MST, a physiological caspase substrate, highly sensitizes apoptosis both upstream and downstream of caspase activation.
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.
Institute for Virus Research, Kyoto University, Kyoto 606-8507, Japan.
K K
KK
Lee
T
T
Ohyama
N
N
Yajima
S
S
Tsubuki
S
S
Yonehara
D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal / Q000378:metabolism; D017209:Apoptosis; D001665:Binding Sites; D017173:Caenorhabditis elegans; D053148:Caspase 3; D020169:Caspases / Q000378:metabolism*; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D003001:Cloning, Molecular; D017124:Conserved Sequence; D003593:Cytoplasm / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D049452:Green Fluorescent Proteins; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D047908:Intracellular Signaling Peptides and Proteins; D019169:Jurkat Cells; D008164:Luminescent Proteins / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D011233:Precipitin Tests; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism* / Q000502:physiology*; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D000090742:Serine-Threonine Kinase 3; D015398:Signal Transduction; D013050:Spectrometry, Fluorescence; D019311:Staurosporine / Q000494:pharmacology; D014162:Transfection; D014407:Tumor Cells, Cultured; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism; D001616:beta-Galactosidase / Q000378:metabolism; D019014:fas Receptor / Q000378:metabolism; D000099317:Hippo Kinases
D000911:Antibodies, Monoclonal; D053148:Caspase 3; D020169:Caspases; D018076:DNA, Complementary; D004791:Enzyme Inhibitors; D049452:Green Fluorescent Proteins; D047908:Intracellular Signaling Peptides and Proteins; D008164:Luminescent Proteins; D017346:Protein Serine-Threonine Kinases; D011993:Recombinant Fusion Proteins; D000090742:Serine-Threonine Kinase 3; D019311:Staurosporine; D014409:Tumor Necrosis Factor-alpha; D001616:beta-Galactosidase; D019014:fas Receptor; C490660:Stk4 protein, mouse; D000099317:Hippo Kinases; C095566:STK4 protein, human; C097835:STK3 protein, human; C490818:Stk3 protein, mouse; C505407:CASP3 protein, human; C505409:Casp3 protein, mouse