Polymerization of the ninth component of complement (C9): formation of poly(C9) with a tubular ultrastructure resembling the membrane attack complex of complement.

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Abstract
The 9th component of complement (C9) has a marked propensity to polymerize. C9 polymers [poly(C9)] formed spontaneously in Veronal-buffered saline upon incubation of purified human C9 for 64 h at 37.degree. C or within 2 h at 46-56.degree. C. Poly(C9) formed at 37.degree. C was visualized by EM as a tubular structure with an internal diameter of 110 .ANG. and a length of 160 .ANG.. Its ultrastructure suggested a dodecameric composition and resembled that of the membrane attack complex of complement. The wider end of the tubular structure was formed by an .apprxeq. 30-.ANG.-thick torus with inner and outer diameters of 110 .ANG. and 220 .ANG., respectively. Because the dimensions of C9 within poly(C9) were 160 .times. 55 .ANG. (maximal) and 20 .ANG. (minimal) and because monomeric C9 has dimensions of approximately 80 .times. 55 .ANG., monomeric C9 probably unfolds during polymerization into tubules. Polymerization also occurred upon treatment of C9 for 1 h at 37.degree. C with 0.6 M guanidine.cntdot.HCl, 0.1 M octyl glucoside or 1.5% sodium deoxycholate. Guanidine.cntdot.HCl-induced C9 polymers consisted of elongated highly curved strands 55-80 .ANG. wide, suggesting that these polymers were formed by globular C9 that had not unfolded.