Search PubMed⌕ Search

Biomedical subjects

L J Stern

Publications and source records attributed to L J Stern.

At least 37 records · Page 2Linked to original sources
PubMed1999

Sodium dodecyl sulfate stability of HLA-DR1 complexes correlates with burial of hydrophobic residues in pocket 1.

Certain class II MHC-peptide complexes are resistant to SDS-induced dissociation. This property, which has been used as an in vivo as well as an in vitro peptide binding assay, is not understood at the molecular level. Here we have investigated the mechanistic basis of SDS stability of HLA-DR1 complexes by using a biosensor-based assay and SDS-PAGE with a combination of wild-type and mutant HLA-DR1 and variants of hemagglutinin peptide HA306-318. Experiments with wild-type DR1 along with previously published results establish that the SDS-stable complexes are formed only when the hydrophobic pocket 1 (P1) is occupied by a bulky aromatic (Trp, Phe, Tyr) or an aliphatic residue (Met, Ile, Val, Leu). To further explore whether the SDS sensitivity is primarily due to the exposed hydrophobic regions, we mutated residue beta Gly86 at the bottom of P1 to tyrosine, presumably reducing the depth of the pocket and the exposure of hydrophobic residues and increasing the contacts between subunits. In direct contrast to wild-type DR1, the peptide-free mutant DR1 exists as an alpha/beta heterodimer in SDS. Moreover, the presence of a smaller hydrophobic residue, such as alanine, as P1 anchor with no contribution from any other anchor is sufficient to enhance the SDS stability of the mutant complexes, demonstrating that the basis of SDS resistance may be localized to P1 interactions. The good correlation between SDS sensitivity and the exposure of hydrophobic residues provides a biochemical rationale for the use of this assay to investigate the maturation of class II molecules and the longevity of the complexes.

Amino Acid Sequence↗
PubMed1999

Empty and peptide-loaded class II major histocompatibility complex proteins produced by expression in Escherichia coli and folding in vitro.

The human class II major histocompatibility complex protein HLA-DR1 has been expressed in Escherichia coli as denatured alpha and beta subunits and folded in vitro to form the native structure. DR1 folding yields are 30-50% in the presence or absence of tight-binding antigenic peptides. The protein produced in this manner is soluble and monomeric with the expected apparent molecular weight. It reacts with conformation-sensitive anti-DR antibodies and exhibits peptide-dependent resistance to SDS-induced chain dissociation and to proteolysis as does the native protein. The observed peptide specificity and dissociation kinetics are similar to those of native DR produced in B-cells and finally the protein exhibits circular dichroism spectra and cooperative thermal denaturation as expected for a folded protein. We conclude that the recombinant DR1 has adopted the native fold. We have folded DR1 in the absence of peptide and isolated a soluble, peptide-free alphabeta-heterodimer. The empty DR1 can bind antigenic peptide but exhibits altered far UV-circular dichroism and thermal denaturation relative to the peptide-bound form.

Amino Acid Sequence↗
PubMed1999

Substitution of aspartic acid at beta57 with alanine alters MHC class II peptide binding activity but not protein stability: HLA-DQ (alpha1*0201, beta1*0302) and (alpha1*0201, beta1*0303).

In class II major histocompatibility complex (MHC) proteins, residue beta57 is usually aspartic acid. Alleles carrying serine, valine, or alanine at this position are strongly correlated with the development of insulin-dependent diabetes mellitus (IDDM). Asp(beta)57 participates in a conserved salt bridge that bridges the alpha and beta subunits in the peptide-binding site. It has been proposed that the correlation between IDDM and MHC alleles lacking Asp(beta)57 may be due to an instability of the protein caused by loss of this salt bridge. Using a pair of HLA-DQ proteins (alpha1*0201, beta1*0302) and (alpha1*0201, beta1*0303) differing only in having aspartic acid or alanine at position beta57, we show that the polymorphism does not have a significant effect on protein stability for either the empty or peptide-loaded forms. However, the circular dichroism spectra indicate that empty and peptide-loaded Alabeta57 proteins display slightly different secondary structures relative to their Aspbeta57 counterparts. A set of three peptides shows different binding affinities for DQ(alpha1*0201, beta1*0302) relative to DQ(alpha1*0201, beta1*0303). We propose that substitution of Asp(beta)57 residue causes a local rearrangement within the DQ peptide-binding site that alters the peptide-binding specificity. This rearrangement may help to explain the previously observed differences in SDS stability between Asp and non-Asp(beta)57 DQ proteins.

Alanine↗
PubMed1998

Comparison of x-ray crystal structures of an acyl-enzyme intermediate of subtilisin Carlsberg formed in anhydrous acetonitrile and in water.

The x-ray crystal structures of trans-cinnamoyl-subtilisin, an acyl-enzyme covalent intermediate of the serine protease subtilisin Carlsberg, have been determined to 2.2-A resolution in anhydrous acetonitrile and in water. The cinnamoyl-subtilisin structures are virtually identical in the two solvents. In addition, their enzyme portions are nearly indistinguishable from previously determined structures of the free enzyme in acetonitrile and in water; thus, acylation in either aqueous or nonaqueous solvent causes no appreciable conformational changes. However, the locations of bound solvent molecules in the active site of the acyl- and free enzyme forms in acetonitrile and in water are distinct. Such differences in the active site solvation may contribute to the observed variations in enzymatic activities. On prolonged exposure to organic solvent or removal of interstitial solvent from the crystal lattice, the channels within enzyme crystals are shown to collapse, leading to a drop in the number of active sites accessible to the substrate. The mechanistic and preparative implications of our findings for enzymatic catalysis in organic solvents are discussed.

Acetonitriles↗
PubMed1998

Enzymatic repair of oxidative damage to human apolipoprotein A-I.

Oxidative damage to apolipoprotein A-I that occurs in vivo commonly involves methionine oxidation, and is accompanied by alterations in structure, lipid association, and cholesterol efflux function. We have used the enzyme peptide methionine sulfoxide reductase (PMSR) to reverse this damage, and shown by a variety of criteria that enzyme treatment restores the primary, secondary, and tertiary structure and lipid association characteristic of the native unoxidized protein. Lipid-associated as well as lipid-free apolipoprotein A-I reacts with PMSR, suggesting that enzymatic reduction of oxidized apolipoprotein A-I in high density lipoproteins can result in restoration of biological activity and the ability to promote cholesterol efflux from cells.

Animals↗
PubMed1998

Organic solvent binding to crystalline subtilisin1 in mostly aqueous media and in the neat solvents.

The X-ray crystal structures of the protease subtilisin Carlsberg in 40% acetonitrile and in 20% dioxane have been determined to at least 2.3 A resolution, and their solvent binding patterns have been compared to those observed in the neat organic solvents. The structures of the protein in the two aqueous-organic mixtures are essentially the same as in pure water, acetonitrile, and dioxane. Interestingly, the enzyme-bound organic solvent molecules tend to congregate in the active site. Three of the five bound acetonitrile molecules observed in the structure of subtilisin in 40% acetonitrile are situated in the enzyme active site, as is the single enzyme-bound dioxane molecule observed in 20% dioxane (whose location is distinct from that of any bound acetonitrile molecule). Furthermore, the organic solvent molecules detected in the enzyme active site in the aqueous-organic mixtures are in the same locations as in the structures in the corresponding neat organic solvents.

Acetonitriles↗
PubMed1997

The class II MHC protein HLA-DR1 in complex with an endogenous peptide: implications for the structural basis of the specificity of peptide binding.

BACKGROUND: Class II major histocompatibility complex (MHC) proteins are cell surface glycoproteins that bind peptides and present them to T cells as part of the mechanism for detecting and responding to foreign material in the body. The peptide-binding activity exhibits allele-specific preferences for particular sidechains at some positions, although the structural basis of these preferences is not understood in detail. We have determined the 2.45 A crystal structure of the human class II MHC protein HLA-DR1 in complex with the tight binding endogenous peptide A2 (103-117) in order to discover peptide-MHC interactions that are important in determining the binding motif and to investigate conformational constraints on the bound peptide. RESULTS: The bound peptide adopts a polyproline II-like conformation and places several sidechains within pockets in the binding site. Bound water molecules mediate MHC-peptide contacts at several sites. A tryptophan residue from the beta 2 'lower' domain of HLA-DR1 was found to project into a pocket underneath the peptide-binding domain and may be important in modulating interdomain interactions in MHC proteins. CONCLUSIONS: The peptide-binding motif of HLA-DR1 includes an aromatic residue at position +1, an arginine residue at position +2, and a small residue at position +6 (where the numbering refers to the normal MHC class II convention); these preferences can be understood in light of interactions observed in the peptide-MHC complex. Comparison of the structure with that of another MHC-peptide complex shows that completely different peptide sequences bind in essentially the same conformation and are accommodated with only minimal rearrangement of HLA-DR1 residues. Small conformational differences that are observed appear to be important in interactions with other proteins.

Amino Acid Sequence↗
PubMed1997

The crystal structure of subtilisin Carlsberg in anhydrous dioxane and its comparison with those in water and acetonitrile.

The x-ray crystal structure of the serine protease subtilisin Carlsberg in anhydrous dioxane has been determined to 2.6-A resolution. The enzyme structure is found to be nearly indistinguishable from the structures previously determined in water and acetonitrile. Small changes in the side-chain conformations between the dioxane and water structures are of the same magnitude as those observed between two structures in different aqueous systems. Seven enzyme-bound dioxane molecules have been detected, each potentially forming at least one hydrogen bond with a subtilisin hydrogen-bond donor or bound water. Two of the bound dioxane molecules are in the active-site region, one in the P2 and another bridging the P1' and P3' pockets. The other five dioxane molecules are located on the surface of subtilisin at interprotein crystal contacts. The locations of the bound solvent in the dioxane structure are distinct from those in the structures in acetonitrile and in water.

Acetonitriles↗
PubMed1996

Crystallographic analysis of endogenous peptides associated with HLA-DR1 suggests a common, polyproline II-like conformation for bound peptides.

The structure of the human major histocompatibility complex (MHC) class II molecule HLA-DR1 derived from the human lymphoblastoid cell line LG-2 has been determined in a complex with the Staphylococcus aureus enterotoxin B superantigen. The HLA-DR1 molecule contains a mixture of endogenous peptides derived from cellular or serum proteins bound in the antigen-binding site, which copurify with the class II molecule. Continuous electron density for 13 amino acid residues is observed in the MHC peptide-binding site, suggesting that this is the core length of peptide that forms common interactions with the MHC molecule. Electron density is also observed for side chains of the endogenous peptides. The electron density corresponding to peptide side chains that interact with the DR1-binding site is more clearly defined than the electron density that extends out of the binding site. The regions of the endogenous peptides that interact with DRI are therefore either more restricted in conformation or sequence than the peptide side chains or amino acids that project out of the peptide-binding site. The hydrogen-bond interactions and conformation of a peptide model built into the electron density are similar to other HLA-DR-peptide structures. The bound peptides assume a regular conformation that is similar to a polyproline type II helix. The side-chain pockets and conserved asparagine residues of the DR1 molecule are well-positioned to interact with peptides in the polyproline type II conformation and may restrict the range of acceptable peptide conformations.

Amino Acid Sequence↗
PubMed1995

Self-release of CLIP in peptide loading of HLA-DR molecules.

The assembly and transport of major histocompatibility complex (MHC) class II molecules require interaction with the invariant chain. A fragment of the invariant chain, CLIP, occupies the peptide-binding groove of the class II molecule. At endosomal pH, the binding of CLIP to human MHC class II HLA-DR molecules was counteracted by its amino-terminal segment (residues 81 to 89), which facilitated rapid release. The CLIP (81-89) fragment also catalyzed the release of CLIP(90-105) and a subset of other self-peptides, probably by transient interaction with an effector site outside the groove. Thus, CLIP may facilitate peptide loading through an allosteric release mechanism.

Amino Acid Sequence↗
PubMed1995

Interference of distinct invariant chain regions with superantigen contact area and antigenic peptide binding groove of HLA-DR.

In the endoplasmic reticulum, MHC class II alpha beta dimers associate with the trimeric invariant chain (li), generating a nine-subunit (alpha beta li)3 complex. In the presence of li, the peptide binding groove is blocked, so that loading with self or antigenic peptides can only occur after proteolytic removal of li in specialized post-Golgi compartments. The class II-associated invariant chain peptide region of li (about residues 81-104) is known to mediate binding to class II molecules and blockade of the groove, but this does not exclude additional contact sites for li. Using a set of overlapping li peptides and recombinant soluble li, we demonstrate here that a large segment of li encompassing approximately residues 71 to 128 interacts with HLA-DR molecules. The N- and C-terminal regions of this li segment appear to bind outside the peptide groove to the contact area for the staphylococcal superantigen Staphylococcus aureus enterotoxin B on the alpha 1 domain. The core region of this segment (residues 95-108) prevents binding of antigenic peptides, probably by interaction with the peptide groove. Occupation of the groove with antigenic peptides abolishes binding not only of the core region, but also that of those li peptides that bind outside the groove. These findings suggest the existence of distinct conformational states of class II molecules, with li binding preferentially to one form.

Amino Acid Sequence↗
PubMed1995

Human class II MHC molecule HLA-DR1: X-ray structure determined from three crystal forms.

The three-dimensional structure of the extracellular region of a 60 kDa class II major histocompatibility glycoprotein, HLA-DR1, was determined to 3.3 A by X-ray crystallography using three crystal forms, each containing two molecules per asymmetric unit. Phases were initially determined to 4.2 A using two crystal forms both containing DR1 from human lymphocytes complexed with a mixture of endogenous peptides. One of these crystal forms also contained a 28 kDa superantigen, Staphylococcus aureus enterotoxin B (SEB), bound to each DR1 molecule. Single-isomorphous replacement phasing followed by iterative two- and fourfold non-crystallographic real-space averaging between the two crystal forms resulted in 4.2 A resolution electron-density maps from which the paths of the polypeptides could be traced. Cryocrystallography and synchrotron radiation were then used to extend the resolution to 3.3 A for the two lymphocyte-derived crystal forms and for a third crystal form grown from DR1 produced in insect cells and complexed in vitro with a specific antigenic peptide. Iterative sixfold non-crystallographic real-space averaging resulted in an electron- density map into which 340 of 371 residues could be fit unambiguously. Crystal contacts and the existence of a parallel dimer of the DR1 alphabeta heterodimer in the three crystal forms are discussed.

Journal Article↗
PubMed1995

Intermediates in the folding of the membrane protein bacteriorhodopsin.

Assembly of proteins within lipid bilayers is essential for the biogenesis and function of biological membranes. Little is known, however, about the underlying mechanism of assembly, and it is not clear whether it is possible to observe individual folding steps for integral membrane proteins either in vivo or in vitro. Fluorescence spectroscopy is used here to follow the time course of folding events for bacteriorhodopsin in mixed detergent/lipid micelles. Transient folding-intermediates are detected and binding of the retinal chromophore occurs at a late stage, when it binds to an apoprotein intermediate.

Apoproteins↗
PubMed1994

MHC class II function preserved by low-affinity peptide interactions preceding stable binding.

Major histocompatibility complex class II molecules and their peptide ligands show unusual interaction kinetics, with slow association and dissociation rates that yield an apparent equilibrium constant of approximately 10(-6)-10(-8) M (refs 1-5). However, there is evidence for a specific, rapidly formed, short-lived complex. The altered migration on SDS-polyacrylamide gel electrophoresis of class II molecules upon stable peptide binding has led to the hypothesis that the two kinetically distinguishable types of class II-peptide complexes correspond to different structures. In accord with this model, we demonstrate here that insect cell-derived HLA-DR1 class II molecules show fast, almost stoichiometric occupancy with rapidly dissociating peptide while remaining sensitive to SDS-induced chain dissociation. The same DR1 molecules slowly and quantitatively form long-lived complexes resistant to SDS-induced denaturation. Surprisingly, low-affinity interaction with peptide protects class II from denaturation at physiological temperature, a finding that has implications for understanding the role of invariant chain in the intracellular behaviour of class II molecules.

Amino Acid Sequence↗
PubMed1994

Binary and ternary complexes between T-cell receptor, class II MHC and superantigen in vitro.

Superantigens are proteins that in association with class II major histocompatibility complex (MHC)-bearing cells can stimulate virtually all T cells that express particular classes of the variable beta-domains of the T-cell receptor (TCR). This mechanism of T-cell activation circumvents the usual requirement for peptide-specific MHC recognition. Staphylococcus aureus enterotoxin B (SEB) is a bacterial superantigen that causes food poisoning and shock. We have characterized the tertiary complex of SEB, a soluble T-cell receptor, and a soluble class II MHC molecule DR1, and the three binary complexes TCR-SEB, SEB-DR1, and the peptide-specific complex DR1-TCR. We report here that in each case the specificity of the interaction among the soluble molecules is the same as observed in biological assays. Native gel electrophoresis and plasmon resonance affinity measurements indicate that SEB-TCR complex can form in the absence of class II MHC and that SEB-TCR interaction increases the binding of DR1. The observation that a superantigen can form complexes with TCR in both the absence and presence of class II MHC may provide a mechanism for its ability to induce anergy in some circumstances and activation in others (reviewed in ref. 8).

Amino Acid Sequence↗
PubMed1994

Three-dimensional structure of a human class II histocompatibility molecule complexed with superantigen.

The structure of a bacterial superantigen, Staphylococcus aureus enterotoxin B, bound to a human class II histocompatibility complex molecule (HLA-DR1) has been determined by X-ray crystallography. The superantigen binds as an intact protein outside the conventional peptide antigen-binding site of the class II major histocompatibility complex (MHC) molecule. No large conformational changes occur upon complex formation in either the DR1 or the enterotoxin B molecules. The structure of the complex helps explain how different class II molecules and superantigens associate and suggests a model for ternary complex formation with the T-cell antigen receptor (TCR), in which unconventional TCR-MHC contacts are possible.

Amino Acid Sequence↗
PubMed1994

Crystal structure of the human class II MHC protein HLA-DR1 complexed with an influenza virus peptide.

An influenza virus peptide binds to HLA-DR1 in an extended conformation with a pronounced twist. Thirty-five per cent of the peptide surface is accessible to solvent and potentially available for interaction with the antigen receptor on T cells. Pockets in the peptide-binding site accommodate five of the thirteen side chains of the bound peptide, and explain the peptide specificity of HLA-DR1. Twelve hydrogen bonds between conserved HLA-DR1 residues and the main chain of the peptide provide a universal mode of peptide binding, distinct from the strategy used by class I histocompatibility proteins.

Amino Acid Sequence↗