Murine binding protein of the fourth component of complement: structural polymorphism and its linkage to the major histocompatibility complex.

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Abstract
The binding protein of the complement component 4 (C4-BP) is a regulatory protein of the complement system with specific affinity for C4. A structural polymorphism of murine C4-BP and its linkage to the major histocompatibility complex of the mouse (H-2) are described. After isoelectric focusing of whole mouse plasma in low-endosmosis agarose, C4-BP was demonstrated as a single precipitin band by overlaying monospecific antiserum on the agarose gel. Two C4-BP patterns were distinguished among many strains of mice on the basis of isoelectric point: C4-BP a type, which has a pH range of 6.5-7.0 (exemplified by B10.cntdot.BR and B10.cntdot.AKM), and C4-BP b type, which has a pH range of 6.3-6.6 (exemplified by B10 and B10.cntdot.M). Genetic crosses between 2 strains bearing distinct C4-BP types demonstrate a C4-BP pattern representative of both types. A linkage study was carried out in which progeny of 2 backcross combinations.sbd.[(B10 .times. B10.cntdot.BR)F1 .times. B10.cntdot.BR] and [(B10.cntdot.AKM .times. B10)F1 .times. B10.cntdot.AKM].sbd.were phenotyped for C4-BP type and serum C4 level. Results were obtained suggesting that C4-BP patterns are inherited by a single codominant locus (C4-Bp) linked to the H-2 complex. The recombination frequency between the C4-Bp locus and the S region was 0.017. By phenotyping appropriate intra-H-2 recombinants of 3 different backgrounds (B10, A and HT), this locus was assigned to the right of the H-2D region.