CHIMERAS OF HUMAN-COMPLEMENT C9 REVEAL THE SITE RECOGNIZED BY COMPLEMENT REGULATORY PROTEIN CD59

CD59 antigen is a membrane glycoprotein that inhibits the activity of the C9 component of the C5b-9 membrane attack complex, thereby protect ing human cells from lysis by human complement. The complement-inhibit ory activity of CD59 is species-selective and is most effective toward C9 derived from human or other primate plasma. By contrast, rabbit C9 , which can substitute for human C9 in the membrane attack complex, me diates unrestricted lysis of human cells. To identify the peptide segm ent of human C9 that is recognized by CD59, rabbit C9 cDNA clones were isolated, characterized, and used to construct hybrid cDNAs for expre ssion of full-length human/rabbit C9 chimeras in COS-7 cells. Ah resul ting chimeras were hemolytically active, when tested against chicken e rythrocytes bearing C5b-8 complexes. Assays performed in the presence or absence of CD59 revealed that this inhibitor reduced the hemolytic activity of those chimeras containing human C9 sequence between residu es 334-415, irrespective of whether the remainder of the protein conta ined human or rabbit sequence. By contrast, when this segment of C9 co ntained rabbit sequence, lytic activity was unaffected by CD59. These data establish that human C9 residues 334-415 contain the site recogni zed by CD59, and they suggest that sequence variability within this se gment of C9 is responsible for the observed species-selective inhibito ry activity of CD59.