Stabilization of thymic glucocorticoid-receptor complexes by the calcium-activated protease inhibitor, calpastatin

We previously described a heat-stable factor from WEHI-7 mouse thymoma, rat liver, spleen, and human chronic lymphocytic leukemia cells that prevents degradation of glucocorticoid-receptor complexes (GRC) in cytosols from rat thymus and acute non-lymphocytic leukemia cells. We now show that the factor has many properties in common with calpastatin, a naturally occurring inhibitor of a family of neutral calcium-activated proteases called calpains. Liver GRC-stabilizing activity and calpastatin activity, in addition to surviving boiling, co-chromatography on columns of DEAE-cellulose ion exchange or agarose A-0.5M gel filtration matrices, and have identical isoelectric points of 5.1. This factor should be especially useful for studying GRC function in the presence of calcium.