ABSTRACT
Antibodies to a soluble liver antigen/liver pancreas (SLA/LP) appear to be highly specific for the diagnosis of autoimmune hepatitis. The SLA/LP target antigen was recently identified as a hitherto unknown gene encoding 474 amino acid residues. The function of this antigen remains unclear, because it does not share sequence homology with proteins of known function stored in any of the publicly accessible databases. Therefore we used a new theoretical method called fold recognition and could show that the SLA/LP sequence is compatible with the architecture of the superfamily of pyridoxal phosphate (PLP; vitamin B6)-dependent transferases. Its function is likely to be that of a serine hydroxymethyltransferase and may be an important enzyme in the thus far poorly understood selenocysteine pathway.
Subject(s)
Autoantigens/physiology , Computational Biology/methods , Hepatitis, Autoimmune/immunology , Amino Acid Sequence/genetics , Autoantigens/genetics , Humans , Models, Molecular , Molecular Sequence DataABSTRACT
Glycoprotein 130 (gp130) is a type I transmembrane protein and serves as the common signal-transducing receptor subunit of the interleukin-6-type cytokines. Whereas the membrane-distal half of the gp130 extracellular part confers ligand binding and has been subject to intense investigation, the structural and functional features of its membrane-proximal half are poorly understood. On the basis of predictions of tertiary structure, the membrane-proximal part consists of three fibronectin-type-III-like domains D4, D5 and D6. Here we describe the bacterial expression of the polypeptides predicted to comprise each of these three domains. The recombinant proteins were refolded from solubilized inclusion bodies in vitro, purified to homogeneity and characterized by means of size-exclusion chromatography and CD spectroscopy. For the first time the prediction of three individual membrane-proximal protein domains for gp130 has been verified experimentally. The three domains do not show intermediate-affinity or high-affinity interactions between each other. Mapping of a neutralizing gp130 monoclonal antibody against D4 suggested a particular functional role of this domain for gp130 activation, because above that an intrinsic tendency for low-affinity oligomerization was demonstrated for D4.
Subject(s)
Antigens, CD/chemistry , Membrane Glycoproteins/chemistry , Animals , Antibodies, Monoclonal , Antigens, CD/genetics , Antigens, CD/immunology , COS Cells , Circular Dichroism , Cross-Linking Reagents , Cytokine Receptor gp130 , Epitope Mapping , Escherichia coli/genetics , Humans , In Vitro Techniques , Membrane Glycoproteins/genetics , Membrane Glycoproteins/immunology , Protein Folding , Protein Structure, Secondary , Protein Structure, Tertiary , Protein Subunits , Receptors, Cytokine/chemistry , Recombinant Proteins/chemistry , Recombinant Proteins/genetics , Recombinant Proteins/immunologyABSTRACT
The typical protein fold of most cytokines is a bundle of four antiparallel helices. This 'four-helical bundle fold' seems to be unique to cytokines and has not been detected in other proteins. Cytokine receptors, however, can be classified as a subfamily of the immunoglobulin superfamily. Cytokines using the same receptor subunits are grouped into cytokine families. The interleukin-6 (IL-6) type cytokine family comprises six members. IL-6 type cytokines may interact with three receptor subunits instead of the usual two subunits. A tetramer would be the simplest model to describe such a receptor complex, but present orthodoxy describes the active complexes of IL-6 and ciliary neurotrophic factor (CNTF) as hexamers. Here, we summarize the structural and biochemical information on IL-6 type cytokines and discuss interactions between cytokine and individual receptor subunits at alternative positions. Contradictory results regarding the stoichiometry and assembly of signaling receptor complexes are rationalized by a new, unique model. The model stipulates that a ligand-induced transition from an active tetrameric to an inactive hexameric complex serves as a molecular switch that turns off cytokine signals in the presence of supraoptimal cytokine concentrations.
Subject(s)
Biopolymers , Interleukin-6/metabolism , Nerve Tissue Proteins/metabolism , Receptors, Cytokine/metabolism , Ciliary Neurotrophic Factor , Fibronectins/chemistry , Models, Molecular , Receptors, Cytokine/chemistryABSTRACT
The transmembrane glycoprotein gp130 is the common signal transducing receptor subunit of the interleukin-6-type cytokines. It is a member of the cytokine-receptor superfamily predicted to consist of six domains in its extracellular part. The second and third domain constitute the cytokine-binding module defined by a set of four conserved cysteines and a WSXWS motif, respectively. The three-dimensional structure of the carboxy-terminal domain of this region was determined by multidimensional NMR. The domain consists of seven beta-strands constituting a fibronectin type III-like topology. The structure reveals that the WSDWS motif of gp130 is part of an extended tryptophan/arginine zipper which modulates the conformation of the CD loop.
