In vitro expression of the beta subunit of human complement component C8

Human complement protein C8 gamma

Human C8 gamma is a 22 kDa subunit of complement component C8, which is one of five components (C5b, C6, C7, C8, C9) that interact to form the cytolytic membrane attack complex (MAC) of complement. C8 contains three nonidentical subunits (alpha, beta, gamma) that are products of different genes. These subunits are arranged asymmetrically to form a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C8 alpha and C8 beta are homologous to C6, C7 and C9 and together these proteins comprise what is referred to as the 'MAC protein family'. By comparison, C8 gamma is distinct in that it belongs to the lipocalin family of small, secreted proteins which have the common ability to bind small hydrophobic ligands. While specific roles have been identified for C8 alpha and C8 beta in the formation and function of the MAC, a function for C8 gamma and the identity of its ligand are unknown. This review summarizes the current status of C8 gamma structure and function and the progress made from efforts to determine its role in the complement system.

Expression and Characterization of Recombinant Subunits of Human Complement Component C8: Further Analysis of the Function of C8α and C8γ

Human C8 is composed of three nonidentical subunits (C8α, C8β, and C8γ) that are encoded in separate genes. In C8 isolated from serum, these are arranged as a disulfide-linked C8α-γ dimer that is noncovalently associated with C8β. In this study, a recombinant form of C8α-γ was expressed independently of C8β in insect cells and COS-7 cells and was shown to be equivalent to serum-derived C8α-γ with respect to its ability to combine with C8β and form functional C8. Also expressed separately were mutant (mut) forms of C8α and C8γ in which the single interchain disulfide bond was eliminated. MutC8α exhibited the ability to combine with C8β and express hemolytic activity, although at a lower level than human C8. Addition of purified mutC8γ increased this activity, presumably by binding to mutC8α. A possible role for C8γ as a retinol binding protein was also investigated. Absorbance spectroscopy and fluorescence emission and quenching revealed no specific binding of retinol to mutC8γ. Together, these results indicate that 1) the biosynthesis and secretion of C8α-γ is not dependent on C8β, which is consistent with in vivo observations in C8β-deficient humans; 2) C8α can be synthesized independently of C8γ; therefore, protection of C8α from premature membrane interactions during biosynthetic processing is not a likely function of C8γ; 3) C8γ enhances but is not required for expression of C8 activity; and 4) C8γ does not bind retinol; therefore, it cannot function as a retinol transport protein.

Expression of the human complement C8 subunits is independently regulated by interleukin 1 beta, interleukin 6, and interferon gamma

The eighth component of human complement (C8) is composed of two subunits which are products from three separate genes. The alpha-gamma- and the beta-subunit of C8 are expressed independently, and are part of the membrane attack complex. C8 is primarily synthesized in the liver. It has been shown in previous studies that the human hepatoma cell line HepG2 constitutively expresses C8, and thus is a suitable model system for studying C8 biosynthesis in vitro. Expression is modulated by the cytokines IL-1 beta, IL-6 and IFN-gamma. The effect of the different cytokines on the expression of these subunits was examined using biosynthetical labelling and immunoprecipitation methods. C8 alpha-gamma is expressed first and secreted independently from C8 beta. After 5 h labelling, the expression is strongly reduced, and the majority of C8 alpha-gamma is found in the supernatant. C8 beta expression exhibits a different pattern with a much slower rate of biosynthesis and secretion. Evidence was obtained for an independent secretion of the C8 beta chain. C8 alpha-gamma expression is strongly enhanced after stimulation with the cytokines IL-6, IFN-gamma and IL-1 beta. In contrast, only IFN-gamma but not IL-6 and IL-1 beta had an increasing effect on the expression of C8 beta. Thus the total amount of assembled functionally active C8 appears to be limited by the rate of C8 beta expression.