Amino-terminal amino acid sequence and chemical and functional properties of a membrane attack complex-inhibitory factor from human erythrocyte membranes.

The protein corresponding to P-18 (Sugita et al. (1988) J. Biochem, 104, 633-637) was isolated from native human erythrocyte, and newly designated membrane attack complex-inhibitory factor (MACIF). The amino-terminal sequence of this protein was determined to be Leu-Gln-Cys-Tyr-Asn-Cys-Pro-Asn-Pro-Thr. Endoglycosidase F digestion of MACIF decreased its molecular weight by about 6K on SDS-PAGE. On the other hand, endoglycosidase H, neuraminidase, or endo-alpha-N-acetylgalactosaminidase treatment had no effect on the molecular weight, indicating that MACIF has complex-type N-linked oligosaccharide chains, but no O-linked chain. MACIF was highly resistant against trypsin digestion and heat treatment. The inhibitory activity of MACIF on the hemolysis of EC5-8 cells was comparable to that on EC5-7 cells, indicating that MACIF inhibited the binding of C9 to the intermediate cells, or the subsequent C9 polymerization.