Channel fluctuations induced by membrane attack complex C5b-9

The assembly of complement (C) components C5b-9 in membranes results in the formation of transmembrane lesions. The C9 component has been shown to be mainly responsible for formation of the ultrastructurally visible tubules associated with C5b-9 complexes. Several studies have disputed the role of C9 polymerization in C-mediated cytolysis on the grounds that C5b-9 lyses cells in the absence of tubular formation. Here, C5b-9 complexes were reconstituted into high-impedance planar lipid bilayers and shown to form channels which are heterogenous in size. The smallest channels had unitary conductances of 15 picoSiemens (pS) in 0.1 M NaCl. The closing of these channels showed voltage-dependence at membrane potentials exceeding 40 mV. These channels were more cation-selective, with K⁺ ions being favored over Na⁺. The 15-pS channels described here are much smaller than the channels attributed previously to either C5b-9 or polymerized C9 complexes but resemble channels formed by the C9b fragment, which does not polymerize into tubules. These results indicate that C5b-9 complexes are capable of damaging membranes by forming initially small ion channels which then aggregate in the membrane to form tubular lesions with much larger conductances. Like C5b-9, C5b-8 also increased membrane permeability. However, this increase in membrane conductance could not be resolved into single channels, suggesting that C5b-8 may induce membrane leakiness by perturbing the packing of membrane lipids, whereas addition of C9 results in authentic production of ion channels.