A novel mechanism for protein delivery: granzyme B undergoes electrostatic exchange from serglycin to target cells.

The molecular interaction of secreted granzyme B-serglycin complexes with target cells remains undefined. Targets exposed to double-labeled granzyme B-serglycin complexes show solely the uptake of granzyme B. An in vitro model demonstrates the exchange of the granzyme from serglycin to immobilized, sulfated glycosaminoglycans. Using a combination of cell binding and internalization assays, granzyme B was found to exchange to sulfated glycosaminoglycans and, depending on the cell type, to higher affinity sites. Apoptosis induced by purified granzyme B and cytotoxic T-cells was diminished in targets with reduced cell surface glycosaminoglycan content. A mechanism of delivery is proposed entailing electrostatic transfer of granzyme B from serglycin to cell surface proteins.