Differential functional expression of the C8 subunits. Primary role of C8 beta in assembly of intact C8

The eighth component of C (C8) is composed of two subunits C8 beta and C8 alpha-gamma, which are non-covalently bound in a 1/1 ratio in the intact molecule. The genes encoding the polypeptide chains composing the subunits demonstrate close genetic linkage. To assess the functional expression of these genes at the protein level, normal human serum and C8-deficient sera were electrophoresed in native polyacrylamide gels following which C8, C8 beta, and C8 alpha-gamma were detected using hemolytic overlays. These experiments demonstrated that normal sera contained free C8 alpha-gamma in addition to intact C8. Free C8 alpha-gamma was not observed when C8 was reconstituted by mixing C8 beta-deficient serum with C8 alpha-gamma-deficient serum in a ratio optimized for C8 activity, suggesting that the free C8 alpha-gamma observed in normal serum was not due to dissociation of intact C8. Inasmuch as this technique did not adequately separate C8 and C8 beta, sera were also examined by anion exchange chromatography. C8 alpha-gamma-deficient serum contained C8 beta in a single peak in the 1.4 ms/cm fall through. C8 beta-deficient serum contained a major peak of C8 alpha-gamma at 7.1 ms/cm and a lesser peak coeluting with C9 at 9.5 ms/cm. Normal serum contained both intact C8 eluting between 2.4 to 5.5 ms/cm and C8 alpha-gamma eluting at 7.1 ms/cm. Free C8 beta was not detectable in normal serum indicating that free C8 alpha-gamma was not due to C8 dissociation. Mixing aliquots from the chromatographic peak of C8 beta activity with the peaks of C8 alpha-gamma activity in C8 beta-deficient serum or in normal serum generated intact C8 hemolytic activity. Non-reducing SDS-PAGE and Western blotting with anti-C8 confirmed the presence of antigenic material of appropriate m.w. in each peak. These findings demonstrate that serum contains excess C8 alpha-gamma relative to C8 beta, despite the equimolar presence of the subunits in intact C8. Thus the availability of C8 beta determines the quantity of C8 produced. Further, these data suggest the possibility that the C8 structural genes may be differentially expressed despite their close genetic linkage.