Doughnut Structure

Cell biology A homopolymeric protein contained in the outer wall of gram-negative bacteria and the outer membrane of mitochondria that is freely permeable to small, < 10 kD molecules
Immunology A multimeric assembly of C9 complement protein monomers inserted into cells, forming a transmembranous pore through which a cell targeted by the complement cascade disintegrates, an event that follows the common pathway of complement activation and prior reversible assembly of a leaky patch