Characterisation of Mcl-1 cleavage during apoptosis of haematopoietic cells.

Mcl-1 is essential for normal haematopoiesis, being required for lymphocyte development and maintenance. Its role in haematopoietic differentiation and development is associated with its function as an anti-apoptotic member of the Bcl-2 family of proteins although the underlining mechanism is poorly understood. We have characterized caspase cleavage of the Mcl-1 protein during apoptosis. Caspase cleavage resulted in the removal of the PEST regions from the protein and generation of a fragment containing the BH-1, -2 and -3 homology domains. Removal of the PEST regions did not appear to alter Mcl-1 stability, suggesting that these regions are not responsible for Mcl-1's short half-life. In addition, unlike cleavage of Bcl-2 and Bcl-X(L), which resulted in pro-apoptotic fragments, cleaved forms of Mcl-1 were unable to induce apoptosis. This novel regulation of Mcl-1 may have important implications not only for its role in apoptosis but also for the essential role it plays in the differentiation and development of haematopoietic cells.