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 caspasecleavable 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.

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The Fas system has been extensively investigated as a model of apoptosis and the caspase cascade has been shown to be a characteristic mechanism of signaling of apoptosis. We have identi®ed and puri®ed a kinase that was activated after the stimulation of Fas on human thymoma-derived HPB ± ALL cells. Partial amino acid sequencing of the puri®ed kinase revealed it to be MST/ Krs, member of the yeast STE20 family of protein kinases. MST/Krs was activated by proteolytic cleavage and proteolytic activation was blocked by the caspase inhibitor, Z-VAD-FK. A mutant MST with Asp?Asn replacement at a putative caspase cleavage site was resistant to either the proteolytic cleavage or the activation of the kinase activity. These ®ndings suggest that proteolytic activation is one activation mechanism of MST and plays a role in apoptosis.

The activation of multiple interleukin-1b converting enzyme-related proteases (caspases) in apoptotic mammalian cells raises questions as to whether the multiple active caspases have distinct roles in apoptotic execution as well as how these proteases are organized in apoptotic signaling pathways. Here we used an a nity-labeling agent, YV(bio)KD-aomk, to investigate the caspases activated during apoptotic cell death. YV(bio)KD-aomk identi®ed six distinct polypeptides corresponding to active caspases in Fas-stimulated Jurkat T cells. On staurosporine treatment, four polypeptides were detected. Competition experiments showed that the labeled caspases have distinct substrate preferences. Stepwise appearance of the labeled caspases in each cell death event was consistent with the view that the activated caspases are organized into protease cascades. Moreover, we found that stepwise activation of caspases similar to that induced by Fas ligation is triggered by exposing non-apoptotic Jurkat cell extracts to caspase-8 (MACH/FLICE/Mch5). Conversely, CrmA protein, a viral suppressor of Fas-induced apoptosis, inhibited the protease activity of caspase-8. Overall, these ®ndings provide evidence that caspase-8, a CrmA-sensitive protease, is responsible for initiating the stepwise activation of multiple caspases in Fas-stimulated cells.

We purified a protein of 32 kDa from human thymoma HPB-ALL cells that was co-purified with a catalytic fragment of MST (mammalian STE-20-like), a kinase of the STE20 family, which is proteolytically activated by caspase in apoptosis (

Mammalian STE20-like kinase (MST) is a member of the yeast STE20-related kinase family and proteolytically activated by caspase during apoptosis. However, its other cellular functions are not known, including its activation mechanism, substrate(s), and subcellular localization. In this report, using anti-MST monoclonal antibodies, we clearly show that endogenous MST is localized in cytoplasm in a leptomycin B-dependent manner. Analyses with serial deletions and point mutations show that MST has two functional nuclear export signals and, unexpectedly, another localization motif for nuclear import. When cells are treated with leptomycin, monomeric MST is accumulated more rapidly in the nucleus than dimeric MST, indicating that dimerization contributes to the cytoplasmic retention of MST. Okadaic acid, an inhibitor of phosphatase 2A, induces activation of MST and translocation into the nucleus. Using phosphopeptide-specific antibody, we directly show that okadaic acid induces phosphorylation in the activation loop of MST, and, once phosphorylated, MST is rapidly translocated to the nucleus. However, kinasedeficient MST does not enter the nucleus, indicating that phosphorylation and activation is required for okadaic acid-induced nuclear translocation. In apoptotic cells, the activation of MST does not require phosphorylation in the activation loop and occurs through the release of C-terminal regulatory domain by caspase-dependent cleavage. Kinase-deficient MST functions dominant-negatively and represses okadaic acid-induced morphological change indicating that MST plays a role in okadaic acid-induced cellular shrinkage. Our identification of cytoplasmic and nuclear localization motifs and phosphorylation-dependent translocation of MST suggests that regulation of localization is important to the biological function of MST, including its effects on cellular morphology.

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Background:The transcriptional coactivator YAP has a dual role, stimulating cell proliferation and promoting apoptosis. Results: YAP is phosphorylated and activated in response to genotoxic stress. Conclusion: The phosphorylation of YAP at multiple sites activates transcription and protects against apoptosis. Significance: Hyperphosphorylation provides a mechanism by which YAP regulates transcription and apoptosis.The transcriptional coactivator Yes-associated protein (YAP) has been implicated in tumorigenesis by regulating cell proliferation and apoptosis. YAP interacts with the transcription factor TEAD and is essential in mediating TEAD-dependent gene expression. Here we show that YAP is hyperphosphorylated and activated in response to genotoxic stress such as UV irradiation and cisplatin treatment. Using high resolution mobility shift assay for phosphorylated proteins, we identified multiple sites of phosphorylation induced by UV irradiation. Pretreatment with p38 and JNK inhibitors completely suppressed the mobility retardation of phosphorylated YAP in UV-irradiated cells. Coimmunoprecipitation experiments showed that the physical interaction of YAP with TEAD was markedly enhanced by UV irradiation or CDDP treatment but suppressed by pretreatment with p38 and JNK inhibitors. Similarly, pretreatment with p38 and JNK inhibitors suppressed the expression of YAP/TEAD target genes, which were elevated on exposure to genotoxic stress. Using phosphomimetic and phosphorylation-deficient YAP mutants, we showed that the coactivator activity of YAP correlated with its state of phosphorylation and sensitivity to cisplatin-induced apoptosis. Our results demonstrate that multisite phosphorylation of YAP induces YAP/TEAD-dependent gene expression and provides a mechanism by which YAP regulates apoptosis differently depending on cellular context.The opposing actions of proliferation and apoptosis control cell numbers in particular tissues and organs and the coordination of cell fate is fundamental to animal development (1). The Hippo signaling pathway plays a key role in controlling both cell proliferation and apoptosis (2). Components of the Hippo pathway such as Merlin, Lats, and MST, are known to contribute to tumorigenesis (3-5). The MST-SAV complex phosphorylates and activates Lats, an NDR family kinase. Lats inhibits YAP, 2 a transcriptional coactivator, via phosphorylation of HXRXX(S/T) motif. This mechanism of regulation is involved in cell contact inhibition and tissue growth control and epithelial-mesenchymal transition. YAP binds to and activates TEAD, a transcription factor essential to the biological functions of . YAP is the candidate oncogene in the human chromosome 11q22 amplicon that is evident in several cancers (9, 10). Elevated YAP levels and increased nuclear localization have been reported in multiple cancerous tissues (11). Moreover, YAP overexpression in MCF10A induces epithelial-mesenchymal transition, a hallmark of tumorigenic transformation (10). YAP overexpression stimulates proliferation and in...