Characterization of the calpain/calpastatin system in human hemopoietic cell lines

As previously suggested by PCR analysis [R. DeTullio, R. Stifanese, F. Salamino, S. Pontremoli, E. Melloni, Characterization of a new p94-like calpain form in human lymphocytes, Biochem. J. 375 (2003) 689-696], a p94-like calpain was now established to be present in six different human cells resembling the various peripheral blood cell types. This protease resulted to be the predominant calpain isoforms whereas the conventional mu- and m-calpains are also expressed although at lower or almost undetectable amounts. The p94-like calpain has been identified by a specific mAb and displays unique features such as: Ca2+ requirement for half maximum activity around 30 microM; no autolytic conversion to a low Ca2+ requiring form and lower sensitivity to calpastatin inhibition. Following cell stimulation, the p94-like calpain undergoes inactivation, a process indicating that the protease is activated and participates in the cell responses to stimuli. The involvement of this protease isoform in immunocompetent cell activation is further supported by its partial recruitment on plasma membranes, the site of action of the conventional calpain forms. The amount of calpain translocated to the membranes correlates to the level of calpastatin which has been shown to control this process through the formation of a complex with calpain, which maintains the protease in the cytosol. These results provide new information on the calpain/calpastatin system expressed in immunocompetent cells and on the functional relationship between the p94-like calpain and the biological function of these cells.