title: Crystal structure of human complement protein C8gamma at 1.2 A resolution reveals a lipocalin fold and a distinct ligand binding site.
abstract: C8gamma is a 22-kDa subunit of human C8, which is one of five components of the cytolytic membrane attack complex of complement (MAC). C8gamma is disulfide-linked to a C8alpha subunit that is noncovalently associated with a C8beta chain. In the present study, the three-dimensional structure of recombinant C8gamma was determined by X-ray diffraction to 1.2 A resolution. The structure displays a typical lipocalin fold forming a calyx with a distinct binding pocket that is indicative of a ligand-binding function for C8gamma. When compared to other lipocalins, the overall structure is most similar to neutrophil gelatinase associated lipocalin (NGAL), a protein released from granules of activated neutrophils. Notable differences include a much deeper binding pocket in C8gamma as well as variation in the identity and position of residues lining the pocket. In C8gamma, these residues allow ligand access to a large hydrophobic cavity at the base of the calyx, whereas corresponding residues in NGAL restrict access. This suggests the natural ligands for C8gamma and NGAL are significantly different in size. Cys40 in C8gamma, which forms the disulfide bond to C8alpha, is located in a partially disordered loop (loop 1, residues 38-52) near the opening of the calyx. Access to the calyx may be regulated by movement of this loop in response to conformational changes in C8alpha during MAC formation.
authors.affiliation: Department of Chemistry and Biochemistry and the School of Medicine, University of South Carolina, Columbia, South Carolina 29208, USA.
authors.forename: Eric
authors.initials: E
authors.lastname: Ortlund
authors.forename: Chasta L
authors.initials: CL
authors.lastname: Parker
authors.forename: Steven F
authors.initials: SF
authors.lastname: Schreck
authors.forename: Steve
authors.initials: S
authors.lastname: Ginell
authors.forename: Wladek
authors.initials: W
authors.lastname: Minor
authors.forename: James M
authors.initials: JM
authors.lastname: Sodetz
authors.forename: Lukasz
authors.initials: L
authors.lastname: Lebioda
mesh_terms: D000209:Acute-Phase Proteins; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry*; D003185:Complement C8 / Q000737:chemistry* / Q000302:isolation & purification; D015938:Complement Membrane Attack Complex / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry*; D004220:Disulfides / Q000737:chemistry; D006801:Humans; D008024:Ligands; D000071068:Lipocalin-2; D054834:Lipocalins; D015513:Oncogene Proteins; D017510:Protein Folding; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D000209:Acute-Phase Proteins; C087169:C8G protein, human; D002352:Carrier Proteins; D003185:Complement C8; D015938:Complement Membrane Attack Complex; D004220:Disulfides; C489506:LCN2 protein, human; D008024:Ligands; D000071068:Lipocalin-2; D054834:Lipocalins; D015513:Oncogene Proteins; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D003545:Cysteine