The ninth component of complement and the pore-forming protein (perforin 1) from cytotoxic T cells: Structural, immunological, and functional similarities

The ninth component of complement (C9) and the pore-forming protein (PFP or perform) from cytotoxic T lymphocytes polymerize to tubular lesions having an internal diameter of 100 Å and 160 Å, respectively, when bound to lipid bilayers. Polymerized C9, assembled by slow spontaneous or rapid Zn ²⁺-induced polymerization, and polyperforin, which is assembled only in the presence of Ca²⁺, constitute large aqueous pores that are stable, nonselective for solutes, and insensitive to changes of membrane potential. Monospecific potydonal antibodies to purified C9 and PFP show cross-reactivity, suggesting structural homology between the two molecules. The structural and functional homologies between these two killer molecules imply an active role for pore formation during cell lysis.