CD59, an LY-6-like protein expressed in human lymphoid cells, regulates the action of the complement membrane attack complex on homologous cells.

Abstract

The lysis of cells by complement requires only the terminal components C5, C6, C7, C8 and C9 and is initiated by the cleavage of C5 to C5b. Sequential addition of C6, C7, C8, and C9 to C5b leads to the formation of the membrane attack complex (MAC)' which, when inserted into the lipid bilayer, can form transmembrane pores (1-5). It is well known that when complement of one species is activated on homologous erythrocytes, lysis is much less efficient than when it is activated on other species of cell, and even among different heterologous cell species the lytic efficiency may be very different. It has long been known that the basis of this variable lytic efficiency is found, at least in part, at the C8 and/or C9 step (6-9). More recently, specific membrane proteins have been described that appear to protect cells from homologous complement . The first of these to be described was the decayaccelerating factor (DAF), a membrane protein of -70 kD molecular mass (10). This protein interferes with the assembly of the C3 converting enzymes both of the classical and alternative pathway (10, 11) and therefore it has only indirect effects on the cell lytic mechanism. A further membrane protein that does restrict homologous lysis, and that has been described both as the C8-binding protein (C8bp) (7, 12) and as homologous restriction factor (HRF) (13), has also been isolated . It seems likely that both these descriptions apply to a single protein of 65 kD molecular mass. In addition, a 55/65-kD MAC-inhibiting protein (MIP) with the capacity to bind C8 and C9 has been identified both on human erythrocyte membranes and in normal human serum (14). The relationship ofthis to HRF/C8bp is not yet clear. Both DAF and HRF/C8bp are bound on cell membranes by a glycolipid anchor (15, 16) and can be eluted from the cell membrane, at least in part, by phosphatidylinositol-specific phospholipase C. These proteins also have the capacity when they are isolated from

Sequence‐specific

“…This is consistent with previous studies of CD59 isolated from urine (Davies et al, 1989;Tomita et al, 1991).…”

“…The CD59 gene codes for a 128-residue polypeptide that is subsequently modified by removal of a signal peptide and of a C-terminal peptide; the C-terminal cleavage occurs during attachment of the GPI membrane anchor (Davies et al, 1989;Okada et al, 1989b;Stefanova et al, 1989). The nascent protein is further modified by the addition of an N-glycan to Asn 18 (Davies et al, 1989;Stefanova et al, 1989;Ninomiya et al, 1992).…”

“…The nascent protein is further modified by the addition of an N-glycan to Asn 18 (Davies et al, 1989;Stefanova et al, 1989;Ninomiya et al, 1992). A preliminary report (Tomita et al, 1991) indicated that Asn 77 is the C-terminal residue of CD59 isolated from urine and that this soluble form of the protein is derived from membrane-bound CD59 by removal of the lipid part of the GPI anchor.…”

“…Whereas the amino acid sequence shows no similarity to other complement components and control proteins, 201 5 it has limited sequence homology to murine LY-6 antigen and the urokinase plasminogen activator receptor (Davies et al, 1989;Stefanova et al, 1989). Motif pattern searching using residues that are conserved among these three proteins revealed further similarities with the snake venom neurotoxin family of proteins (Harrison, 1992).…”

Activity after Site-Directed Mutagenesis of CD59 on Complement-Mediated Cytolysis

“…CD59 was first identified as a regulator of the terminal of complement, which acted by binding to the C8/C9 components, thus preventing the formation of MAC, to interfere with C9 membrane insertion, polymerization and pore formation (3)(4)(5). The precursor of human CD59 is a single peptide composed of 128 amino acids deduced from its cDNA sequence (6,7). The mature protein consists of 77 amino acids arranging in a single cysteine-rich domain composed of two antiparallel β-sheets, 5 protruding surface loops and a short helix (8)(9)(10).…”