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Asymmetric thermograms of defatted albumin, alone and in the presence of two model drugs, have been obtained in phosphate buffers at three pH values. The albumin is less thermally stable in the N form, but is protected by both drugs. The nonsteroidal antiinflammatory benoxaprofen offers more protection than warfarin against thermal denaturation.

The cDNA sequence encoding mature human C9 protein and its signal peptide was cloned into three expression vectors for expression in COS-7 (mammalian), Spodoptera frugiperda IPLB-SF-21AE (insect), and Saccharomyces cerevisiae (yeast) cells. In addition, C9 cDNA encoding only the mature protein was fused to the yeast invertase leader sequence (SUC2) and cloned for expression in yeast. Under optimal conditions COS-7 and IPLB-SF-21AE cells secreted recombinant C9 (rC9) at concentrations of about 111 and 700 ng C9/ml culture supernatant, respectively. By comparison S. cerevisiae, whether transformed with C9 cDNA containing its native or yeast invertase leader sequence, secreted only very small amounts of rC9 (5-10 ng/ml). However, upon lysis concentrations of up to 500 ng/mg dry wt were found in yeast cells transformed with C9 cDNA. SDS-PAGE followed by Western blot analysis revealed COS-7 cell and S. cerevisiae expressed rC9 to have a MW similar to that of native C9 purified from human serum, while rC9 from IPLB-SF-21AE cells was about 4 kDa smaller. No hemolytic activity of S. cerevisiae secreted rC9 could be detected and the specific hemolytic activity of S. cerevisiae intracellular rC9 was also very low. However, the specific hemolytic activities of COS-7 and IPLB-SF-21AE secreted rC9 were indistinguishable from that of purified native human C9. Thus, for future studies on the structure and function of C9 where the production of large quantities of mutant protein would be desirable, the baculovirus-insect cell expression system appears to offer considerable advantages.

The effects of sequential additions of purified human complement proteins C5b-6, C7, C8, and C9 to assemble the C5b-9 membrane attack complex (MAC) of complement on electrical properties of planar lipid bilayers have been analyzed. The high resistance state of such membranes was impaired after assembly of large numbers of C5b-8 complexes as indicated by the appearance of rapidly fluctuating membrane currents. The C5b-8 induced conductance was voltage dependent and rectifying at higher voltages. Addition of C9 to membranes with very few C5b-8 complexes caused appearance of few discrete single channels of low conductance (5–25 pS) but after some time very large (greater than 0.5 nS) jumps in conductance could be monitored. This high macroscopic conductance state was dominated by 125-pS channels having a lifetime of approximately 1 s. The high conductance state was not stable and declined again after a period of 1–3 h. Incorporation of MAC extracted from complement-lysed erythrocytes into liposomes and subsequent transformation of such complexes into planar bilayers via an intermediate monolayer state resulted in channels with characteristics similar to the ones produced by sequential assembly of C5b-9. Comparison of the high-conductance C5b-9 channel characteristics (lifetime, ion preference, ionic-strength dependence) with those produced by poly(C9) (the circular or tubular aggregation product of C9) as published by Young, J.D.-E., Z.A. Cohn, and E.R. Podack. (1986. Science [Wash. DC]. 233:184–190.) indicates that the two are significantly different.

Human complement protein C9 is shown to be a metalloprotein that binds 1 mol of Ca2+/mol of C9 with a dissociation constant of 3 micron as measured by equilibrium dialysis. Incubation with EDTA removes the bound calcium, resulting in a apoprotein with decreased thermal stability. This loss in stability leads to aggregation and, therefore, to loss of hemolytic activity upon heating to a few degrees above the physiological temperature. Heat-induced aggregation of apoC9 can be prevented by salts that stabilize proteins according to the Hofmeister series of lyotropic ions, suggesting that the ion in native C9 may ligand with more than one structural element or domain of the protein. Ligand blotting indicates that the calcium binding site is located in the amino-terminal half of the protein. Removal of calcium by inclusion of EDTA in assay mixtures has no effect on the hemolytic activity of C9, and its capacity to bind to C8 in solution, or to small unilamellar lipid vesicles at temperatures at or below the physiological range. Although we do not know yet the precise structural and functional role of the bound calcium, it is clear that it provides thermal stability to C9 and it may have a function in regulation of membrane insertion.

Recent experiments with membrane-restricted, photoactivatable probes indicated a preferential labeling of C9 within the assembled membrane attack complex (MAC) of complement, suggesting a direct role for C9 in the interaction of the MAC with membrane lipids. To further characterize the lipid-binding sites on C9, we have now used C9 that has been cleaved by alpha-thrombin. This enzyme cleaves C9 at one site but the newly generated peptides, C9a and C9b, respectively, remain noncovalently associated and the cleaved protein suffers no loss in hemolytic activity. When cleaved C9 was incorporated into the MAC during assembly on phospholipid vesicles and photolabeled, subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and fluorography revealed that only the larger fragment C9b, but not the smaller fragment C9a, became labeled. C9 attached alone to vesicles through heat aggregation in the absence of the precursor complex C5b-8 is also accessible to the hydrophobic photolabel. When cleaved C9 is used in the heat-induced assembly on vesicles and the polymerized C9 is photolabeled, the label associates again predominantly with C9b and not C9a. These results not only show that, within C9 polymers or within the assembled MAC, C9 possesses a two-domain structure, but also lend considerable support to the structure proposed for C9 by Biesecker et al. (Biesecker, G., Gerard, C., and Hugli, T. E. (1982) J. Biol. Chem. 257, 2584-2590) who classified C9a as hydrophilic and C9b as hydrophobic.