Perforin is a pore forming protein of the vertebrate immune system. It is released by cytotoxic T lymphocytes to form 10-20 nm wide holes in the membranes of virus infected or cancerous target cells. These perforations enable diffusion of co-secreted granzymes into the target cells, which next trigger apoptotic cell death.
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, short prepore oligomers on the membrane surface. These short oligomers can subsequently convert to membrane pores and recruit adfitional prepores to further grow in the membrane. Most of the resulting arc- and ring-shaped perforin pores contain between 10 - 30 subunits.
Using a similar experimental approach, we next set out to study and quantify the interaction of perforin with different lipid substrates. In particular, perforin binding was refractory on densely packed, raft-like lipids, and formed dysfunctional protein aggregations on membranes containing the negatively charged lipid phosphatidylserine. Such lipid properties are found in T lymphocyte membranes at the site of perforin release, and thus have important implications in protecting the immune cells from self-harm.