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.

Conformational properties of the prion octa-repeat and hydrophobic sequences.

We have used circular dichroism to study synthetic peptides from two important regions of the prion protein: the N-terminal octa-repeat domain and a highly conserved hydrophobic section. Our results show that the octa-repeat sequence in free solution can adopt a non-random, extended conformation with properties similar to the poly-L-proline type II left-handed helix. We also show that the conformation can be changed by temperature, organic solvents (e.g. acetonitrile) and on binding to phospholipid vesicles. We compared CD data from two peptides corresponding to the hydrophobic region between residues 106 and 136 which contained either methionine or valine at position 129. This variation represents a common polymorphism in humans which has been shown to influence predisposition towards iatrogenic and sporadic CJD. There was no detectable difference between the CD spectra of these peptides irrespective of the solvent conditions we used.

De novo design and structure-activity relationships of peptide emulsifiers and foaming agents.

A series of eight amphipathic peptides (8, 11, 15, 2 x 18, 22, 26, 29 amino acids in length) were designed to investigate the effects of amino acid composition, peptide length and secondary structure on surface activity assessed as emulsification and foaming activity. The potential for alpha-helix formation at the hydrophobic/hydrophilic interface was maximized through the use of helix-forming amino acids, a relatively large hydrophobic surface of 200 degrees of arc and ion pairs between basic and acidic amino acids on the hydrophilic surface. Emulsification activity increased rapidly between 11 and 22 residues as alpha-helicity in aqueous solution increased. Despite their small size, the peptides produced exceptionally stable emulsions, compared with proteins. Foaming activity was enhanced by the presence of aromatic amino acids and the activity of the best peptide examined was superior to that of bovine serum albumin and beta-lactoglobulin.

Further characterization of the acid-soluble phosphoprotein (SDS/PAGE apparent molecular mass of 22 kDa) in rat fat-cells by peptide sequencing and immuno-analysis: effects of insulin and isoprenaline.

1. Earlier studies have shown that exposure of fat-cells to insulin results in the rapid increased phosphorylation of an acid-soluble protein which migrates as a doublet on SDS/PAGE with an apparent molecular mass of close to 22 kDa; agents such as isoprenaline, which increase cell concentrations of cyclic AMP, also increase phosphorylation, but to a lesser extent [Belsham, Brownsey, Hughes and Denton (1980) Diabetologia 18, 307-312; Diggle and Denton (1992) Biochem. J. 282, 729-736]. 2. The protein has been purified from rat epididymal adipose tissue, and the sequences of six tryptic peptides were determined. All six peptides are present in the deduced sequence of a protein of similar properties, designated PHAS-I by Hu, Pang, Kong, Velleca and Lawrence [(1994) Proc. Natl. Acad. Sci. U.S.A. 91, 3730-3734]. Hence the proteins are the same or extremely similar. 3. A rabbit anti-peptide antibody has been raised against one of the peptides (AGGDESQFEMD). The antibody was found to be highly specific for the phosphorylated and non-phosphorylated forms of the acid-soluble 22 kDa protein in Western blots and by immunoprecipitation. Studies with the antibody preparation have shown that both phosphorylated and non-phosphorylated forms of the protein appear to be exclusively located in the cytoplasm, and that exposure of cells to isoprenaline causes increased phosphorylation of the same acid-soluble 22 kDa protein as does insulin treatment. 4. Western blots carried out with the antibody preparation indicate that the protein is also present in other insulin-sensitive tissues, including liver, skeletal muscle, heart and brown adipose tissue. The protein was also detected in lung and spleen, but not brain and kidney. It is concluded that the protein may play an important role in some of the actions of insulin.

The solution structures of the first and second transmembrane-spanning segments of band 3.

We have studied the structures of synthetic peptides which correspond to the proposed first and second membrane-spanning segments of the human red cell anion transporter (band 3). The peptides, which were acetylated at their N-termini and amidated at the C-termini, comprise the 20 amino acids of residues 405-424 and 21 amino acids of residues 436-456 of the human band 3 sequence. The solution structures of the peptides in trifluoroethanol were studied by two-dimensional NMR spectroscopy. Characteristic NOEs were observed indicating that the peptides adopted a predominantly alpha-helical structure in trifluoroethanol solution. Dynamical simulated annealing using the program XPLOR was employed for the structure calculations. The amide exchange rates in trifluoroethanol have also been measured and are consistent with an alpha-helical structure for the peptides.

NMR solution structure of a cytoplasmic surface loop of the human red cell anion transporter, band 3.

The membrane domain of the human red cell anion transport protein, band 3, is too large to be studied by solution nuclear magnetic resonance spectroscopy (NMR), and its amphiphilic nature requires the use of detergents for solubilization. An alternative approach is to divide the protein into smaller (trans-membrane or surface loop) domains for NMR study. We report the structure of a 46-residue synthetic peptide that corresponds to the cytoplasmic surface loop connecting the putative 12th and 13th trans-membrane spans (residues 796-841) in the 14 span model of band 3. This peptide was shown by circular dichroism (CD) to be 38% helical in 30% trifluoroacetic acid. Two regions of helix (one close to the N-terminus of the peptide and one close to the C-terminus of the peptide) were identified by NMR. Long-range nuclear Overhauser effect (NOE) cross-peaks showed the two helices to be in near proximity. The helices were separated by a proline-rich loop that exhibited local order but was mobile with respect to the rest of the peptide. We discuss how the NMR structure of this loop fits the current models of band 3 structure and topology and the results of recent mutagenesis experiments. A cyclic version of this peptide was synthesized and studied by CD, but NMR studies were not possible due to the low solubility of this peptide.

Spectroscopic studies of the interaction of Ca2+-ATPase-peptides with dodecyl maltoside and its brominated analog.

The transmembrane sector of sarcoplasmic reticulum Ca2+-ATPase comprises ten putative transmembrane spans (M1-M10) in current topology models. We report here the structure and properties of three synthetic peptides with a single Trp representing the M6 and M7 regions implicated in Ca2+ binding: peptide M6 (amino acid residues 785-810), peptide M7-L (amino acid residues 808-847) corresponding to loop 6-7 and the majority of span M7, and peptide M7-S (amino acid residues 818-847) which contains a shorter version of loop 6-7 than M7-L. After uptake of the peptides in the hydrophobic environment of dodecyl maltoside micelles, the peptides gain a significant amount of secondary structure, as indicated by their CD spectra. However, the alpha-helical content of M6 is lower than would be expected for a classical transmembrane segment. For M7-L peptide, the L6-7 loop is subject to specific proteolytic cleavage by proteinase K, as in intact Ca2+-ATPase. The formation of the peptide-detergent complexes was followed from the resulting fluorescence intensity changes, either enhancement using n-dodecyl beta-D-maltoside or quenching using the recently introduced brominated analog of n-dodecyl beta-D-maltoside: 7,8-dibromododecyl beta-maltoside [de Foresta, B., Legros, N., Plusquellec, D., le Maire, M. & Champeil, P. (1996) Eur J. Biochem. 241, 343-354]. Our results indicate that M7-L and M7-S are completely taken up by the detergent micelles. In contrast, the M6 peptide, which is highly water soluble, is more loosely associated with the detergent, as is also demonstrated by size-exclusion chromatography. The location of Trp in micelles was evaluated from the quenching observed in mixed micelles of n-dodecyl beta-D-maltoside/7,8-dibromododecyl beta-maltoside, using tryptophan octyl ester and solubilized Ca2+-ATPase as reference compounds. We conclude that W832 in M7 appears to be located near the surface of the micelle, in agreement with its membrane interfacial localization predicted in most Ca2+-ATPase topology models. In contrast, our data suggest that W794 in M6 has a deeper insertion in the micelle although not to the extent predicted by current models of Ca2+-ATPase and the rather short alpha-helix span of M6 may lead to exposure of a significant part of the C-terminal of this peptide to the micelle surface. The results are discussed in relation to the proposed roles of these membrane segments in active transport of Ca2+ ions, in particular, the demonstration that M6 does not behave as a classical transmembrane helix may be correlated with the evidence, from site-directed mutagenesis, that this transmembrane segment should be essential in Ca2+ binding.