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Spontaneous somatic mutations. Structural studies on mutant immunoglobulins
Abstract:
The precise alterations in the protein amino acid sequences of the immunoglobulin heavy chains of spontaneously arisen MOPC 21 mutant clones IF2 and IF1 have been determined. All the cyanogen bromide fra-ments of both heavy chains have been isolated and compared to the wild type CNBr fragments. For IF2, there is an internal deletion, from the wild type sequence, of 96 amino acids, from residues 121 to 215 inclusive. Moreover, in IF2, there are no disulfide bonds formed between heavy and light chains, presumably because of the deletion of the CH1 pseudosubunit. There are no other alterations in its covalent structure. For IF1, there is a deletion of the COOH-terminal 83 amino acids (residues 358 to 440, inclusive). Although IF1 heavy chain behaves on dodecyl sulfate-polyacryl-amide gels as if it were only 10 to 15 residues shorter than wild type, no other amino acid sequence differences from wild type are found. IF1 arose most likely by a nonsense mutation of a serine codon. For IF2, whose deletion is like that seen in some human heavy chain disease proteins, the most likely explanation is an error of recombination. The structure of IF2 suggests that the heavy chain variable region ends at a position homologous to residue 120 of the MOPC 21 heavy chain.
Insights
Researchers precisely mapped mutations in immunoglobulin heavy chains of MOPC 21 mutant clones. IF2 has a 96-amino acid internal deletion, while IF1 shows an 83-amino acid COOH-terminal deletion.
Area of Science:
- Immunology
- Molecular Biology
- Protein Chemistry
Background:
- Immunoglobulin heavy chains are crucial for antibody function.
- Understanding mutations in immunoglobulin genes provides insights into protein structure and disease.
Purpose of the Study:
- To determine the precise amino acid sequence alterations in immunoglobulin heavy chains of MOPC 21 mutant clones IF2 and IF1.
- To compare these alterations to the wild-type sequence and elucidate the mechanisms of mutation.
Main Methods:
- Isolation and comparison of cyanogen bromide fragments of heavy chains from mutant and wild-type MOPC 21.
- Analysis of amino acid sequences to identify deletions and other structural changes.
- Assessment of disulfide bond formation between heavy and light chains.
Main Results:
- Mutant IF2 exhibits an internal deletion of 96 amino acids (residues 121-215) and lacks heavy-light chain disulfide bonds, likely due to CH1 deletion.
- Mutant IF1 shows a deletion of 83 COOH-terminal amino acids (residues 358-440), with minimal apparent size difference on gels.
- IF1's mutation is likely a nonsense mutation, while IF2's deletion suggests an error of recombination, similar to human heavy chain diseases.
Conclusions:
- The study precisely defines the molecular basis of mutations in MOPC 21 immunoglobulin heavy chains.
- IF2's deletion pattern resembles human heavy chain diseases, suggesting a common mechanism.
- The findings provide structural insights into immunoglobulin heavy chain variability and potential disease-related alterations.