Assembly of complement components C5b-8 and C5b-9 on lipid bilayer membranes: visualization by freeze-etch electron microscopy.
We have visualized by freeze-etch electron microscopy the macromolecular complexes of complement, C5b-8 and C5b-9, respectively, assembled on synthetic phospholipid bilayers. These complexes were formed sequentially by using purified human complement components C5b-6 followed by C7, C8, and C9. Complexes of C5b-8 were observed on the external surface (ES) of vesicles as 12-nm particles that tended to form polydisperse aggregates. The aggregates were sometimes of a regular chainlike structure containing varying numbers of paired subunits. Etching of vesicles containing C5b-9 complexes revealed on the ES large rings of approximately 27-nm outer diameter. One or two knobs usually were attached to the perimeter of the rings. Splitting of the membrane resulted in partitioning of the C5b-9 with the outer leaflet. Thus, round holes of approximately 17-nm diameter were present in the protoplasmic face (PF), and raised circular stumps of a matching size were present on the exoplasmic face (EF) of C5b-9 vesicles. C5b-9 complexes were frequently localized in regions of the lowest lipid order. That is, in micrographs of the EF and ES, single C5b-9 complexes were located where the ripples of the P beta' phase bend or reach a dead end, and linear arrays of C5b-9 complexes outlined disclination-like structures in the lattice; the holes in the PF mirrored this distribution. The membrane immediately surrounding C5b-9 rings was often sunk inwardly over an area much larger than that of the ring itself.(ABSTRACT TRUNCATED AT 250 WORDS)
McCloskey
M A
MA
Department of Physiology and Biophysics, University of California, Irvine 92717.
Dankert
J R
JR
Esser
A F
AF
D015060:1,2-Dipalmitoylphosphatidylcholine; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins / Q000378:metabolism / Q000648:ultrastructure*; D005456:Fluorescent Dyes; D005613:Freeze Etching; D008051:Lipid Bilayers; D008854:Microscopy, Electron / Q000379:methods; D010714:Phosphatidylethanolamines; D010718:Phosphatidylserines
D015938:Complement Membrane Attack Complex; D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D010714:Phosphatidylethanolamines; D010718:Phosphatidylserines; C042295:complement C5b-8 complex; D015060:1,2-Dipalmitoylphosphatidylcholine; C050294:1-palmitoyl-2-oleoylglycero-3-phosphoserine; C042077:N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethanolamine; D003165:Complement System Proteins