C4 from C4-high and C4-low mouse strains have identical sequences in the region corresponding to the isotype-specific segment of human C4.

The human complement component C4 isotypes, C4A and C4B, show a substantial and biologically important difference in chemical reactivity. Murine C4 from different mouse strains has recently been reported to have a comparable difference in reactivity. In human C4, the difference in reactivity has been attributed to the effect(s) of one or more of only four amino acid residues, within a six-residue-long segment of the alpha subunit, which distinguish the two isotypes. In the present study, we sought to assess the role of the corresponding four amino acids in mouse C4 in the strain-specific modulation of C4 reactivity. In order to compare the sequences of the corresponding region in murine C4 among different mouse strains, we used the polymerase chain reaction method to amplify an approximately 229-bp segment of the murine gene that includes the codons for these four amino acid residues. Because the difference in chemical reactivity of murine C4 has been reported to be between C4 from strains which express different levels of C4, we examined sequences from mouse strains C57BL/6, DBA/2, C3H/He, B10.BR and CBA/J; these represent two C4-high strains and three C4-low strains. The amplified segments were cloned into the pUC19 vector and 20 independent clones from each mouse strain were sequenced. Across the entire amplified segment, our results revealed expected isotypic differences between C4 and its nonfunctional isotype in the mouse, Slp, as well as allelic differences among the C4 and Slp genes. However, all of these differences were quite distant from the amino acid residues corresponding to the human isotype-specific residues and those corresponding residues were identical in all five mouse strains. This result indicates that any strain-specific difference in chemical reactivity of murine C4 must be due to a mechanism distinct from that operating in human C4.