Cytotoxic cell granule-mediated apoptosis: perforin delivers granzyme B-serglycin complexes into target cells without plasma membrane pore formation.

The mechanism underlying perforin (PFN)-dependent delivery of apoptotic granzymes during cytotoxic cell granule-mediated death remains speculative. Granzyme B (GrB) and perforin were found to coexist as multimeric complexes with the proteoglycan serglycin (SG) in cytotoxic granules, and cytotoxic cells were observed to secrete exclusively macromolecular GrB-SG. Contrary to the view that PFN acts as a gateway for granzymes through the plasma membrane, monomeric PFN and, strikingly, PFN-SG complexes were shown to mediate cytosolic delivery of macromolecular GrB-SG without producing detectable plasma membrane pores. These results indicate that granule-mediated apoptosis represents a phenomenon whereby the target cell perceives granule contents as a multimeric complex consisting of SG, PFN, and granzymes, which are, respectively, the scaffold, translocator, and targeting/informational components of this modular delivery system.