CLUSTERIN, THE HUMAN APOLIPOPROTEIN AND COMPLEMENT INHIBITOR, BINDS TO COMPLEMENT-C7, C8-BETA, AND THE B-DOMAIN OF C9

Clusterin is a heterodimeric multifunctional protein expressed in a va riety of tissues and cells. It forms high density lipid complexes in p lasma and participates in the control of the lytic activity of the lat e complement complex (TCC, C5b-9). Together with vitronectin, clusteri n binds to the nascent amphiphilic C5b-9 complex, rendering it water s oluble and lytically inactive. To define the interactions that underli e the complement-inhibitory function of clusterin, we have examined th e binding interactions between [I-125]clusterin and the isolated compo nents of the complex, C5b-6, C7, C8, and C9 and vitronectin. By using ligand blotting in the presence of Tween, specific binding of the labe led clusterin with C7, the beta-subunit of C8 and C9 was detected. Bin ding to C9 was competed by polymerized C9, but not by C8, C7, C6, and CD59, suggesting that the conformational change occurring during the h ydrophilic-amphiphilic transition of C9 exposes the interaction site f or clusterin. When thrombin-treated C9 was analyzed, clusterin was fou nd to recognize the C9b fragment containing the hydrophobic membrane i nteraction segment. Both subunits of clusterin interact with C9 and ar e similarly potent in inhibiting C5b-9-mediated hemolysis and Zn++-ind uced C9 polymerization. These results show that clusterin exerts its i nhibitory effect by interacting with a structural motif common to C7, C8alpha, and C9b.