Decay-accelerating factor (DAF) of human erythrocytes is a glycoprotein with a M(r) of 65,000 that is anchored in the membrane via a glycolipid tail. During the purification of DAF, two lower m.w. forms were noted. DAF-A had an M(r) of 63,000, and DAF-B had an M(r) of 55,000. In a fluid phase assay, both forms accelerated the decay of the classical and the alternative C3 convertases with a specific activity similar to that of DAF. However, the decay-accelerating activity for the cell-bound C3 convertases was abolished, suggesting that neither could insert into E membranes and therefore that the glycolipid tail is altered. Analysis by molecular sieve high-pressure liquid chromatography demonstrated that DAF-A eluted with a M(r) of ~450,000, similar to native DAF, and was thus in an aggregated form. In contrast, DAF-B eluted as a monomer with a M(r) of ~60,000. DAF-A, but not DAF-B, bound to a hydrophobic column. To further characterize these two forms, surface-labeled human erythrocytes were incubated with phosphatidyl inositol-specific phospholipase C or papain. The phospholipase inefficiently released a form of DAF that was slightly larger (M(r) of 64,000) than DAF-A. Papain efficiently released a 55,000 fragment that had the same M(r) as DAF-B. To determine if DAF was cleaved by endogenous enzymes, surface-labeled erythrocytes were incubated with leukocytes. The kinetics of the leykocyte-induced degradation was similar to those observed with papain, and the released fragment aligned on seizing gels with the papain-derived fragment. We hypothesize that endogenous phospholipases and proteases cleave DAF to produce fragments similar to DAF-A and DAF-B, respectively.
|Number of pages||8|
|Journal||Journal of Immunology|
|State||Published - Jan 1 1987|