The role of calpain in the regulation of CFTR turnover
Résumé
The level of mature native 170 kD at plasma membrane is under the control of a selective proteolysis catalyzed by calpain. The product of this limited digestion, consisting of discrete fragments still associated by strong interactions, is removed from the plasma membrane and internalized in vesicles addressed to an additional degradation. This process can be monitored by the accumulation of a 100 kD fragment in growing human leukaemic T cell line and in human circulating lymphocytes. In reconstructed systems and in intact cells, the conversion of native CFTR into 100 kD fragment linearly correlated to calpain activation and was prevented by addition of synthetic calpain inhibitors. Reduction in Ca2+ influx by blocking the NMDA receptor Ca2+ channel inhibited conversion of native 170 kD into 100 kD fragment, whereas the endosome acidification blocker promoted accumulation of the 100 kD CFTR digested form. An important role in calpain-mediated turnover of CFTR is exerted by HSP90, which, associating to the protein channel, modulates the degradative effect of calpain through a selective protection. Altogether these results indicate that CFTR turnover is initiated by calpain activation, induced by an increased Ca2+ influx and, following internalization of the cleaved channel protein, it is completed by the lysosomal proteases. These findings are providing new insights in the understanding of the molecular mechanisms responsible for the defective functions of ion channels in human pathologies.
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