Ultrastructure of the first component of human complement: electron microscopy of the crosslinked complex

Electron micrographs are shown of the first component of human complement (C1) which has been crosslinked with a water-soluble carbodiimide to prevent dissociation into its C1q and C1r2C1s2 subunits. Two projections of the crosslinked molecule are seen in the electron micrographs, which are called "top" and "profile." In both views, the C1q heads are visible. From the top, the C1r2C1s2 tetrameric subunits appears to be located centrally on the C1q and folded to form a compact mass obscuring most of the arms and central bundle. In profile, the tetramer appears to be located in the region of the arms between the C1q heads and the central bundle. Both the heads and the rod-like central bundle appear to be free of C1r2C1s2 in these profile projections. Sometimes it is possible to count more than six domains in the region of the C1q heads, as though a portion of the tetramer had unfolded to protrude among the heads.