Complete covalent structure of a human immunoglobulin D: sequence of the lambda light chain

Semi-extended solution structure of human myeloma immunoglobulin D determined by constrained X-ray scattering

Human immunoglobulin D (IgD) occurs most abundantly as a membrane-bound antibody on the surface of mature B cells (mIgD). IgD possesses the longest hinge sequence of all the human antibody isotypes, with 64 residues connecting the Fab and Fc fragments. A novel rapid purification scheme of secreted IgD from the serum of an IgD myeloma patient using thiophilic (T-gel) and lectin affinity chromatography gave a stable, homogeneous IgD preparation. Synchrotron X-ray scattering and analytical ultracentrifugation of IgD identified the solution arrangement of its Fab and Fc fragments, and thereby its hinge structure. The Guinier X-ray radius of gyration R(G) of 6.9(+/-0.1)nm showed that IgD is more extended in solution than the immunoglobulin subclass IgA1 (R(G) of 6.1-6.2nm). Its distance distribution function P(r) showed a single peak at 4.7nm and a maximum dimension of 23nm. Velocity experiments gave a sedimentation coefficient of 6.3S, which is similar to that for IgA1 at 6.2S. The complete IgD structure was modelled using molecular dynamics to generate IgD hinge structures, to which homology models for the Fab and Fc fragments were connected. Good scattering curve fits were obtained with 18 semi-extended best fit IgD models that were filtered from 8500 trial models. These best-fit models showed that the IgD hinge does not correspond to an extended polypeptide structure. The averaged solution structure arrangement of the Fab and Fc fragments in IgD is principally T-shaped and flexible, with contribution from Y-shaped and inverted Y-shaped structures. Although the linear sequence of the IgD hinge is much longer, comparison with previous scattering modelling of IgA1 and IgA2(m)1 suggests that the hinge of IgA1 and IgD are more similar than might have been expected, Both possess flexible T-shaped solution structures, probably reflecting the presence of restraining O-linked sugars.

BIT, an immune antigen receptor-like molecule in the brain

We previously found a brain-specific glycoprotein in the rat brain. It postnatally increases and is rich in the mature brain. We cloned cDNA of this protein. It is composed of a signal peptide, a V-type immunoglobulin domain, two C1-type immunoglobulin domains, a transmembrane segment and a cytoplasmic region containing two tyrosine-based activation motifs (TAM) that are variants of the antigen receptor signaling motifs. The overall structure is similar to those of immune antigen receptors. This molecule, BIT (brain immunoglobulin-like molecule with TAMs), is a major endogenous substrates of brain tyrosine kinases in vitro. Cerebral cortical neurons could extend their neurites on BIT-coated substrate and anti-BIT monoclonal antibody specifically inhibited the effect. These findings and our recent study concerning BIT signal transduction mechanism suggest that BIT, an immune antigen receptor-like molecule of the brain, functions as a membrane signaling molecule that may participate in cell-cell interaction.

Complete sequence of the genes encoding the VH and VL regions of low- and high-affinity monoclonal IgM and IgA1 rheumatoid factors produced by CD5+ B cells from a rheumatoid arthritis patient

We have characterized the VH and VL genes of three low-affinity polyreactive and two high-affinity monoreactive IgM and IgA1 rheumatoid factor (RF) mAb generated using circulating CD5+ B cells from a single rheumatoid arthritis patient. We found that four and one RF mAb utilized genes of the VHIV and VHIII families, respectively. The VHIV gene usage by these RF mAb differs from the preferential VHIII, VHI, and, to a lesser extent, VHII gene usage by the IgM with RF activity found in patients with mixed cryoglobulinemia, Waldenstrom's macroglobulinemia, and other monoclonal gammopathies. In addition, in contrast to the preponderant kappa L chain usage by the RF in these patients, a lambda L chain was utilized by all RF mAb from our rheumatoid arthritis patient. Two RF mAbs utilized V lambda I, two V lambda IV, and one V lambda III L chains. The VH genes of the two low-affinity polyreactive IgM RF mAb were in germline configuration. When compared with the deduced amino acid sequence of the putatively corresponding genomic segment, the VH gene of the high-affinity monoreactive IgM RF mAb displayed five amino acid differences, all of which are in the complementarity determining regions (CDR), possibly the result of a process of somatic point mutation and clonal selection driven by Ag. The unavailability of the corresponding genomic VH segment sequences made it impossible to infer whether the VH genes utilized by the two IgA1 RF were in a germline or somatically mutated configuration. Sequencing of the genes encoding the H chain CDR3 (D segments) revealed that all three low-affinity polyreactive RF mAb displayed a much longer D segment (36-45 bases) than their high-affinity monoreactive counterparts (15-24 bases), raising the possibility that a long D segment may be one of the factors involved in antibody polyreactivity.

Definition of the human immunoglobulin variable lambda (IGLV) gene subgroups

Comparison of 60 human immunoglobulin variable lambda (IGLV) sequences allowed us to define seven subgroups designated V lambda I to V lambda VII. We demonstrate that all lambda proteins sequenced so far fall into the subgroups I, II, III and VI, and that the lambda regions previously assigned to subgroups IV and V belong, in fact, to subgroups III and II, respectively. Four sequences not belonging to any of the subgroups I, II, III and VI define the new subgroups IV, V and VII. Interestingly, these subgroups show a higher homology to rabbit or mouse V lambda genes than to the other human V lambda subgroups. By comparison of the proteins either with the sequences deduced from the germ-line genes or with the consensus sequences, the rate of amino acid changes due to somatic mutations or allelic variations was evaluated in several lambda proteins. Framework and complementarity-determining regions of the human IGLV genes and proteins were delineated. Alignment of the lambda sequences shows that functional V-J rearrangement occurs, with or without deletion of nucleotides encoding one or two amino acids at the 3' end of the V gene. Diversity of the third complementarity-determining region is due to somatic mutations and to flexible V-J junction, but there is no evidence of N-diversity in the human lambda locus.

Analysis of the mechanism, rate, and sites of proteolytic cleavage of human immunoglobulin D by high-pressure liquid chromatography

The high susceptibility of human immunoglobulin D to proteolytic degradation affects its biological function, metabolism, and immunoassay. High-pressure liquid chromatography was used to investigate the mechanism and rate of limited proteolytic cleavage of IgD and also to identify, isolate, and quantify the reaction products. Within 1 to 5 min, tryptic digestion of native IgD almost quantitatively yields a labile Fab fragment, a stable Fc fragment, and a highly charged peptide derived from the hinge region. A galactosamine-rich glycopeptide from the hinge region increases inversely as the Fab is largely degraded to a series of peptides within 1 hr. In contrast, the Fc and the high-charge peptide resist proteolysis for more than 24 hr. The initial sites of cleavage of IgD occur in the hinge region at exposed secondary structures predicted to be beta-turns. Concomitant with removal of the galactosamine-rich glycopeptide at its carboxyl terminus, the Fd fragment is rapidly and rather randomly degraded, but the light chain is somewhat more resistant than the Fd section of the delta heavy chain. This study of the rapid rate of proteolysis of IgD explains the rarity with which intact IgD is found in human sera. It also raises questions about immunoassay of IgD, which is usually measured with antisera against Fc. In vivo, proteolytic cleavage initiates the catabolism of circulating IgD and also affects the role and fate of IgD as an antigen receptor on the B-cell membrane.

Purification of glycopeptides of human plasma proteins by high-performance liquid chromatography

The combination of gel permeation chromatography and high-performance liquid chromatography proves to be very effective for the purification of high-molecular-weight glycopeptides containing a single glycan, that have been difficult to separate by other procedures. In order to facilitate comparison of the chromatographic properties of glycopeptides derived from a variety of proteins and having different structures, identical procedures were used for their purification. The method was applied to a series of human plasma proteins, including immunoglobulin D, ceruloplasmin, hemopexin, beta-2-glycoprotein I, 3.1S alpha-2-leucine-rich glycoprotein, and alpha-1-B-glycoprotein. All the purified glycopeptides were placed in the protein structure of these plasma proteins. In several cases the carbohydrate structure has been determined by collaborating groups. Immunoglobulin D is the first example of a glycoprotein whose entire primary structure has been defined by utilizing a a single protein source. Furthermore, hemopexin and 3.1S alpha-2-leucine-rich glycoprotein were both found to contain GalN oligosaccharide, which had not previously been identified in these proteins. The method was also used to identify the oligosaccharide that is missing in a carbohydrate variant of ceruloplasmin.

Structure of human hemopexin: O-glycosyl and N-glycosyl sites and unusual clustering of tryptophan residues

The primary structure of human hemopexin is being deduced from sequence analysis of a series of peptides obtained from chemical and enzymatic digests of the protein. Human hemopexin consists of about 440 amino acid residues. It has five sites of attachment of glucosamine oligosaccharides at the signal sequence of Asn-X-Thr/Ser. A unique structural feature is the virtual blocking of the amino-terminal threonine residue, which is O-linked to a galactosamine oligosaccharide that has not previously been identified in this protein. The galactosamine oligosaccharide and one glucosamine oligosaccharide are located in the amino-terminal region, three of the glucosamine oligosaccharides are in the middle region, and one glucosamine oligosaccharide is in the carboxyl-terminal region of the protein. Two of the five glucosamine oligosaccharides are present in a histidine-rich sequence of the middle region of the protein, in which the histidines flank beta-turns presumably at the surface of hemopexin. Clusters of tryptophan residues occur in four regions, each of which contains three or four tryptophan residues separated by 0-12 other residues. This clustering is significant because both histidine and tryptophan have been implicated in the binding of heme. A computer analysis did not identify significant matches of human hemopexin to any protein, including cytochromes and other heme-binding proteins, which suggests that the human hemopexin gene evolved from a unique primordial gene differing from those of other heme-binding proteins.