title: Crystal structure of the MACPF domain of human complement protein C8 alpha in complex with the C8 gamma subunit.
abstract: Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that assemble on bacterial membranes to form a porelike structure referred to as the "membrane attack complex" (MAC). C8 contains three genetically distinct subunits (C8 alpha, C8 beta, C8 gamma) arranged as a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C6, C7 C8 alpha, C8 beta, and C9 are homologous. All contain N- and C-terminal modules and an intervening 40-kDa segment referred to as the membrane attack complex/perforin (MACPF) domain. The C8 gamma subunit is unrelated and belongs to the lipocalin family of proteins that display a beta-barrel fold and generally bind small, hydrophobic ligands. Several hundred proteins with MACPF domains have been identified based on sequence similarity; however, the structure and function of most are unknown. Crystal structures of the secreted bacterial protein Plu-MACPF and the human C8 alpha MACPF domain were recently reported and both display a fold similar to those of the bacterial pore-forming cholesterol-dependent cytolysins (CDCs). In the present study, we determined the crystal structure of the human C8 alpha MACPF domain disulfide-linked to C8 gamma (alphaMACPF-gamma) at 2.15 A resolution. The alphaMACPF portion has the predicted CDC-like fold and shows two regions of interaction with C8 gamma. One is in a previously characterized 19-residue insertion (indel) in C8 alpha and fills the entrance to the putative C8 gamma ligand-binding site. The second is a hydrophobic pocket that makes contact with residues on the side of the C8 gamma beta-barrel. The latter interaction induces conformational changes in alphaMACPF that are likely important for C8 function. Also observed is structural conservation of the MACPF signature motif Y/W-G-T/S-H-F/Y-X(6)-G-G in alphaMACPF and Plu-MACPF, and conservation of several key glycine residues known to be important for refolding and pore formation by CDCs.
authors.affiliation: Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Slade
authors.forename: Leslie L
authors.initials: LL
authors.lastname: Lovelace
authors.forename: Maksymilian
authors.initials: M
authors.lastname: Chruszcz
authors.forename: Wladek
authors.initials: W
authors.lastname: Minor
authors.forename: Lukasz
authors.initials: L
authors.lastname: Lebioda
authors.forename: James M
authors.initials: JM
authors.lastname: Sodetz
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D003185:Complement C8 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D015938:Complement Membrane Attack Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018360:Crystallography, X-Ray; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: C087169:C8G protein, human; D003185:Complement C8; D015938:Complement Membrane Attack Complex; D021122:Protein Subunits