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MST, a physiological caspase substrate, highly sensitizes apoptosis both upstream and downstream of caspase
Abstract:
The human serine/threonine kinase, mammalian STE20-like kinase (MST), is considerably homologous to the budding yeast kinases, SPS1 and STE20, throughout their kinase domains. The cellular function and physiological activation mechanism of MST is unknown except for the proteolytic cleavage-induced activation in apoptosis. In this study, we show that MST1 and MST2 are direct substrates of caspase-3 both in vivo and in vitro. cDNA cloning of MST homologues in mouse and nematode shows that caspase-cleaved sequences are evolutionarily conserved. Human MST1 has two caspase-cleavable sites, which generate biochemically distinct catalytic fragments. Staurosporine activates MST either caspase-dependently or independently, whereas Fas ligation activates it only caspase-dependently. Immunohistochemical analysis reveals that MST is localized in the cytoplasm. During Fas-mediated apoptosis, cleaved MST translocates into the nucleus before nuclear fragmentation is initiated, suggesting it functions in the nucleus. Transiently expressed MST1 induces striking morphological changes characteristic of apoptosis in both nucleus and cytoplasm, which is independent of caspase activation. Furthermore, when stably expressed in HeLa cells, MST highly sensitizes the cells to death receptor-mediated apoptosis by accelerating caspase-3 activation. These findings suggest that MST1 and MST2 play a role in apoptosis both upstream and downstream of caspase activation.
Insights
Mammalian STE20-like kinase (MST) is cleaved by caspase-3 during apoptosis. This study reveals MST1 and MST2 function both upstream and downstream of caspase activation in programmed cell death.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- The cellular function of mammalian STE20-like kinase (MST) remains largely unknown.
- MST is homologous to yeast kinases SPS1 and STE20.
- MST activation is primarily understood through proteolytic cleavage during apoptosis.
Purpose of the Study:
- To investigate the role of MST1 and MST2 in apoptosis.
- To determine if MST is a substrate of caspase-3.
- To elucidate the activation mechanisms and cellular localization of MST during apoptosis.
Main Methods:
- In vivo and in vitro assays to identify MST as a caspase-3 substrate.
- cDNA cloning of MST homologues in mouse and nematode.
- Immunohistochemical analysis of MST localization.
- Transient and stable expression of MST1 in HeLa cells.
Main Results:
- MST1 and MST2 are direct substrates of caspase-3.
- Caspase-cleaved sequences in MST are evolutionarily conserved.
- MST activation can be caspase-dependent or independent, depending on the stimulus.
- Cleaved MST translocates to the nucleus during Fas-mediated apoptosis.
- MST expression sensitizes cells to death receptor-mediated apoptosis.
Conclusions:
- MST1 and MST2 play a dual role in apoptosis, acting both upstream and downstream of caspase activation.
- MST cleavage by caspase-3 is a conserved mechanism.
- MST's nuclear translocation suggests a role in nuclear events during apoptosis.