Planar lipid bilayer membranes (BLM) have been used to study three features of the C5b-9 membrane attack sequence, namely: 1) whether the C5b-9 complex is assembled outside or within the membrane, 2) whether membrane thickness plays a role in the action of the cytolytic sequence, and 3) whether the terminal components increase membrane permeability in discrete steps, i.e., quanta, which would suggest trans-membrane channels.
In the first study involving “CIS/TRANS” experiments, we examined the process of C5b-9 assembly with respect to “sidedness”. The results show that the cytolytic sequence is divided into two reaction pairs, C5b, 6 plus C7 and C5b-8 plus C9. The reaction between the partners of each pair requires their presence on the same side of the membrane. However, the coupling between the pairs, i.e., between the C5b, 6, 7 complex and C8, can occur from opposite sides of the membrane, indicating that it occurs within the lipid interior of the bilayer.