-101), respectively, with respect to the interaction with HO-1. These observations indicated that CAV(82-101) and hemin share a common binding site within the HO-1 protein. The identified caveolin binding motif (FLLNIELF) of rat HO-1 is incomplete compared to the proposed consensus sequence. The affinity between HO-1 and CAV(82-101), however, was almost completely or remarkably eliminated by replacement of Phe(207) and/or Phe(214) with Ala, indicating that HO-1 binds to caveolin-1 via this motif. Among the peptide fragments derived from CAV(82-101), i.e., CAV(82-91), CAV(87-96), CAV(92-101), and CAV(97-101), CAV(92-101) and CAV(97-101) are able to inhibit the HO-1 activity to a similar extent; thus, the five-amino acid sequence (residues 97-101) is considered to be a minimum sequence for binding to HO-1.

Modifications on amphiphilicity and cationicity of unnatural amino acid containing peptides for the improvement of antimicrobial activity against pathogenic bacteria

The widespread natural sources-derived cationic peptides have been reported to reveal bacterial killing and/or growth-inhibiting properties. Correspondingly, a number of artificial peptides have been designed to understand antibacterial mechanism of the cationic peptides. These peptides are expected to be an alternative antibiotic against drug-resistant pathogenic bacteria because major antimicrobial mechanism of cationic peptides involves bacterial membrane disorder, although those availabilities have not been well evaluated. In this study, cationic peptides containing Aib were prepared to evaluate the availability as an antimicrobial agent, especially against representative pathogenic bacteria. Among them, BRBA20, consisting of five repeated Aib-Arg-Aib-Ala sequences, showed strong antibacterial activity against both Gram-negative and Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus. Additionally, growth of Serratia marcescens and multidrug-resistant Pseudomonas aeruginosa, known as proteases-secreting pathogenic bacteria, were also completely inhibited by BRBA20 under 20 µg/ml peptide concentrations. Our results suggested availabilities of Aib-derived amphiphilicity and protease resistance in the design of artificial antimicrobial peptides. Comparing BRBA20 with BKBA20, it was also concluded that Arg residue is the preferred cationic source than Lys for antimicrobial action of amphiphilic helices.

A comparative study on conformation and ligand binding of the neuronal uncoupling proteins

Mitochondrial uncoupling proteins of the nervous system (UCPs 2, 4, and 5) have potential roles in the function and protection of the central nervous system (CNS). In the absence of structural information, conformations of the hexahistidine-tagged versions of all five human UCPs in liposomes were investigated for the first time, using far- and near-UV CD and fluorescence spectroscopy. Highly pure UCPs 1-5 were reconstituted in detergents and stable small unilamellar vesicles, appropriate for spectroscopic studies. All UCPs formed dominantly helical conformations in negatively charged phospholipid vesicles (palmitoyloleoylphosphatidylcholine/palmitoyloleoylphosphatidylglycerol, 7:3 molar ratio). UCPs 2 and 5 exhibited comparable helical conformations with possible association in lipid bilayers, whereas UCP4 had a different helical profile that can be related to its less associated form. Interaction of reconstituted UCPs with GDP and GTP, inhibitors of the prototypic UCP1, was detected by near-UV CD and fluorescence spectroscopy, utilizing the sensitivity of these techniques to microenvironments around Trp residues close to the nucleotide binding site. Binding of UCP4 to purine nucleotides was also different from other UCPs. Binding of fatty acids, activators of proton transport in UCPs, to UCPs could not be unambiguously detected, implying a nonbinding conformation/orientation of the proteoliposomes. Interaction of CoA with UCPs was comparable to nucleotide binding, suggesting a possible binding of this molecule at the nucleotide binding site. Despite dissimilar primary sequences, neuronal UCPs share common structural and functional properties with UCPs 1 and 3, supporting a common physiological role in addition to their specific roles in the CNS.