C1 fixation and classical complement pathway activation by a fragment of the Cmu4 domain of IgM

A 56 residue fragment derived from a Waldenstrome IgM protein and consisting of 24 residues of the amino-terminal portion of the Cmu4 domain disulfide bonded to 32 residues of the carboxy-terminal region of the loop has been shown to fix active C1 (C1) in a C1-fixation assay. Cleavage of the disulfide bond within the CH4 fragment resulted in a marked decrease of C1-fixing ability, although the isolated A and B fragments did retain a limited ability to fix C1. Upon incubation with normal human serum the intact CH4 fragment and equal molar amounts of the isolated A and B peptides consumed C4 suggesting that the C1- activating determinant of IgM remains intact in these three fragments. Furthermore, on a molar basis the intact or the reduced CH4 fragment consumed C4 as effectively as each of its component chains suggesting that transient binding of C1 by the individual A and B peptide chains is sufficient to activate C1. On the basis of these observations it is proposed that a classical complement fixation function, i.e. C1 binding and activation, can be localized within a region of the IgM molecule corresponding to the Cmu4 domain.