Calreticulin, a component of the endoplasmic reticulum and of cytotoxic lymphocyte granules, regulates perforin-mediated lysis in the hemolytic model system

Cytotoxic lymphocytes kill virally infected cells with specialized cytotoxic granules containing perforin, a protein that forms toxic pores in the target cell membrane. These specialized cytotoxic granules also contain calreticulin, an endoplasmic reticulum chaperone protein. The calcium-independent association of perforin and calreticulin prompted our evaluation of calreticulin's potential to function as a regulatory molecule that protects cytotoxic lymphocytes from their own perforin. We report here that 10(-7) M calreticulin blocked perforin-mediated lysis in the hemolytic model system using erythrocytes as targets. Previously, we found that millimolar levels of calcium in the hemolytic assays dissociate high-affinity perforin-calreticulin complexes, which makes it unlikely that perforin associates with calreticulin in solution when hemolysis is blocked. Calreticulin may affect perforin at the erythrocyte membrane. We observed calcium-dependent binding of calreticulin to erythrocyte membranes with a K-d Of 2.7 x 10(-7) M and a saturation average of 10(5) molecules calreticulin per erythrocyte. At concentrations that blocked hemolysis, calreticulin occupied many of the calreticulin membrane-binding sites and was in molar excess of perforin. These observations open the possibilities that membrane-bound calreticulin prevents hydrophobic entry of perforin into membranes and (or) prevents perforin from assembling into polyperforin pores.