Binding of the lipocalin C8gamma to human complement protein C8alpha is mediated by loops located at the entrance to the C8gamma ligand binding site.
Human C8 is one of five complement components (C5b, C6, C7, C8 and C9) that interact to form the membrane attack complex (MAC). C8 is composed of a disulfide-linked C8alpha-gamma heterodimer and a noncovalently associated C8beta chain. C8alpha and C8beta are homologous to C6, C7 and C9, whereas C8gamma is the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small molecule. In C8gamma, the calyx opening is surrounded by four loops that connect beta-strands. Loop 1 is the largest and contains Cys40 that links to Cys164 in C8alpha. To determine if these loops mediate binding of C8alpha prior to interchain disulfide bond formation in C8alpha-gamma, the loops were substituted separately and in combination for the corresponding loops in siderocalin (NGAL, Lcn2), a lipocalin that is structurally similar to C8gamma. The siderocalin-C8gamma chimeric constructs were expressed in E. coli, purified, and assayed for their ability to bind C8alpha. Results indicate at least three of the four loops surrounding the entrance to the C8gamma calyx are involved in binding C8alpha. Binding near the calyx entrance suggests C8alpha may restrict and possibly regulate access to the C8gamma ligand binding site.
Chiswell
Brian
B
Department of Chemistry and Biochemistry and School of Medicine, University of South Carolina, Columbia, 29208, USA.
Slade
Daniel J
DJ
Sodetz
James M
JM
D000595:Amino Acid Sequence; D002352:Carrier Proteins / Q000378:metabolism*; D003185:Complement C8 / Q000378:metabolism*; D015938:Complement Membrane Attack Complex / Q000378:metabolism; D006801:Humans; D054834:Lipocalins; D011993:Recombinant Fusion Proteins / Q000378:metabolism
C087169:C8G protein, human; D002352:Carrier Proteins; D003185:Complement C8; D015938:Complement Membrane Attack Complex; C498925:LCN8 protein, human; D054834:Lipocalins; D011993:Recombinant Fusion Proteins