Decay-accelerating factor

Abstract: Decay-accelerating factor (DAF), a glycoprotein that is anchored to the cell membrane by phosphatidylinositol, binds activated complement fragments C3b and C4b, thereby inhibiting amplification of the complement cascade on host cell membranes. Here, we report the molecular cloning of human DAF from HeLa cells. Analysis of DAF complementary DNAs revealed two classes of DAF messenger RNA, one apparently derived from the other by a splicing event that causes a coding frameshift near the C terminus. The apparent 'intron' sequence contains an Alu family member and encodes contiguous protein sequence. Two DAF proteins are therefore possible, having divergent C-terminal domains which differ in their hydrophobicity. Both mRNAs are found on polysomes, suggesting that both are translated. We propose that the major (90%) spliced DAF mRNA encodes membrane-bound DAF whereas the minor (10%) unspliced DAF mRNA may encode secreted DAF and we present expression data supporting this. The deduced DAF sequence contains four repeating units homologous to a consensus repeat found in a recently described family of complement proteins.

Abstract: Decay-accelerating factor (DAF) is a 70,000 Mr membrane protein that inhibits amplification of the complement cascade on the cell surface, and protects cells from damage. Purified DAF can be reincorporated into the membrane of red cells and is functional. DAF is deficient in paroxysmal nocturnal hemoglobinuria (PNH), a disease characterized by increased sensitivity of erythrocytes to complement lysis. We show here that DAF is part of a newly described family of membrane proteins anchored to the lipid bilayer by means of phosphatidylinositol (PI). Treatment with PI-specific phospholipase C (PIPLC) releases 70-80, 60, and 10% of cell surface DAF from mononuclear cells, neutrophils, and erythrocytes, respectively. The PIPLC-released DAF (DAF-S) is slightly smaller (67,000 Mr) than the membrane form. DAF and DAF-S cannot be distinguished antigenically. Furthermore, DAF-S has lost its ability to significantly inhibit the C3-convertase, as well as its ability to incorporate into cell membranes. Since DAF can only inhibit C3-convertase endogenously, i.e., within the membrane of the same cell, it is likely that the loss of activity of DAF-S is causally related to its inability to reincorporate in the lipid bilayer. As shown by others, the complement-sensitive red cells from PNH patients lack acetylcholinesterase, which is also anchored to the membrane by PI (9). Thus it is possible that the molecular defect in PNH lies in the biosynthetic pathways leading to the attachment of PI to the polypeptide chains, in the transport of these proteins to the surface, or in their release by the action of endogenous phospholipases. From a practical standpoint the specific release of DAF by PIPLC could facilitate killing of tumor cells by amplifying the effects of the complement cascade on the surface of antibody-sensitized cells.

Abstract: Echoviruses are human pathogens belonging to the picornavirus family. Decay-accelerating factor (DAF) is a glycosylphosphatidylinositol (GPI)-anchored surface protein that protects cells from lysis by autologous complement. Anti-DAF monoclonal antibodies prevented echovirus 7 attachment to susceptible cells and protected cells from infection. HeLa cells specifically lost the capacity to bind echovirus 7 when treated with phosphatidylinositol-specific phospholipase C, an enzyme that releases GPI-anchored proteins from the cell surface, indicating that the virus receptor, like DAF, is a GPI-anchored protein. Although Chinese hamster ovary cells do not bind echovirus 7, transfectants expressing human DAF bound virus efficiently, and binding was prevented by pretreatment with an anti-DAF monoclonal antibody. Anti-DAF antibodies prevented infection by at least six echovirus serotypes. These results indicate that DAF is the receptor mediating attachment and infection by several echoviruses.

Abstract: The alternative pathway of complement activation provides the host with a humoral component of the natural defense mechanisms against infectious agents. The six plasma proteins involved in recognition and activation perform a continuous surveillance function that does not require specific antibody to recognize potential pathogens. Discrimination between host and foreign particles occurs because activation of the system is under strict control by regulatory plasma and membrane proteins of the host. Full activation only occurs when the function of these regulators is dampened on the surface of foreign particles. Organisms sensitive to attack by the alternative pathway include bacteria, fungi, certain viruses, virus-infected cells, some tumor cell lines, and human erythrocytes lacking the decay accelerating factor (DAF) in their membrane. Activation of the pathway on these particles results in the deposition of large numbers of C3b molecules and assembly of the cytolytic membrane attack complex. These events trigger, through a variety of mediators, several cellular responses including leukocyte chemotaxis, release of vasoactive amines, hydrolytic enzymes and of arachidonic acid metabolites, adhesion, and phagocytosis.