Electron microscopy of two homologous giant proteins revealed that complement factor C3 and αi-inhibitor 3 have a common structural motif of a semicircularly bent string 18–20 nm long with two or three bumps indicating globular domains. C3 had a structure similar to the letter C with a small but distinct hole in the center. α1-Inhibitor 3 was a more complete ring sometimes ajar at one corner. When the latter was treated with a proteinase, it became slightly flattened and adopted a squarish C-shape.
