The switch point in mu heavy chains of human IgM immunoglobulins

Use of human IgM derived fragments to study structures responsible for protein A-reactivity

The protein A-binding site of human IgM was studied by affinity chromatography on SpA-Sepharose using fragments derived from a human monoclonal SpA-reactive IgM, Iz. Neither Fabmu nor (Fc) 5mu fragments were retained on the column but IgM reactivity was unaffected by thermic treatment during proteolysis. Products intermediate between IgM and (Fc) 5mu fragments produced during shorter proteolysis showed a reactivity related to their content in Fabmu regions. On the other hand mild reduction of IgM Iz to monomeric subunits results in a dramatic loss of SpA-affinity. However these subunits, like F(ab') 2mu but unlike Fab'mu fragments, showed a significant interaction with the column. Thus, the principal requirement for SpA reactivity with IgM Iz seems to be related to the presence of Fabmu regions in a polymeric state resembling native IgM.

Amino acid sequence of the variable regions of heavy chains from two idiotypically cross-reactive human IgM anti-gamma-globulins of the Wa group

The amino acid sequences of the variable regions of the heavy chains derived from two idiotypically related human monoclonal rheumatoid factors are reported. The sequences were obtained through automated Edman degradations of the intact, pyroglutamate aminopeptidase cleaved heavy chains and peptides produced from digestion of the pentameric IgM with CNBr. The peptides generated from the CNBr reaction were further digested by trypsin and the Staphylococcus aureus V8 protease. Comparisons of these sequences and those of the light chains from these molecules [Andrews, D. W., & Capra, J. D. (1981) Biochemistry (preceding paper in this issue)] suggest that the idiotypic determinant(s) in this system may reside in the framework portions of these molecules or in their J segments.

A rapid procedure for the isolation of human IgM myeloma proteins

A two-step rapid procedure for the isolation of human IgM paraproteins from plasma samples is described. It involves gel filtration on Ultrogel AcA 34 followed by anion exchange chromatography on DEAE-Sepharose CL6B. The overall yield of IgM is greater than 70% and the final product is pure as judged by immunoelectrophoresis and SDS-polyacrylamide gel electrophoresis. The procedure is applicable to both euglobulin and cryoglobulin IgM proteins of a wide range of electrophoretic mobilities.

Amino acid sequence of the variable region of a human mu chain: location of a possible JH segment

The complete amino acid sequence of th variable (V) region of the mu heavy chain of a human IgM immunoglobulin (Cam) has been determined. The strategy for sequence determination involved sequenator analysis of the CNBr cleavage products of the succinylated carboxymethylated Fab mu fragment, and of tryptic peptides of the CNBr polypeptides and thermolytic subpeptides. The variable region of this heavy chain (VH) belongs to the VHIII subgroup; it has greater than 70% homology with other VHIII sequences and contains the VHIII marker peptide, Phe-Thr-Ile-Ser-Arg (residues 67-71). As more sequences have been published, the number of subgroup-specific residues has diminished to the point that no position is absolutely subgroup specific. An analysis of the available human VH sequences in the V/C switch region showed the likelihood of a human JH segment (residues 101-113) analogous to the J segments in mouse light chains. The JH region is highly conserved, has striking homology to proposed mouse JH regions, and has significant homology to known mouse J lambda and J kappa segments.

Lactoperoxidase-catalyzed iodination of human IgM. Differences between 7 S IgM and 19 S IgM and between cell surface 7 S IgM and serum 7 S IgM

We have studied the lactoperoxidase-catalyzed iodination of human IgM and have measured the ratio of radioactivity incorporated into the mu chain to that incorporated into the L chain (i.e. the mu/L ratio). Both 7 S and 19 S IgM were examined. The ratio of radioactivity was found to be larger for 7 S IgM than for 19 S IgM for all four of the monoclonal IgM proteins examined. The data suggest that some tyrosines of the mu chain which are buried and not available for iodination in 19 S IgM become exposed on conversion of 19 S IgM to 7 S IgM. The mu/L ratio for the IgM found on the cell surface of RPMI 8392 cells was significantly smaller than the ratios for all of the five 7 S IgM proteins studied in solution. It appears, therefore, that a portion of the mu chain of the cell surface IgM of the RPMI 8392 cells is buried in the membrane.

Complete covalent structure of a human IgA1 immunoglobulin

The complete covalent structure has been determined for a human myeloma IgA1 immunoglobulin. This protein has unique features in the amino acid sequence and disulfide bridge structure of the variable (V) and constant (C) regions of both the alpha heavy and the lambda light chains, and in the number and loci of oligosaccharides. Whereas C region domains of heavy chains have evolved independently over eons, recent isotypic variations have occured in lambda light chains and possibly in alpha heavy chains.

Chemical studies on a monoclonal immunoglobulin from a patient with carcinoma of the colon

Structural studies were carried out on a monotypic immunoglobulin (Ig) isolated from a patient suffering from a colon tumor. Results indicated that the light (L) chain of this protein belonged to the VkappaII subgroup and was devoid of known Inv allotypic determinants, whereas the heavy (H) chain variable (V) region belonged to the VHIII subgroup and its constant (C) region was of the gamma1 subclass and was Gm (a+Z+). The amino acid sequence of a total of 106 residues has been determined for this molecule. An extra cysteine was present at the fourth hypervarible region of the heavy chain. Preliminary results indicated that the Fc fragment of this protein did not include the inter-heavy-chain disulfide bonds.