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.
Laboratory of Cellular Physiology and Immunology, Rockefeller University, New York, NY 10021.
J D
JD
Young
T M
TM
Young
D002463:Cell Membrane Permeability / Q000276:immunology; D003186:Complement C9 / Q000502:physiology; D015938:Complement Membrane Attack Complex / Q000502:physiology*; D003165:Complement System Proteins / Q000502:physiology; D004553:Electric Conductivity; D006801:Humans; D007473:Ion Channels / Q000502:physiology*; D008563:Membrane Lipids / Q000378:metabolism
D003186:Complement C9; D015938:Complement Membrane Attack Complex; D007473:Ion Channels; D008563:Membrane Lipids; C042295:complement C5b-8 complex; D003165:Complement System Proteins