INTERACTION BETWEEN A CA2-BINDING PROTEIN CALRETICULIN AND PERFORIN, A COMPONENT OF THE CYTOTOXIC T-CELL GRANULES()

Calreticulin is a component of cytotoxic T-lymphocyte and NK lymphocyt e granules. We report here that granule-associated calreticulin termin ates with the KDEL endoplasmic reticulum retrieval amino acid sequence and somehow escapes the KDEL retrieval system. In perforin knock-out mice calreticulin is still targeted into the granules, Thus, calreticu lin will traffic without perforin to cytotoxic granules. In the granul es, calreticulin and perforin are associated as documented by (i) copu rification of calreticulin with perforin but not with granzymes and (i i) immunoprecipitation of a calreticulin-perforin complex using specif ic antibodies. By using calreticulin affinity chromatography and prote in ligand blotting we show that perforin binds to calreticulin in the absence of Ca2+ and the two proteins dissociate upon exposure to 0.1 m M or higher Ca2+ concentration. Perforin interacts strongly with the P -domain of calreticulin (the domain which has high Ca2+-binding affini ty and chaperone function) as revealed by direct protein-protein inter action, ligand blotting, and the yeast two-hybrid techniques, Our resu lts suggest that calreticulin may act as Ca2+-regulated chaperone for perforin. This action will serve to protect the CTL during biogenesis of granules and may also serve to regulate perforin lytic action after release.