CD59 (also called 1F-5Ag, H19, HRF20, MACIF, MIRL, P-18 or protectin) inhibits formation of membrane attack complex (MAC), thus protecting cells from complement mediated lysis. It has a signalling role, as a GPI-anchored molecule, in T cell activation and appears to have some role in cell adhesion through CD2 (controversial). CD59 associates with C9, inhibiting incorporation into C5b-8 preventing terminal steps in polymerisation of the (MAC) in plasma membranes. Genetic defects in GPI-anchor attachment that cause a reduction or loss of both CD59 and CD55 on erythrocytes produce the symptoms of the disease paroxysmal nocturnal haemoglobinuria (PNH). A variety of GPI-linked cell-surface glycoproteins are composed of one or more copies of a conserved domain of about 100 amino-acid residues [], []. Among these proteins, U-PAR contains three tandem copies of the domain, while all the others are made up of a single domain.As shown in the following schematic, this conserved domain contains 10 cysteine residues involved in five disulphide bonds - in U-PAR, the first copy of the domain lacks the fourth disulphide bond.+------+ +------------------------+ +---+| | | | | |xCxxCxxxxxxCxxxxxCxxxxxCxxxxxxxxxxxxxxxxxxCxxxxCxxxxxxxxxxxxxxCCxxxCxxxxxxxx| | | |+---------------------+ +--------------+'C': conserved cysteine involved in a disulphide bond.
