Apoptosis is a highly orderly process, which is important for development, maintaining tissue normality as well as the prevention of diseases. The mechanism of apoptosis is very complicated. It is known that mitochondria play a crucial role by releasing pro-apoptotic molecules from intermembrane space under certain apoptotic stimuli. A key step to trigger the release of mitochondrial intermembrane proteins is the translocation of Bax from the cytosol to mitochondria outer membrane to form aggregates. The mechanisms for this process, however, have not been fully understood yet. Apparently, other Bcl-2 family proteins also play important roles in regulating Bax translocation and aggregation. Among them, Mcl-1 is one of the major negative regulators. Distinct from other anti-apoptotic prot...[ Read more ]

Apoptosis is a highly orderly process, which is important for development, maintaining tissue normality as well as the prevention of diseases. The mechanism of apoptosis is very complicated. It is known that mitochondria play a crucial role by releasing pro-apoptotic molecules from intermembrane space under certain apoptotic stimuli. A key step to trigger the release of mitochondrial intermembrane proteins is the translocation of Bax from the cytosol to mitochondria outer membrane to form aggregates. The mechanisms for this process, however, have not been fully understood yet. Apparently, other Bcl-2 family proteins also play important roles in regulating Bax translocation and aggregation. Among them, Mcl-1 is one of the major negative regulators. Distinct from other anti-apoptotic proteins, Mcl-1 must be rapidly degraded for the apoptotic progression. In this study, we would like to answer two major questions: (1) What are the mechanisms by which Mcl-1 exerts the anti-apoptotic function? (2) What are the mechanisms by which Mcl-1 is eliminated during apoptosis?

Using various GFP-tagged deletion mutants, we found that different domains of Mcl-1 play different roles in regulating apoptosis. We then found that the anti-apoptotic function of Mcl-1 is mainly through inhibiting the activation of Bax. This inhibitory effect is through Mcl-1 interaction with activators of Bax rather than directly interacting with Bax itself. This is different from the action of other anti-apoptotic proteins, such as Bcl-XL, which was demonstrated to inhibit Bax by direct interaction. Furthermore, Mcl-1 appears to suppress mitochondria calcium uptake by inhibiting p32, which was demonstrated here to be associated with the mitochondrial uniporter. This action can partially account for the anti-apoptotic function of Mcl-1. Finally, we found a novel motif (VTLISFG) residing in BH1 domain that appears to be responsible for the degradation of the Mcl-1 protein. We believe that such a mechanism may act together with poly-ubiquitination to regulate Mcl-1 degradation. The detailed mechanism, however, still needs to be determined.