Perforin is a key effector protein of the vertebrate immune system. Secreted as soluble monomers by cytotoxic T lymphocytes and natural killer cells, perforin selectively forms oligomeric transmembrane pore assemblies on the surface of virus infected and cancerous cells. These 10-20 nm wide pores allow diffusion of co-secreted granzymes into the target cells, which next trigger apoptotic cell killing. To understand the pathway of perforin pore assembly on/in target membranes, we have visualised different stages of this process by atomic force and electron microscopy. Initially, perforin forms intermediate, prepore oligomers of up to 8 subunits on the membrane surface. These short oligomers can subsequently convert to membrane pores with a tighter subunit packing. These pore assemblies next recruit further prepore oligomers from the membrane surface to grow the pore size. Most of the resulting arc- and ring-shaped perforin pores contain between 10 - 30 subunits. To identify mechanisms by which immune cells are protected from self-harm by perforin, we have investigated how perforin pore assembly depends on the lipid composition of the target membrane. Perforin binding is affected by the packing density of lipid molecules in the membrane: It does not or hardly binds to raft-like, liquid ordered lipid domains. Furthermore, negatively charged membrane surfaces, i.e., rich in phosphatidylserine, allow perforin binding and oligomerization of short intermediate assemblies, but subsequently trap these assemblies in a non-porating, dysfunctional state. These findings coincide with reports of increased lipid packing and phosphatidylserine exposure on the surface of activated cytotoxic T lymphocytes
