The architectural transition of human complement component C9 to poly(C9)

Several regions of C9 including three cysteine-rich modules homologous to those in thrombospondin (TS), the low density lipoprotein receptor (LDL), the epidermal growth factors (EDGF), as well as two middle sections of the polypeptide chain were expressed in bacteria. Antibodies derived from these segments were used to probe the relative exposure of epitopes in C9 and poly(C9) using ELISAs. The results indicated that the TS and LDL modules are fully exposed in both monomer and polymer; however, the middle region of the polypeptide chain is buried in the monomer but external in the polymer. Using specified conditions, Fab fragments to the TS and LDL modules did not block C9 polymerization, but those to the middle region of the polypeptide chain and to some extent to the EDGF module did so. Immuno-electron microscopy of poly(C9) indicated that the C9 polypeptide chain assumes a 'U' shape, in which the TS and LDL modules are located on the upper rim. The EDGF module is located on the lower edge of the upper rim, and midsection of the polypeptide chain constructs the barrel of the tubule. Computer assisted contrast enhancement of select electron micrograph images of poly(C9) allowed the clear visualization of each subunit. These were seen to have a volute shape. The upper rim is composed of whorls that are apparently not in lateral contact. It is concluded that the TS and LDL modules do not participate directly in polymerization but cover the hydrophobic central region of the polypeptide chain in the monomer. As a consequence of circular polymerization the midsection of the polypeptide chain becomes exposed as each C9 lengths to fashion a volute form. reserved.