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J F Collawn

Publications and source records attributed to J F Collawn.

35 records · Page 2Linked to original sources
PubMed1989

Substitutions engineered by chemical synthesis at three conserved sites in mitochondrial cytochrome c. Thermodynamic and functional consequences.

Analogues of the 39-residue CNBr fragment of horse cytochrome c (66-104) have been prepared by total chemical synthesis. Conformationally assisted ligation of these peptides with the native cytochrome c fragment 1-65 (homoserine lactone form) occurred in high yield. Semisynthetic protein molecules of the expected molecular weight were obtained that had folded structures similar to the native molecule as shown by spectral properties and by cross-reactivity with a panel of monoclonal antibodies sensitive to the three-dimensional integrity of cytochrome c. Point mutations were introduced into the horse sequence at three strongly conserved sites: Tyr67, Thr78, and Ala83. The contributions of these 3 residues to the stability of the heme crevice were estimated by titration of the 695 nm absorption due to coordination of ferric iron by the sixth ligand methionine sulfur. The roles of these residues in catalysis of electron transfer and in establishing the value of the redox potential of cytochrome c were also investigated. The hydroxyl group of Tyr67 modulates the spectral properties of the heme and has a profound influence on its redox properties, but hydrogen bonding involving this phenolic hydroxyl does not stabilize the heme crevice. In contrast, we find that Thr78 is strongly stabilizing and that asparagine is not an adequate substitute for this residue because of the greater entropic cost of burying its side chain. The low biological activity of analogues modified at this position, despite normal redox potentials, imply a role for Thr78 in the electron transfer mechanism. The replacement of Ala83 by proline induces a similar phenomenon. An involvement of this residue in the catalysis of electron transfer provides an explanation of the low reactivity of plant mitochondrial cytochromes c in mammalian redox systems.

Amino Acid Sequence↗
PubMed1989

An analysis of the physical properties of peptides that influence the pigeon cytochrome c specific T lymphocyte response.

The ability to modify T cell responses was analyzed using synthetic peptide analogues of the T cell determinant for pigeon cytochrome c. Although the B10.A T cell proliferative response is directed to residues 95-104, residues to the amino-terminal side of this determinant influence antigen-specific T cell recognition. The proposed role of this non-determinant leader sequence has been to stabilize the core determinant in a helical conformation. Previous studies from our laboratory, however, using non-native leader sequences that were designed to examine the changes to T cell recognition invoked when the determinant was made more or less helical, amphipathic, or lipid binding in character than the native determinant. The structure of each analogue in aqueous, non-polar (TFE) and lipid environments was determined by circular dichroism. The ability of each antigen analogue to bind to phospholipid membranes and to stimulate two different pigeon cytochrome c T cell hybridomas, 2B4 and 22.D11, was also investigated. Our findings suggest that neither helicity or amphipathicity are necessary features of T cell recognition but that electrostatic interactions involving either the lipid membrane or the I-Ek molecule may influence T cell stimulation.

Animals↗
PubMed1988

Monoclonal antibodies as probes of conformational changes in protein-engineered cytochrome c.

Determination of the nature of the antigen-antibody complex has always been the ultimate goal of three-dimensional epitope mapping studies. Various strategies for epitope mapping have been employed which include comparative binding studies with peptide fragments of antigens, binding studies with evolutionarily related proteins, chemical modifications of epitopes, and protection of epitopes from chemical modification or proteolysis by antibody shielding. In this study we report the use of protein engineering to modify residues in horse cytochrome c that are in or near the epitopes of four monoclonal antibodies specific for this protein. The results demonstrate not only that site-specific changes in the antigen binding site dramatically affect antibody binding, but, more importantly, that some of the site-specific changes cause local and long-range perturbations in structure that are detected by monoclonal antibody binding at other surfaces of the antigen. These findings emphasize the role of native conformation in the stabilization of the interaction between protein antigens and high affinity monoclonal antibodies. Furthermore, the results demonstrate that monoclonal antibodies are more sensitive probes of changes in conformation brought about by protein engineering than low resolution spectroscopic methods such as circular dichroism, where similar spectra are observed for all the analogues. These findings suggest a role for monoclonal antibodies in detecting conformational changes invoked by nonconservative amino acid substitutions or substitutions of evolutionarily conserved residues in protein-engineered or recombinant proteins.

Animals↗
PubMed1987

Isolation and partial amino acid sequence of three subunit species of porcine spleen ferritin: evidence of multiple H subunits.

A partial amino acid sequence for three different subunits of the iron storage protein, ferritin, has been determined. Ferritin (Mr approximately 480,000) was isolated from porcine spleen and dissociated into its component subunits (Mr approximately 20,000). The subunits, in turn, were separated into three fractions by reversed-phase HPLC. The fractions appeared to be of equal size by sedimentation velocity, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and size-exclusion chromatography in 6 M guanidinium chloride. All three fractions were shown to be monomeric and to have no covalently attached carbohydrate (J. F. Collawn et al. (1984) Arch. Biochem. Biophys. 233, 260-266). Determination of the amino acid sequence of the C-terminal 70-80 residues from each of the fractions demonstrated three different sequences. Comparison with human liver H and L subunit sequences indicates that two of the porcine ferritin subunits are H-type subunits and one is an L-type subunit. Application of the Chou-Fasman algorithm on the three partial sequences suggests that these respective regions from each of the three subunits would probably adopt the same conformation.

Amino Acid Sequence↗
PubMed1986

Evidence that H-enriched human placental ferritin is structurally similar to L-enriched ferritins of other tissues.

Ferritin was purified from normal full-term placenta, and the native structure and subunit composition were characterized. Reversed-phase high-performance liquid chromatographic analysis of the placental ferritin subunits suggested the presence of three subunit types. Using acid urea gel electrophoresis and amino acid analysis, these subunits were tentatively identified as two H-type and one L-type. The relative proportions of the subunit types were approx. 23% H-1, 33% H-2 and 44% L. The native structure of placental ferritin as judged by circular dichroism and fluorescence spectroscopy was quite similar to that of ferritin isolated from horse spleen, a source that is composed predominantly of L subunits. These results are consistent with a ferritin tetracosameric structure whose H and L subunits fit into 24 equivalent sites interchangeably because the secondary and tertiary structures of the two subunit types are very similar.

Amino Acids↗
PubMed1985

Hydrogen-tritium exchange by apoferritin and ferritin.

The out-exchange kinetics of tritium from apoferritin, ferritin of various iron contents, and apoferritin subunits were examined. The exchange kinetics indicated no detectable conformational differences in the tetracosamer with and without hydrous ferric oxide in the internal cavity of the molecule. The data for apoferritin subunits were markedly different from those for the tetracosameric state. The exchange kinetics for apoferritin were consistent with a rapid exchange of water between the internal cavity of the protein and the bulk solvent outside the protein shell.

Apoferritins↗
PubMed1985

A comparison by HPLC of ferritin subunit types in human tissues.

Ferritin was isolated from human liver and spleen. Reversed phase high performance liquid chromatography of the ferritin subunits from each tissue yielded the same three chromatographic fractions. Physical and chemical characterization of the three fractions indicated that they represented at least two, perhaps three, chemically distinct subunits.

Amino Acids↗
PubMed1984

A chemical and physical comparison of ferritin subunit species fractionated by high-performance liquid chromatography.

Selected chemical and physical properties were measured for different forms of ferritin subunits which had been separated by reverse-phase high-performance liquid chromatography. Ferritin subunits from porcine spleen behaved, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, as though they were approximately Mr 2000 larger than equine spleen ferritin, whereas no difference in size was observed by gel chromatography in 6 M guanidinium chloride. All subunit species exhibited similar isoelectric focusing properties. In contrast to previous reports, no carbohydrate could be found associated with any of the isolated subunit species. Thus, the aberrant behavior of the porcine ferritin subunits between the two empirical molecular weight estimation methods appears to be the result of factor(s) other than protein intrinsic charge or covalently attached carbohydrate.

Animals↗
PubMed1984

The distribution of ferritin subunit types in porcine tissues.

Ferritin was isolated from porcine heart, liver and spleen. Reversed phase high performance liquid chromatography of the ferritin subunits yielded three chromatographic fractions. The relative proportions of the three chromatographic fractions were different for each tissue ferritin. These results support the model which proposes a combination of (at least) two subunit types as the basis for the existence of isoferritins.

Amino Acids↗
PubMed1983

A single column method to measure hydrogen--tritium exchange by gel chromatography.

Employment of a commercially integrated gel chromatography system together with the utilization of cross-linked polyacrylamide as the chromatographic medium simplifies the methodology of hydrogen--tritium exchange measurements. The system described allows the execution of hydrogen--tritium exchange measurements with as little as 0.5 mg protein per time point and with only a single pass of sample through the column for out-exchange times of less than 1 min to at least 24 h. The accuracy and precision of this system are comparable to those of existing methodologies.

Chemical Phenomena↗
PubMed1982

Purification and macromolecular properties of a sialic acid-specific lectin from the slug Limax flavus.

A lectin (LFA) which is highly specific for sialic acid has been purified from the slug Limax flavus by a combination of ammonium sulfate fractionation and affinity chromatography on bovine submaxillary mucin coupled to Sepharose 4B. The affinity-purified lectin appeared homogeneous by electrophoresis in the presence of sodium dodecyl sulfate. Below 1 mg/ml at pH 7, LFA exists as a species of Mr = 44,000 which is composed of two equal sized subunits. Above 1 mg/ml, the protein solution was observed to behave as a rapidly associating-dissociating system. N-acetylneuraminic acid and N-glycolylneuraminic acid gave a 50% inhibition of agglutination of erythrocytes by LFA at 0.13 and 0.81 mM, respectively. Galactose, N-acetylgalactosamine, galactosamine, glucose, N-acetylglucosamine, glucosamine, mannose, arabinose, xylose, fucose, glucuronic acid, alpha-methyl-D-glucoside, alpha-methyl-D-mannoside, lactose, and sucrose were ineffective inhibitors at concentrations up to 10-25 mM. Bovine submaxillary mucin, a sialoprotein, was a potent inhibitor of hemagglutination by LFA. Upon treatment of the mucin with neuraminidase, loss of inhibitory activity was observed which was proportional to the loss of sialic acid from the mucin.

Amino Acids↗
PubMed1980

Behavior of glycopolypeptides with empirical molecular weight estimation methods. 1. In sodium dodecyl sulfate.

The influence of the presence of oligosaccharide branches was examined with respect to the behavior of glycopolypeptides in empirical molecular weight estimation methods in the presence of sodium dodecyl sulfate (NaDodSO4). This examination was conducted by comparing the gel chromatographic and gel electrophoretic behaviors in the presence of NaDodSO4 of 13 glycopolypeptides of known chemical and physical properties to those of regular polypeptides. Errors in the gel chromatographic molecular weight for glycopolypeptides in NaDodSO4 varied from -22% to +10% and indicated that the hydrodynamic behavior of the glycopolypeptide--NaDodSO4 complex could not be correlated with the amount of carbohydrate in the glycopolypeptide. NaDodSO4 binding measurements on a number of the glycopolypeptides suggest that the polypeptide moiety binds the nominal weight ratio of NaDodSO4, while the carbohydrate portion exhibits little or no NaDodSO4 binding. As has been reported by others, the polyacrylamide gel electrophoretic behavior of glycopolypeptide--NaDodSO4 complexes yielded abnormally high molecular weight estimates. In general, the error of these estimates diminished with decreasing porosity of the gel; however, each glycopolypeptide behaved in a unique fashion. Treatment of the electrophoretic data by any of several empirical means provided no reliable way to correct for the glycopolypeptides' aberrant behavior.

Chromatography, Gel↗
PubMed1980

Behavior of glycopolypeptides with empirical molecular weight estimation methods. 2. In random coil producing solvents.

The effects of oligosaccharide branch chains on the hydrodynamic behavior of reduced glycopolypeptides was examined by gel chromatography in random coil producing solvents. This entailed a comparison of the gel chromatographic behavior in the presence of concentrated guanidinium chloride of 16 glycopolypeptides of known physical and chemical properties to that of regular polypeptides. For most of the glycopolypeptides employed, the presence of oligosaccharide branches sufficiently perturbed the dimensions of the unfolded glycopolypeptide such that its effective hydrodynamic radius was the same as that of a linear polypeptide of the same total mass. For this reason, gel chromatography in random coil producing solvents appears to be the most reliable empirical method to obtain a first approximation of the molecular weight of a glycopolypeptide. Glycopolypeptides rich in N-acetylneuraminic acid, and thus possessing low isoionic points, exhibited more pronounced deviations in their electrophoretic behavior in the presence of 8 M urea than those glycopolypeptides whose ionic properties were similar to those of the polypeptide standards employed.

Carbohydrate Conformation↗
PubMedDate not supplied

Stabilization of helical structure in two 17-residue amphipathic analogues of the C-terminal peptide of cytochrome C.

The conformations of two 17-residue peptide analogues derived from the C-terminal sequence of pigeon cytochrome c (native sequence = KAERADLIAYLKQATAK) were examined in aqueous and lipid environments by CD spectroscopy. The two analogues, KKLLKKLIAYLKQATAK (K peptide) and EELLEELIAYLKQATAK (E peptide), were made amphipathic with respect to helical segregation by substituting a 6-residue sequence at the N-terminus of the native peptide. Their structures were compared to the native peptide under aqueous conditions of varying pH and temperature, and in the presence of liposomes composed of phosphatidylcholine and phosphatidylserine in the ratio of 9:1. The results indicated that the native peptide remains unstructured under all the conditions examined even though this region of the native molecule is surface exposed and helical. The E peptide, however, was helical under aqueous conditions at 25 degrees C from pH 2-10 with a maximum helicity at pH 4 (54% helix from analysis of CD data). The ellipticity of the E peptide at pH 4 and 8 was concentration dependent, indicating an aggregation phenomenon. In studies in which the CD spectrum was measured at different temperatures, the E peptide became more helical at lower temperatures at pH 4 but not at pH 8. Upon interaction with a lipid membrane in the form of liposomes, there appeared to be a slight destabilization in the structure of the E peptide. The K peptide in an aqueous environment behaved like the native peptide in that it was structureless at all pHs and temperatures examined. In the presence of liposomes, however, this peptide had a high helical content (75% helix from analysis of CD data). These findings suggest that while stabilization of the helix dipole with negative charges at the N-terminus are important in inducing helical conformation in the E peptide, hydrophobic interactions created during aggregation appear to provide the principal stabilizing force. The results with the K peptide demonstrate that the positive N-terminal sequence of this peptide is able to interact with the negatively charged head groups in the phospholipid membrane in such a fashion as to stabilize a helical structure that is not apparent in an aqueous environment alone.

Amino Acid Sequence↗
PubMedDate not supplied

Structural requirements for high efficiency endocytosis of the human transferrin receptor.

Wild-type and mutant human transferrin receptors (TR) have been expressed in chicken embryo fibroblasts using a helper-independent retroviral vector. By functional studies of the mutant TRs, we have identified the tetrapeptide sequence, YXRF, in the cytoplasmic tail of the receptor as the internalization signal required for high efficiency endocytosis and shown that transplanted internalization signals from the low density lipoprotein receptor (LDLR) and the cation-independent mannose-6-phosphate receptor (Man-6-PR) are able to promote rapid internalization of the human TR. A six-residue LDLR signal, FDNPVY, is required for activity in TR, whereas a four-residue Man-6-PR signal, YSKV, is sufficient. These data indicate that internalization signals are interchangeable self-determined structural motifs and that signals from type I membrane proteins are active in a type II receptor. Putative internalization signals in the cytoplasmic tails of other receptors and membrane proteins can be identified based on the sequence patterns of the LDLR, Man-6-PR, and TR signals. Two such putative four-residue internalization signals, one from the poly-Ig receptor and one from the asialoglycoprotein receptor, were tested for activity by transplantation into TR and were found to promote high efficiency internalization. These results suggest that an exposed tight turn is the conformational motif for high efficiency endocytosis.

Amino Acid Sequence↗