Protectin, a complement regulatory protein, also known as CD59, or MIRL (membrane inhibitor of reactive lysis) is a human gene and protein. CD59 inhibits the complement membrane attack complex by binding C5b678 and preventing C9 from binding and polymerizing. It is present on "self" cells to prevent complement from damaging them. CD59, along with CD55 are not present in the condition paroxysmal nocturnal hemoglobinuria. Viruses such as HIV, human cytomegalovirus and vaccinia incorporate host cell CD59 into their own viral envelope to prevent lysis by complement.
CD59a is potent inhibitor of the complement membrane attack complex (MAC) action. CD59a acts by binding to the C8 and/or C9 complements of the assembling MAC, thereby preventing incorporation of the multiple copies of C9 required for complete formation of the osmolytic pore. This inhibitor appears to be species-specific. CD59a is involved in signal transduction for T-cell activation complexed to a protein tyrosine kinase. The soluble form from urine retains its specific complement binding activity, but exhibits greatly reduced ability to inhibit MAC assembly on cell membranes.
CD59a is homologous restriction factor (18 kD) binding through C8G to the membrane attack complex of complement as a membrane inhibitor of cell lysis
CD59a functions as a co-receptor in natural killer cell activation
CD59a inhibits assembly of the lytic membrane attack complex of complement by incorporation into the forming complex