Interaction between the C8α-γ and C8β Subunits of Human Complement C8:  Role of the C8β N-Terminal Thrombospondin Type 1 Module and Membrane Attack Complex/Perforin Domain

Abstract

Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that interact to form the cytolytic membrane attack complex (MAC). It is an oligomeric protein composed of a disulfide-linked C8alpha-gamma heterodimer and a noncovalently associated C8beta chain. C8alpha and C8beta are homologous; both contain an N-terminal thrombospondin type 1 (TSP1) module, a low-density lipoprotein receptor class A (LDLRA) module, an extended central segment referred to as the membrane attack/perforin (MACPF) domain, an epidermal growth factor (EGF) module, and a second TSP1 module at the C-terminus. In this study, the segment of C8beta that confers binding specificity toward C8alpha-gamma was identified using recombinant C8beta constructs in which the N- and/or C-terminal modules were deleted or exchanged with those from C8alpha. Constructs were tested for their ability to bind C8alpha-gamma in solution and express C8 hemolytic activity. Binding to C8alpha-gamma was found to be dependent on the TSP1 + LDLRA + MACPF segment of C8beta. Within this segment, the TSP1 module and MACPF domain are principally involved and act cooperatively to mediate binding. Results from activity assays suggest that residues within this segment also mediate binding and incorporation of C8 into the MAC.

Complement-Mediated Regulation of Apolipoprotein E in Cultured Human RPE Cells

“…Class A domains, which are thought to bind ligands, are, therefore, referred to as complement type repeats. 59–64 The LDLR class A domains of these complement proteins may play a role in ApoE association with MAC on the cell surface. Further studies will be required to determine whether LDLR class A domains present in MAC-associated proteins bind directly to ApoE.…”

Structural homology and expression tendency of the natural immune response of the terminal complement components to inoculations in pigs: a review

“…The C6 FIMAC module promotes the interaction of C6 with C5 enabling a more efficient bimolecular coupling ultimately leading to the formation of the C5b-6 complex (DiScipio et al 1999) and binding of C7 FIMAC to the C345C domain in C5 is essential for incorporation of C7 into C5b-6 (Thai and Ogata 2005). The principal binding site for C9 lies within the MACPF domain of C8A (Slade et al 2006), while the binding specificity between C8B and C8A-G subunits is determined by a cooperative interaction of the asparaginyl-terminal TSP1 module and MACPF (Musingarimi et al 2002). Site-specific antibodies directed to the LDLa homology unit in C9 cross-reacted with C6 and C7 (Tschopp et al 1986).…”

Structure of Human Complement C8, a Precursor to Membrane Attack

“…S6). These additional domains are thought to have a role in coordinating the complex as well as mediating hemolytic activity 20 . Given the small size of the domains and the resolution of the reconstruction, we have not included them in our refinement.…”