Perforin pores in the endosomal membrane trigger release of endocytosed granzyme B to the cytosol of target cells

How the pore-forming protein perforin delivers apoptosis-inducing granzymes to the cytosol of target cells is uncertain. Perforin induces a transient Ca(2+) flux in the target, which triggers a damaged cell membrane repair process. As a consequence, both perforin and granzymes are endocytosed into enlarged endosomes, called gigantosomes. Here we show that perforin forms pores in the gigantosome membrane, allowing endosomal cargo, including granzymes, to be gradually released. After about 15 minutes, gigantosomes rupture, releasing their remaining content. Thus, perforin delivers granzymes by a two-step process that first involves transient pores in the cell membrane that trigger granzyme and perforin endocytosis and then pore formation in endosomes to trigger cytosolic release.