Complement activation by IgM: evidence for the importance of the third constant domain of the mu heavy chain

We have isolated and analyzed the DNA encoding the mu heavy chain constant region of a mutant IgM which is defective in initiating complement-dependent cytolysis. By assaying the expression of mu genes which were constructed in vivo from mutant and normal gene segments, we have mapped the mutation into a 555-base pair segment. In this segment there is one nucleotide change, such that the mutant mu gene encodes serine rather than the normal proline at amino acid position 436 in the third constant domain. We have used site-directed mutagenesis to revert this mutation to the normal sequence and shown that this substitution results in the production of IgM with the normal phenotype.