BINDING OF HUMAN AND RAT CD59 TO THE TERMINAL COMPLEMENT COMPLEXES

CD59-antigen (protectin) is a widely distributed glycolipid-anchored i nhibitor of complement lysis. CD59 interacts with complement component s C8 and C9 during assembly of the membrane attack complex (MAC). To e valuate species specificity of these interactions we have in the prese nt study examined cross-species binding of isolated human and rat CD59 to the terminal complement components C8 and C9. By using primarily s oluble CD59 isolated from urine (CD59(U)) potentially non-specific bin ding interactions of the phospholipid portion of the membrane forms of CD59 could be avoided. Sucrose density gradient ultracentrifugation a nalysis showed that human CD59(U) bound to both human and rat C8 in th e SC5b-8 complexes. Similar binding occurred when rat CD59(U) was used . The degree of binding did not significantly differ between the heter ologous and homologous CD59-C8 combinations. C9 from both species inhi bited the binding of CD59 to soluble SC5b-8. In ligand blotting analys is human and rat CD59(U) bound to human and rat C8 alpha gamma-subunit and C9. Binding of human and rat CD59(U) was stronger to human than r at Cg. In plate binding assays the erythrocyte form of CD59 (CD59(E)) bound to both human and rat C8. Binding of CD59(E) to heterologous C9 was considerably weaker than to homologous C9. Our results imply that the reciprocal binding sites between C8 and CD59 and to a lesser degre e between CD59 and C9 are conserved between human and rat. Interaction s of CD59 with the terminal C components are thus species selective bu t not 'homologously restricted'.